Article

Ground-Water Surface-Water Interactions and Long Term Change in Riverine Riparian Vegetation in the Southwestern United States

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Abstract

Riverine riparian vegetation has changed throughout the southwestern United States, prompting concern about losses of habitat and biodiversity. Woody riparian vegetation grows in a variety of geomorphic settings ranging from bedrock-lined channels to perennial streams crossing deep alluvium and is dependent on interaction between ground-water and surface-water resources. Historically, few reaches in Arizona, southern Utah, or eastern California below 1530 m elevation had closed gallery forests of cottonwood and willow; instead, many alluvial reaches that now support riparian gallery forests once had marshy grasslands and most bedrock canyons were essentially barren. Repeat photography using more than 3000 historical images of rivers indicates that riparian vegetation has increased over much of the region. These increases appear to be related to several factors, notably the reduction in beaver populations by trappers in the 19th century, downcutting of arroyos that drained alluvial aquifers between 1880 and 1910, the frequent recurrence of winter floods during discrete periods of the 20th century, an increased growing season, and stable ground-water levels. Reductions in riparian vegetation result from agricultural clearing, excessive ground-water use, complete flow diversion, and impoundment of reservoirs. Elimination of riparian vegetation occurs either where high ground-water use lowers the water table below the rooting depth of riparian species, where base flow is completely diverted, or both. We illustrate regional changes using case histories of the San Pedro and Santa Cruz Rivers, which are adjacent watersheds in southern Arizona with long histories of water development and different trajectories of change in riparian vegetation.

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... 1). Streams in Canyon de Chelly were historically wide, shallow and braided, with native cottonwood and willows along the margins ( Webb and Leake, 2006; Cadol and others, 2011). Some of the most downstream reaches still remain wide and shallow however, the tributary canyon streams have narrowed and downcut 1 to 5 meters (m) over the last 50 years. ...
... Climate change imposes additional challenges by further reducing discharge, introducing variability in seasonal precipitation patterns, and increasing temperatures (Palmer and others, 2008;Seager and others, 2012). Collectively, these changes in a river or stream's annual hydrology affects surface and groundwater dynamics, fluvial processes, and the linked aquatic and riparian responses, particularly in arid regions (Palmer and others, 1997;Webb and Leake, 2006;Osterkamp and Hupp, 2010;Perry and others, 2012). Recognizing the inherent ecosystem services that riparian and aquatic habitats provide, society increasingly supports restoring the functionality of riparian and aquatic ecosystems (National Research Council, 1992). ...
... By Webb and Leake, 2006;Cadol and others, 2011). Some of the most downstream reaches still remain wide and shallow however, the tributary canyon streams have narrowed and downcut 1 to 5 meters (m) over the last 50 years. ...
... Hydrological processes driven by groundwater and surface water interactions strongly influence the ecological functioning and sustainability of riparian ecosystems (Webb and Leake 2006). In arid regions where evapotranspiration (ET) greatly exceeds annual rainfall, these systems are clearly groundwater dependent Newman et al. 2006;Cleverly et al. 2015). ...
... Similar to the reduction in overall river flows, peak snowmelt runoff in the MRG during our 2000-2013 monitoring period was substantially lower than the post-dam mean, likely as a result of prevailing drought (Fig. 4). Cottonwoods are more susceptible than saltcedar to reduced flow and altered timing of peaks (Braatne et al. 2007;Merritt and Poff 2010), which are typical hydrological responses downstream of dams (Webb and Leake 2006;Burke et al. 2009). Although Cochiti Dam operations may have extended peak flow timing later into the growing season (Fig. 4), many free-flowing, snowmelt-driven rivers are experiencing earlier peak flows, partially uncoupling seasonal flow from seedling establishment (Rood et al. 2008). ...
... Groundwater regimes necessary to regenerate and sustain aridland cottonwood-dominated riparian ecosystems need to be well characterized and ecosystem response to changes in groundwater availability carefully monitored Webb and Leake 2006;Stromberg et al. 2007). Ideally, informed flow management could enable cottonwood seedling recruitment (Mahoney and Rood 1998). ...
Article
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Hydrological processes drive the ecological functioning and sustainability of cottonwood-dominated riparian ecosystems in the arid southwestern USA. Snowmelt runoff elevates groundwater levels and inundates floodplains, which promotes cottonwood germination. Once established, these phreatophytes rely on accessible water tables (WTs). In New Mexico’s Middle Rio Grande corridor diminished flooding and deepening WTs threaten native riparian communities. We monitored surface flows and riparian WTs for up to 14 years, which revealed that WTs and surface flows, including peak snowmelt discharge, respond to basin climate conditions and resource management. WT hydrographs influence the composition of riparian communities and can be used to assess if potential restoration sites meet native vegetation tolerances for WT depths, rates of recession, and variability throughout their life stages. WTs were highly variable in some sites, which can preclude native vegetation less adapted to deep drawdowns during extended droughts. Rates of WT recession varied between sites and should be assessed in regard to recruitment potential. Locations with relatively shallow WTs and limited variability are likely to be more viable for successful restoration. Suitable sites have diminished greatly as the once meandering Rio Grande has been constrained and depleted. Increasing demands on water and the presence of invasive vegetation better adapted to the altered hydrologic regime further impact native riparian communities. Long-term monitoring over a range of sites and hydroclimatic extremes reveals attributes that can be evaluated for restoration potential.
... Riparian areas throughout Canyon de Chelly National Monument in northeastern Arizona were invaded early in the 20th century and by 2005 tamarisk and Russian olive dominated the floodplains ( fig. 1). Streams in Canyon de Chelly were historically wide, shallow and braided, with native cottonwood and willows along the margins ( Webb and Leake, 2006; Cadol and others, 2011). Some of the most downstream reaches still remain wide and shallow however, the tributary canyon streams have narrowed and downcut 1 to 5 meters (m) over the last 50 years. ...
... Climate change imposes additional challenges by further reducing discharge, introducing variability in seasonal precipitation patterns, and increasing temperatures (Palmer and others, 2008;Seager and others, 2012). Collectively, these changes in a river or stream's annual hydrology affects surface and groundwater dynamics, fluvial processes, and the linked aquatic and riparian responses, particularly in arid regions (Palmer and others, 1997;Webb and Leake, 2006;Osterkamp and Hupp, 2010;Perry and others, 2012). Recognizing the inherent ecosystem services that riparian and aquatic habitats provide, society increasingly supports restoring the functionality of riparian and aquatic ecosystems (National Research Council, 1992). ...
... By Webb and Leake, 2006;Cadol and others, 2011). Some of the most downstream reaches still remain wide and shallow however, the tributary canyon streams have narrowed and downcut 1 to 5 meters (m) over the last 50 years. ...
... Climate change imposes additional challenges by further reducing discharge, introducing variability in seasonal precipitation patterns, and increasing temperatures (Palmer and others, 2008;Seager and others, 2012). Collectively, these changes in a river or stream's annual hydrology affects surface and groundwater dynamics, fluvial processes, and the linked aquatic and riparian responses, particularly in arid regions (Palmer and others, 1997;Webb and Leake, 2006;Osterkamp and Hupp, 2010;Perry and others, 2012). Recognizing the inherent ecosystem services that riparian and aquatic habitats provide, society increasingly supports restoring the functionality of riparian and aquatic ecosystems (National Research Council, 1992). ...
... By Webb and Leake, 2006;Cadol and others, 2011). Some of the most downstream reaches still remain wide and shallow however, the tributary canyon streams have narrowed and downcut 1 to 5 meters (m) over the last 50 years. ...
Technical Report
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The Las Vegas Wash is a small, historically ephemeral stream in the Lower Colorado River Basin, draining the 1,600 square-mile Las Vegas Valley watershed in southern Nevada and now contributing about 200,000 acre-feet per year of return flow water to Lake Mead. Base flows range diurnally between about 200 and 300 cubic feet per second (cfs) mostly because of highly treated wastewater contributions resulting from municipal and industrial water uses. Urban runoff into upstream tributaries and shallow groundwater drainage contribute lesser amounts of base flow to the Las Vegas Wash, while stormwater has historically swollen flows above 10,000 cfs. By the late 1990’s, increasing base flows and episodic storm flows eroded more than 8,000,000 cubic yards of sediment from the channel (Buckingham and others, 2004), nearly eliminating more than 2,000 acres of wetland and riparian habitat that had been created since the 1950's and degrading the ecosystem services they provided. The Southern Nevada Water Authority (SNWA) established a citizen advisory committee in 1997 to obtain recommendations on how to prevent further degradation to the Las Vegas Wash. By 1998, the multi-stakeholder Las Vegas Wash Coordination Committee was formed and SNWA was tasked with leading the effort to stabilize the Las Vegas Wash through an adaptive management process. More than $125,000,000 has been spent on stabilizing the Las Vegas Wash by designing and constructing erosion control structures (also referred to as “weirs”) with improvements inuring to the benefit of various ecosystem services. The weirs are constructed to widen the wetted stream channel, decrease its depth, and reconnect the channel to the floodplain (fig. 1). These conditions support the reestablishment of wetland and riparian vegetation which further armors the channel from erosion in addition to the ecosystem benefits provided (i.e. improvements to water quality and wildlife habitat).
... Therefore, riparian ecosystems in arid environments are highly dynamic in terms of changes on land use and land cover [24,25]. These changes often result in the modification of the structure and function of riparian vegetation due to active change (e.g., the clearing of riparian vegetation to establish agricultural areas) or changes introduced by constant pressure posed by human activities (e.g., Cattle Ranching) [11,26,27]. For arid environments, there is a lack of information regarding the assessment of disturbance, however, it is of utmost importance to understand how different disturbance-causing activities are changing riparian habitats in Sonora, and how these changes could be compromising the provision of ecosystem services. ...
... Riparian Vegetation in all sites shows an herbaceous cover of 50% or more, this can be due to the fact that field sampling was done during the growing season. Shrub cover is variable among all sites, the lowest percentages in Aguilar Ranch, Cienega and San Juan Ranch can be due to the constant presence of cattle and the negative effect it has on shrubs [4,11,26,[52][53][54][55]. Trees cover more than 30% of the area in most sites ( Figure 4), which indicates certain stability among the canopy. ...
Article
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Transformation or modification of vegetation distribution and structure in arid riparian ecosystems can lead to the loss of ecological function. Mexico has 101,500,000 ha of arid lands, however there is a general lack of information regarding how arid riparian ecosystems are being modified. To assess these modifications, we use eight sites in the San Miguel River (central Sonora) to analyze (1) riparian vegetation composition, structure and distribution using field sampling and remote sensing data from Unmanned Aerial Vehicles (UAV); (2) productivity (proxies), using vegetation indices derived from satellite data; and (3) variability posed by riparian vegetation and vegetation adjacent to riparian habitats. The development of a simple yet informative Anthropogenic-disturbance Index (ADI) allowed us to classify and describe each study site. We found sharp differences in vegetation composition and structure between sites due to the absence/presence of obligate-riparian species. We also report significant difference between EVI (Enhanced Vegetation Index) values for the dry season among vegetation types that develop near the edges of the river but differ in composition, suggesting that land cover changes form obligate-riparian to facultative-riparian species can lead to a loss in potential productivity. Finally, our tests suggest that sites with higher disturbance present lower photosynthetic activity.
... Riparian forests support rich biodiversity and act as corridors between forest habitats and populations (Fremier et al. 2015;de la Fuente et al. 2018). Riparian habitats provide home for a variety of organisms, ranging from plants to algae and insects to fish to migratory birds and resident animals (Webb and Leake 2006). They are recognized as the important sites for migratory and breeding birds (McClure et al. 2015). ...
... Riparian zones of South Africa are vulnerable to the widespread invasion of Eucalyptus camaldulensi (Forsyth et al. 2004;Tererai et al. 2013;). All riparian ecosystems in the Southwest USA, are at risk for decline (Tellman et al. 1997) and the invasion of non-native species, mainly tamarisk (Tamarix ramosissima), posed a great concerns for sustainability and ecological health of these ecosystems (Webb and Leake 2006). ...
Article
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Riparian zones are among the most valuable ecosystems on the earth. They act as the ecological engineers that improve river health through delivering a range of ecosystem functions. Stream bank stabilization, pollutant and sediment buffering, temperature regulation, provision of energy to river food webs and communities, groundwater recharge and provision of ecological corridors and habitat for wildlife, are among major ecosystem functions of riparian zones that play a great role in river health. Besides these ecosystem functions, riparian zones also provide various ecosystem goods and services for human well-being. But in the current scenario, riparian zones are under severe threat due to agricultural activities, urbanization, river flow alteration, overexploitation, climate change, pollution, and biological invasion. In the present and probable future scenarios of declining river health and global environmental changes, there is a pressing need of an integrated approach for managing riparian zones. This review article aims to advocate an integrated approach for riparian zone management based on various components such as riparian condition assessment, policy framework, stakeholder's participation, management practices, legislation, and awareness. Authors also discussed riparian zones in context of their concepts, features, functions, and threats.
... Historic observations indicate that entrenchment occurs on the order of years to decades and can lead to loss of productive agricultural lands and infrastructure along alluvial corridors. Moreover, the lowering of the alluvial aquifer associated with arroyo entrenchment leads to loss of riparian habitat and floodplain cienega environments (Webb and Leake, 2006). ...
... Because precipitation and evapotranspiration are the two of most important controlling factors affecting soil water content. Even though the groundwater remains stable and is not a water source for grass (Geris et al., 2017), it is a critical factor controlling the evapotranspiration rate (Webb and Leake, 2006) and supporting soil water retention in upper soil layers. ...
... In contrary to the present findings, Klimo et al. (2013) observed that the herb layer has shown the most resgistered responses due to changes in the groundwater table following river regulation. The negative influence of river regulation on the riparian vegetation can attribute to subsidence of the water table as riparian species in particular are sensitive to changes in groundwater levels ( Webb and Leake, 2006). Mature trees can survive for a short period under rapid drawdown, while saplings cannot (Shafroth et al. 2000). ...
Article
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Construction of dams across rivers is a common practice worldwide. However, obstructing free-flowing rivers could result in many negative impacts on riparian ecosystems, and are highly site-specific. In the present study, we evaluate the potential impacts of river regulation on the riparian vegetation by enumerating the vegetation at different distances from the dam/ reservoir complex from upstream to downstream reaches of a tributary of the river Mahaweli in Sri Lanka. The tree-dominated riparian vegetation has been evaluated using belt transects located in the immediate and 8 km away from the reservoir/dam towards both upstream and the downstream of the tributary (US-0, US-8; DS-0, DS-8, respectively). A total of 150 species belonging to 58 families were identified, of which 43% were tree species. Approximately 77% of species were found exclusively in the upstream while 8% were recorded in the downstream. Both canopy and understory layers showed a significant decline in terms of richness, abundance, diversity and stem density from upstream to downstream. Leuceana leucocephala (Lam.) de Wit, an invasive tree species, showed higher relative abundance in downstream reaches, while riparian species such as Terminalia arjuna (Roxb.) Wight & Arn. and Pongamia pinnata (L.) showed no marked decline from upstream to downstream. The conditions triggered by reduced water discharges and severe river bank erosion may have contributed to these changes. The results indicate that the extreme water management practices in storage reservoirs have incurred negative impacts on the composition of the riparian vegetation. Introduction of well-coordinated flow management practices may help to mitigate some of these negative impacts.
... Hazardous wastes and heavy metal are detrimental to all life forms (Dixit et al., 2015;Sharma, Katnoria, Kaur, & Nagpal, 2015). Organic forms of heavy metals such as mercury and lead can accumulate in living tissues; the health implication ranges from low immunocompetence, failed reproduction to high mortality in a number of taxa (Falq et al., 2011;Fritsch et al., 2010;Hollamby et al., 2004;Kerby, Richards-Hrdlicka, Storfer, & Skelly, 2010;Scheifler et al., 2006;Webb & Leake, 2006). Unprecedented levels of heavy metals and their negative role in biodiversity loss and habitat degradation pose major ecological concerns (Ayangbenro & Babalola, 2017;Kibria, 2016;Sharma et al., 2015). ...
... Even though our study is set in a very different climatic setting, the prerequisites for the existence of a cut-and-fill system in the semi-arid setting are remarkably similar to those found at Wangi Creek, including i) (high) sediment delivery, ii) large magnitude flood events, and iii) a stable surface (summarized in Tucker, et al. (2006)). In addition, our study highlights three important factors to consider: firstly, the important role of the shallow floodplain water table through feedbacks with the stabilizing effects of vegetation, an effect also observed by Webb, et al. (2006) in semi-arid cut-and-fill systems. The fact that feedbacks between groundwater and riparian vegetation can control channel morphodynamics has also been found in other settings and river types (Bätz, et al., 2016), and might have been overlooked in the past. ...
Article
The modern distribution of monsoonal rainforest in the Australian tropics is patchy and is mainly associated with river corridors and groundwater springs, which indicates a strong dependence on hydrologic and geomorphic conditions. While their present distribution is well known, very little data exists on past spatial and temporal dynamics of these ecosystems, or their medium‐ to longer‐term controls. Factors such as (i) fire frequency and type, and/or (ii) hydroclimatic conditions (e.g. droughts) have been proposed to control riverine corridor rainforest extent. Recent observations, however, also suggest an additional (iii) geomorphic control induced by alluvial knickpoint migration. Sediment sequences provide valuable archives for the reconstruction of longer‐term (i) floodplain sedimentary dynamics, (ii) local vegetation history, and (iii) catchment‐wide fire histories. This study investigates such a sediment sequence at Wangi Creek, and shows that a phase of aggradation, lasting ~4000 years, was recently disrupted by channel incision and floodplain erosion. The aggradational phase is characterized by sand deposition with average vertical floodplain accretion rates of 0.8 cm/year and includes phases of soil development. The recent incisional phase has changed hydro‐geomorphic conditions and caused widespread degradation of vegetation, erosion and lowering of the macro‐channel surface. While there is no evidence in our data for an erosional event of similar magnitude since the onset of late Holocene floodplain aggradation, Wangi Creek experienced significant erosion and incision immediately before ~4000 years, providing the first evidence for a tropical cut‐and‐fill river system. We hence argue that phases of aggradation mainly controlled by biotic processes alternate and depend on feedbacks with incision phases controlled mainly by abiotic processes. The results show that eco‐hydro‐geomorphic feedbacks may play a crucial role in the medium to longer‐term history of tropical fluvial systems and need to be considered when interpreting fluvial archives with regards to climate, fire or human induced change.
... Alluvial valleys throughout the semi-arid American Southwest are dissected by 5-40-mdeep arroyos, which are entrenched channels characterized by near-vertical walls and flat channel bottoms (Bryan, 1925). Rapid nearsynchronous historic arroyo entrenchment in the late-eighteenth century and early nineteenth century was one of the most significant geomorphic events in the region, causing former floodplains to become terraces and leading to a decline in local water tables, changes in riparian vegetation, and altered channel morphology (e.g., Webb and Leake, 2006). Early work and observations of stratigraphic exposures in the newly formed arroyo walls lead to speculations of driving mechanisms and indicated repeated prehistoric entrenchment and aggradation (Bryan, 1925;Bailey, 1935;Antevs, 1952;Judson, 1952). ...
Article
Many alluvial valleys in the American Southwest are entrenched within continuous arroyos, and stratigraphic evidence indicates that these fluvial systems experienced repeated periods of entrenchment and aggradation during the mid- to late-Holocene. Previous research suggests arroyo dynamics were regionally quasi-synchronous, implying that they were driven by allogenic forcing due to hydroclimatic fluctuations. However, several of these interpretations rely on records with limited age control and include distal correlations across the American Southwest. While hydroclimatic variability must exert some role, autogenic mechanisms related to catchment-specific geomorphic thresholds are hypothesized to partially control the timing of arroyo dynamics. If driven by autogenic processes, episodes of arroyo cutting and filling may not be regionally contemporaneous. Recent improvements in dating methods permit more detailed reconstructions of the timing and evolution of arroyo dynamics, allowing for a more nuanced assessment of these competing hypotheses. Here we present a uniquely large and focused chronostratigraphic data set from two alluvial reaches of Kanab Creek, located in the Grand Staircase region of southern Utah. Episodes of prehistoric arroyo cutting and filling are reconstructed from 27 sites through recognition of soils and buttressed unconformities in the arroyo-wall stratigraphy, and age control derived from 54 optically stimulated luminescence (OSL) ages and 50 radiocarbon ages. Our chronostratigraphic data set indicates five periods of channel aggradation occurred since ca. 6.0 ka, with each interrupted by an episode of arroyo entrenchment. Repeated aggradation to a similar channel elevation suggests attainment of a threshold profile, and comparison of the pre-entrenchment longitudinal profile with the modern arroyo channel demonstrates that changes between end-member entrenched and aggraded states are expressed in channel concavity and slope. We propose that arroyo dynamics are partially driven by sediment supply and the rate of channel aggradation, and that these systems must approach complete re-filling before they become sensitive to incision. Entrenchment itself appears to be associated with rapid transitions from pronounced decadal-scale aridity to pluvial (wetter) periods. Not all such hydroclimatic fluctuations are associated with arroyo entrenchment, which highlights the importance of threshold controls on the behavior of these systems. The collective period of “dynamic instability” characterized by epicycles of arroyo entrenchment and aggradation did not initiate until the mid-Holocene, when a climatic shift toward warmer and drier conditions likely increased fine-grained sediment supply to the fluvial system.
... Many smaller surface streams within arid regions, such as the Colorado Plateau, have already experienced extensive hydrologic and thermal changes due to direct water diversions and regional groundwater drawdown (e.g. Stromberg et al. 1996;Webb and Leake 2006). These impacts are evident in declining native fish populations, where nonnative salmonids, including brown trout, were introduced in the last century (reviewed in Whiting et al. 2014), and are proliferating in places such as the Colorado River in Grand Canyon following the construction of Glen Canyon Dam (Gloss and Coggins 2005). ...
Article
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Water infrastructure updates at Grand Canyon National Park (GRCA) provide an opportunity to restore natural flow to Bright Angel Creek, adding an additional ~20% to baseflow. This creek provides habitat for endangered humpback chub (Gila cypha) and invasive brown trout (Salmo trutta). We assess how increased flow may alter habitat and how that change may impact native and nonnative species using physical habitat modeling and statistical analysis of stream temperature data. We used System for Environmental Flow Analysis to calculate the change in habitat area for both species in the lower 2.1 km of the creek before and after the increased flow. Results indicate a slight increase in available habitat for juveniles of both species and a slight decrease for spawning brown trout. We used regression modeling to relate daily average air temperature to stream temperature and periods of increased discharge during water system maintenance were used to model the temperatures during likely future conditions. Both high and low stream temperature were dampened due to the added water resulting in fewer days with suitable spawning temperature and more days with suitable growth temperature for humpback chub. Fewer suitable days for growth upstream but more suitable days downstream, were predicted for brown trout. Compared to other streams that sustain populations of humpback chub, flow conditions for Bright Angel Creek provide fewer days throughout the year with suitable temperatures, particularly during the winter months. Juvenile humpback chub rearing may improve through the restoration of flow however the presence of predatory brown trout complicates the net beneficial impact.
... Hydrology is a function of climate inputs, watershed characteristics, and local stream reach features. Riparian vegetation, which is often more directly relevant to terrestrial vertebrates, is denoted by structure and composition, which is a function not only of local hydrology, but also streamside geomorphology and history (Shafroth and others, 2002;Webb and Leake, 2006). ...
Technical Report
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We modeled the current and future breeding ranges of seven bird and five reptile species in the Southwestern United States with sets of landscape, biotic (plant), and climatic global circulation model (GCM) variables. Our modeling approach relied on conceptual models for each target species to inform selection of candidate explanatory variables and to interpret the ecological meaning of developed probabilistic distribution models. We employed logistic regression and maximum entropy modeling techniques to create a set of probabilistic models for each target species.We considered climatic, landscape, and plant variables when developing and testing our probabilistic models. Climatic variables included the maximum and minimum mean monthly and seasonal temperature and precipitation for three time periods. Landscape features included terrain ruggedness and insolation. We also considered plant species distributions as candidate explanatory variables where prior ecological knowledge implicated a strong association between a plant and animal species.Projected changes in range varied widely among species, from major losses to major gains.
... The role of a higher water table in enhanced vegetation colonization and succession has been established through assessments of the role of beavers, which are instrumental in creating river-meadow sequences (Westbrook et al., 2011). Absence of beaver has strongly altered meadow configuration and channel-complexity patterns in the western U.S. (Webb and Leake, 2006). Restoration to Stage 0 may mimic these patterns and facilitate the restoration of the processes needed to sustain them, namely repopulation by beaver, which was shown at SFMR Transect 5 with eDNA results. ...
Article
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Degraded floodplains and valley floors are restored with the goal of enhancing habitat for native fish and aquatic-riparian biota and the protection or improvement of water quality. Recent years have seen a shift toward "process-based restoration" that is intended to reestablish compromised ecogeomorphic processes resulting from site-or watershed-scale degradation. One form of process-based restoration has developed in the Pacific Northwest, United States, that is intended to reconnect rivers to their floodplains by slowing down flows of sediment, water, and nutrients to encourage lateral and vertical connectivity at base flows, facilitating development of dynamic, self-forming, and self-sustaining river-wetland corridors. Synergies between applied practices and the theoretical work of Cluer and Thorne in 2014 have led this form of restoration to be referred to regionally as restoration to a Stage 0 condition. This approach to rehabilitation is valley scale, rendering traditional monitoring strategies that target single-thread channels inadequate to capture pre-and post-project site conditions, thus motivating the development of novel monitoring approaches. We present a specific definition of this new type of rehabilitation that was developed in collaborative workshops with practitioners of the approach. Further, we present an initial synthesis of results from monitoring activities that provide a foundation for understanding the effects of this approach of river rehabilitation on substrate composition, depth to groundwater, water temperature, macroinvertebrate richness and abundance, secondary macroinvertebrate production, vegetation conditions, wood loading and configuration, water inundation, flow velocity, modeled juvenile salmonid habitat, and aquatic biodiversity.
... A more immediate anthropogenic cause of moisture deficits is the damming and diversion of rivers (Chapters 3 and 4). Where water is diverted in large quantities, as is common for rivers dammed for agricultural and other consumptive uses, as well as hydropower, reduced stream discharge can diminish recharge and exchange with local groundwater aquifers (Webb and Leake, 2006). Depressed water tables can increase water stress for riparian plants and result in reduced growth, higher mortality, and dieback of tree crowns (Fig. 5.4;Stromberg and Patten, 1992;Rood et al., 2000). ...
... The consequences for fauna of increasing scour and temperatures in particular are likely to be negative, especially for salmonids (Battin et al. 2007). Alterations to hydrodynamics and flow regimes associated with climate change may also change the relative influence of ground and surface water on vegetation composition, reinforcing structural changes in riparian forest and habitat quality (Webb and Leake 2006). These changes are particularly important due to the influence of riparian habitat on stream temperature and habitat suitability for a suite of other organisms. ...
Technical Report
Located on American coast of the Pacific Ocean, Humboldt Bay is the largest bay between San Francisco Bay (CA) and Coos Bay (OR). The United States Fish and Wildlife Service’s (USFWS) Humboldt Bay National Wildlife Refuge (HBNWR) is comprised of several units in the vicinity of Arcata and Eureka (CA) established to conserve coastal habitats for diverse suite of wildlife and plants. The 9,502 acres within the refuge’s authorized boundary contains freshwater, brackish, and salt marsh; agricultural wetlands; intertidal mudflats; eelgrass beds; and some of the most pristine dune habitats in the western United States (U.S. Fish and Wildlife Service 2009). The 2009 HBNWR Comprehensive Conservation Plan (CCP) (U.S. Fish and Wildlife Service 2009), details the desired future conditions of the refuge, identifies general goals and objectives, and provides long‐ range guidance and management direction for achieving these conditions. As part of its Natural Resource Management Planning (NRMP) process, the refuge is currently undergoing an effort to conduct a more detailed analysis of refuge threats, refine refuge goals and objectives, and develop more specific management strategies and actions than those contained within the CCP. The efforts detailed within this report represent an effort on behalf of HBNWR staff, USFWS Inventory and Monitoring Program Staff, and affiliates of the University of California, Davis John Muir Institute of the Environment effort to fulfill both Department of Interior and USFWS mandates to incorporate climate change considerations in planning. This report is meant to compliment an additional effort undertaken by USFWS staff to characterize climatic trends for the Refuge particularly with respect to a number of hydrological factors of interest (Esralew and Michehl 2015). Differences in area of interest, climate models evaluated, resolution of climatic data, and downscaling methodology used to achieve that resolution make direct comparison between the two efforts difficult. In general, trends for temperature (and related metrics) and the general variability in precipitation values are similar between the two efforts. Actual point estimates vary likely due to the choice of models used, the mechanisms for downscaling, and the temporal period evaluated. https://ecos.fws.gov/ServCat/DownloadFile/107917
... The water required by riparian trees for reproduction, growth, and survival is accessed from surface flows, groundwater, and precipitation (Kolb et al. 1997;Snyder and Williams 2000). These water sources are also linked to one another through natural hydrological processes (Webb and Leake 2006). Periodic shortages in these sources occur naturally across climatic cycles, but for over a century shortages have been caused or exacerbated by surface flow diversion, groundwater withdrawal, and reservoir storage (Phillips et al. 2011;Summitt 2013). ...
... Expansion of riparian vegetation is common among many regulated rivers in the southwestern United States (Webb and Leake, 2006;Mortenson and Weisberg, 2010) due to alterations of sediment transport and flood frequencies associated with flow regulation (Schmidt and Wilcock, 2008). Since the completion of Glen Canyon Dam, riparian vegetation has expanded along the Colorado River corridor in response to alterations in the flow regime (Turner and Karpiscak, 1980;Waring, 1995;Webb and others, 2002;Ralston, 2005Ralston, , 2010Ralston and others, 2008;Sankey and others, 2015a) (fig. ...
Article
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Sandbars along the Colorado River are used as campsites by river runners and hikers and are an important recreational resource within Grand Canyon National Park, Arizona. Regulation of the flow of river water through Glen Canyon Dam has reduced the amount of sediment available to be deposited as sandbars, has reduced the magnitude and frequency of flooding events, and has increased the magnitude of baseflows. This has caused widespread erosion of sandbars and has allowed native and non-native vegetation to expand on open sand. Previous studies show an overall decline in campsite area despite the use of controlled floods to rebuild sandbars. Monitoring of campsites since 1998 has shown changes in campsite area, but the factors that cause gains and losses in campsite area have not been quantified. These factors include, among others, changes in sandbar volume and slope under different dam flow regimes that include controlled floods, gullying caused by monsoonal rains, vegetation expansion, and reworking of sediment by aeolian processes. Using 4-band aerial imagery and digital elevation models (DEMs) derived from total-station survey data, we analyzed topographic and vegetation change at 35 of 37 long-term monitoring sites (2 sites were excluded because topographic measurements do not overlap with measurements of campsite area) using data collected between 2002 and 2009 to quantify the factors affecting the size of campsite area. Over the course of the study period, there was a net loss in campsite area of 2,431 square meters (m2). We find that (1) 53 percent of the net loss was caused by topographic change associated with controlled floods and erosion of those flood deposits, (2) 47 percent of the net loss was caused by increases in vegetation cover, the majority of which occurred in high-elevation campsite area, and (3) gullying was significant at certain sites but overall was a minor factor. Sites in critical reaches—sections of river where campsites are infrequent or where there is high demand by river runners—were subjected to more erosion and changes in sandbar slope than sites in noncritical reaches, suggesting that campsite area is less stable in those reaches. There was also a greater increase in vegetation cover at sites in noncritical reaches than at sites in critical reaches. Our results show a continuation of sandbar erosion and vegetation encroachment that has been occurring at campsites since construction of the dam.
... In many studies, emphasis has been placed on the role of vegetation in facilitating channel, island, and floodplain development during the relatively lowmagnitude floods that occur during "building" phases (e.g., Schumm and Lichty, 1963;Burkham, 1972;Osterkamp and Costa, 1987;Lisle, 1989;Hooke and Mant, 2000;Rountree et al., 2001;Greenbaum and Bergman, 2006). In the southwestern United States, groundwater-surface water interactions, influenced by catchment management activities, have also been shown to facilitate riparian vegetation growth (Webb and Leake, 2006). While the conditions and process thresholds (e.g., velocities, shear stress) that give rise to riparian vegetation removal during floods in dryland rivers still warrant further investigation (Thornes, 1994), flood-related vegetation losses have been studied in relation to catastrophic floods in small limestone streams in central Texas (Baker, 1977), as part of the "arroyo cycle" phenomena in alluvial streams in the southwestern United States (e.g., Graf, 1983;Hereford, 1984Hereford, , 1993, and in sand-bed streams in the Great Plains of the United States (Friedman et al., 1996a(Friedman et al., , 1996b. ...
Article
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High-magnitude flood events are among the world?s most widespread and significant natural hazards and play a key role in shaping river channel-floodplain morphology and riparian ecology. Developing conceptual and quantitative models for the response of bedrock-influenced dryland rivers to such floods is of growing scientific and practical importance but in many instances modelling efforts are hampered by a paucity of relevant field data. Here, we combine extensive aerial and field data with hydraulic modelling to document erosion, deposition and vegetation changes that have occurred during successive, cyclone-driven, extreme floods along a 50 km long reach of the bedrock-influenced Sabie River in the Kruger National Park (KNP), eastern South Africa. Aerial LiDAR and photography obtained after extreme floods in 2000 and 2012 (discharges >4000 m3 s-1) were used to generate Digital Elevation Models (DEMs) and provide the boundary conditions for hydraulic modelling (flow shear stresses for three discharges up to 5000 m3 s-1). For the Sabie River study reach as a whole, DEM differencing revealed that the 2012 flood resulted in net erosion of ~1,219,000 m3 (~53 mm m-2). At the sub-reach scale, however, more complex spatial patterns of erosion, deposition and vegetation change occurred, as largely controlled by differences in channel type (e.g. degree of bedrock and alluvial exposure) and changing hydraulic conditions (shear stresses widely >1000 N m-2 across the river around peak flow). The impact of flood sequencing and relative flood magnitude is also evident; in some sub-reaches, remnant islands and vegetation that survived the 2000 floods were removed during the smaller 2012 floods owing to their wider exposure to flow. These findings are synthesised to refine and extend a conceptual model of bedrock-influenced dryland river response that incorporates flood sequencing, channel type and sediment supply influences. In particular, with some climate change projections indicating the potential for future increases in the frequency of cyclone-generated extreme floods in eastern southern Africa, the Sabie and other KNP rivers may experience additional sediment stripping and vegetation removal. Over time, such rivers may transition to a more bedrock-dominated state, with significant implications for ecological structure and function and the associated ecosystem services. These findings contribute to an improved analysis of the KNP rivers in particular, but also to growing appreciation of the global diversity of dryland rivers and the relative and synergistic impacts of extreme floods
... small change (less than 10 cm) in wetland water table level may have profound impacts on wetland structure and other ecosystem functions (Webb and Leake, 2006). ...
Article
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The southeastern United States hosts extensive forested wetlands, providing ecosystem services including carbon sequestration, water quality improvement, groundwater recharge, and wildlife habitat. However, these wetland ecosystems are dependent on local climate and hydrology, and are therefore at risk due to climate and land use change. This study develops site-specific empirical hydrologic models for five forested wetlands with different characteristics by analyzing long-term observed meteorological and hydrological data. These wetlands represent typical cypress ponds/swamps, Carolina bays, pine flatwoods, drained pocosins, and natural bottomland hardwood ecosystems. The validated empirical models are then applied at each wetland to predict future water table changes using climate projections from 20 general circulation models (GCMs) participating in Coupled Model Inter-comparison Project 5 (CMIP5) under the Representative Concentration Pathways (RCPs) 4.5 and 8.5 scenarios. We show that combined future changes in precipitation and potential evapotranspiration would significantly alter wetland hydrology including groundwater dynamics by the end of the 21st century. Compared to the historical period, all five wetlands are predicted to become drier over time. The mean water table depth is predicted to drop by 4 to 22 cm in response to the decrease in water availability (i.e., precipitation minus potential evapotranspiration) by the year 2100. Among the five examined wetlands, the depressional wetland in hot and humid Florida appears to be most vulnerable to future climate change. This study provides quantitative information on the potential magnitude of wetland hydrological response to future climate change in typical forested wetlands in the southeastern US.
... Other examples of alternative states described for river ecosystems include riverine wetlands with wide, shallow channels and extensive emergent vegetation versus minimally vegetated, gravel-bed channels in desert rivers of the south-western United States, with shifts between states triggered by changes in the magnitude and frequency of floods (Heffernan, 2008) and changes in riparian groundwater levels (Webb and Leake, 2006). Historical removal of riparian vegetation and in-stream wood along the Cann River in south-eastern Australia resulted in substantial increases in channel cross-sectional area and lateral mobility, and changed the affected lengths of channel from a sediment storage zone to a sediment source to downstream regions (Brooks et al., 2003). ...
Article
We demonstrate how land use can drive mountain streams in the Southern Rockies across a threshold to induce an alternative state of significantly reduced physical complexity of form and reduced ecological function. We evaluate field data from 28 stream reaches in relatively laterally unconfined valleys and unmanaged forest that is either old-growth forest or naturally disturbed younger forest, and 19 stream reaches in managed forest with past land use. We evaluate potential differences in stream form, as reflected in channel planform, cross-sectional geometry, and instream wood loads, and stream function, as reflected in pool volume and storage of organic carbon. Field data indicate a threshold of differences in stream form and function between unmanaged and managed stream reaches, regardless of forest stand age, supporting our hypothesis that the legacy effects of past land use result in an alternative state of streams. Because physical complexity that increases stream retentiveness and habitat can maintain aquatic-riparian ecosystem functions, the alternative physical state of streams in managed watersheds creates a physical template for an alternative ecological state with reduced pool volume, organic carbon storage, and ecosystem productivity. We recommend maintaining riparian forests that can supply large wood to streams as a stream restoration technique in historically forested stream segments.
... In semiarid areas, the GW-SW interaction is highly complex and poorly understood. Several studies have also focussed on hydroecological and biogeochemical aspects of hydrology (Webb and Leake, 2006). ...
Article
The identification of specific aquifers that supply water to river systems is fundamental to understanding the dynamics of the rivers' hydrochemistry, particularly in arid and semiarid environments where river flow may be discontinuous. There are multiple methods to identify the source of river water. In this study of the River Andarax, in the Southeast of Spain, an analysis of natural tracers (physico-chemical parameters, uranium, radium and radon) in surface water and groundwater indicates that chemical parameters and uranium clearly identify the areas where there is groundwater-surface water interaction. The concentration of uranium found in the river defines two areas: the headwaters with U concentrations of 2 μg L(-1) and the lower reaches, with U of 6 μg L(-1). Furthermore, variation in the (234)U/(238)U isotopic ratio allowed us to detect the influence that groundwater from the carbonate aquifer has on surface water in the headwaters of the river, where the saline content is lower and the water has a calcium bicarbonate facies. The concentration of (226)Ra and (222)Rn are low in the surface waters: <1.6 × 10(-6) μg L(-1) and <5.1 × 10(-12) μg L(-1), respectively. There is a slight increase in the lower reaches where the water has a permanent flow, greater salinity and a calcium-magnesium-sulphate facies. All this is favoured by the influence of groundwater from the detritic aquifer on the surface waters. The results of this study indicate the utility in the use of physico-chemical and radiological data conjointly as tracers of groundwater-surface water interaction in semiarid areas where the lithology of aquifers is diverse (carbonate and detritic) and where evaporitic rocks are present.
... A similar study is done in different parts of the world. For example Krause, Bronstert, and Zehe 2007 in Germany, Webb and Leake 2006 in the Southwest United States. ...
Thesis
Groundwater dynamics of Morava river floodplain forest in right bank alluvium are profoundly affected by increasing climatic extremes. Soil and hydrological conditions are also changed by the hydraulic engineering measures taken in the Morava river floodplain in the 2nd period of 20th century. The study area of the floodplain of the lower course of Morava river which is also facing to lack of precipitation a lower amount of underground water. The aim of the thesis is to evaluate the trend of the dynamics of underground water and soil moisture in the model area in floodplain forest near Mikulčice. Comparing the result with previous results provides the actual trend of underground water and soil moisture changes. The year 2018 was an extremely dry year in comparison with previous years. GIS method was used to evaluate stratigraphy of surface fluvial sediments and hydroisohips of underground water throughout the period of study. Key Words- Groundwater, Soil moisture, Morava river alluvium, Mikulčice floodplain forest, GIS application, Czech Republic
... Global sea level will continue to rise at a speed of 4-9 mm and 10-20 mm per year under the RCP2.6 and RCP8.5 scenarios [3], consequently, 20-78% of the world's wetlands will be submerged, including coastal wetlands [4,5]. Different inundation depths can affect plant growth, reproduction and biomass allocation by altering sediment and water quality, and they have an important impact on the species diversity and ecosystem stability [6][7][8]. Understanding vegetation response to inundation depth in an estuarine wetland is of great significance for predicting ecological restoration and sustainable development of estuarine wetlands in the future. Inundation depth can directly affect the physiological process of plants and profoundly change the ecological strategy of individual plants. ...
Article
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Inundation shifts plant growth, species diversity and ecosystem stability, but it remains unclear how inundation depth shapes the phenotypic variability of clonal plants in an estuary wetland. To reveal the response of clonal plant populations to inundation depth, we calculated the variation of Phragmites australis using shoot height, leaf length, leaf width, leaf biomass and panicle length in the Liaohe estuary wetland. Reproductive allocation was defined by the ratio of panicle length to shoot height. Linear regression showed that shoot height, leaf length, leaf width, leaf biomass and panicle length were negatively correlated with inundation depth, while the ratio of panicle length to shoot height was negatively correlated with inundation depth (p < 0.0001). Based on data regarding the statistical differences of plant phenotypic traits among P. australis, we recognized populations had generated variation differentiation. Compared with other functional traits, the coefficient of variation of leaf-related traits were at a high level. Therefore, leaf parameters would be the most suitable, and they increased the area and weight to support the action of plants during floods. Multivariate statistical analysis suggested that P. australis populations in the Liaohe estuary wetland were divided into two phenotypic clusters, consistent with geographical distance and morphological similarity. Our results provide a novel perspective on the ecological strategy of cloned plants under inundation change and offer theories for the conservation and restoration of estuarine wetland ecosystems.
... Woody deciduous plant species are also an important indicator of riparian condition and channel evolution potential (Webb & Leake 2006, Polvi et al. 2014, MacFarlane et al. 2017. Woody species including Salix, Populus, Alnus, Cornus and Rosa species contribute immensely to riparian bank stability (Polvi et al. 2014) with fast growing, net-like root systems. ...
... The historic riparian vegetation of the middle Gila River was similar to the lower San Pedro River, but is now dominated by mostly non-natives like Tamarix spp. (Webb and Leake, 2006). Soil composition of the San Pedro River Valley is diverse, like most broad valley systems (Green et al., 2009). ...
... Therefore, wetlands play an important role in regulating the C cycle at a global scale . The hydrological regime is a key process in wetland ecosystems, which not only affects the structure and function of wetland ecosystems but also controls the C sink and source of wetlands (Webb and Leake 2006;Jimenez et al. 2012;Rasmussen et al. 2018). Hydrological regimes have been altered by global climate change in terrestrial ecosystems during recent years, resulting in increases in intensity and frequency of extreme rainfall, thereby increasing the risk of prolonged inundation (IPCC 2013;Trenberth 2011;Westra et al. 2014). ...
Article
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AimsClimate change (extreme rainfall) and water management activities have led to variation in hydrological regimes, especially inundation, which may alter the function and structure of wetlands as well as wetland-atmosphere carbon (C) exchange. However, the degree to which different inundation depths (standing water depth above the soil surface) affect ecosystem CH4 fluxes, ecosystem respiration (Reco) and net ecosystem CO2 exchange (NEE) remains uncertain in wetland ecosystems.Methods We conducted a field inundation depth manipulation experiment (no inundation, i.e. only natural precipitation; 0, water-saturated; 5, 10, 20, 30 and 40 cm inundation depth) in a freshwater wetland of the Yellow River Delta, China. The CH4 fluxes, Reco and NEE were measured with a static chamber technique during the growing seasons (May–October) of 2018 and 2019.ResultsInundation depth significantly increased plant shoot density, above-water level leaf area index (WLAI), above-water level plant shoot height (WHeight), aboveground and belowground biomass of the dominant grass Phragmites australis in both years. Meanwhile, inundation depth increased the CH4 fluxes, Reco (except for 0 cm) and NEE compared to no inundation, which could be attributed partly to the increased plant productivity (shoot density, WLAI, WHeight, biomass). Additionally, the CH4 fluxes, Reco or NEE exhibited parabolic responses to inundation depth. Furthermore, global warming potential (GWP) was significantly decreased under different inundation depths during the growing season, especially from 5 to 40 cm inundation depth in 2019. NEE was the largest contributor to the seasonal GWP, which indicates that the inundated wetlands are a net sink of C and have a cooling climate effect in the Yellow River Delta.Conclusions Inundation depth substantially affects the magnitude of CH4 fluxes, Reco and NEE, which were correlated with altered plant traits in wetland ecosystems. Inundation depth could mitigate greenhouse gas emissions in the P. australis wetlands during the growing season. Inundation depth-induced ecosystem C exchange should be considered when estimating C sequestration capacity of wetlands due to climate change and water management activities, which will assist to accurately predict the impact of hydrological regimes on C cycles in future climate change scenarios.
... In arid and semi-arid basins with limited water resources, human activities are closely related to the development and utilization of water resources, such as the construction of reservoirs, the diversion of river channels, the networking of river channels and canals, the transfer of water across basins, and the exploitation of GW, interfering with the SW and GW circulation to the native state, and changing the recharge-discharge relationship between rivers and groundwater aquifers [71,72], causing a decline in the exchange of SW and GW in some basins, complicating the evolution of the relationship between SW and GW, and producing a series of ecological and environmental problems [73,74]. ...
Article
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The interaction of surface water (SW) and groundwater (GW) is becoming more and more complex under the effects of climate change and human activity. It is of great significance to fully understand the characteristics of regional SW–GW circulation to reveal the water circulation system and the effect of its evolution mechanism to improve the rational allocation of water resources, especially in arid and semi-arid areas. In this paper, Yinchuan Plain is selected as the study area, where the SW–GW interaction is intensive. Three typical profiles are selected to build two-dimensional hydrogeological structure models, using an integrated approach involving field investigation, numerical simulation, hydrogeochemistry and isotope analysis. The SW–GW transformation characteristics are analyzed with these models, showing that geological structure controls the SW–GW interaction in Yinchuan Plain. The SW–GW flow system presents a multi-level nested system including local, intermediate and regional flow systems. The runoff intensity and renewal rate of different flow systems are evidently different, motivating evolution of the hydro-chemical field; human activities (well mining, agricultural irrigation, ditch drainage, etc.) change the local water flow system with a certain impacting width and depth, resulting in a variation of the hydrological and hydro-chemical fields. This study presents the efficacy of an integrated approach combining numerical simulation, hydrogeochemistry and isotope data, as well as an analysis for the determination of GW-SW interactions in Yinchuan Plain.
... Therefore, colmation of river beds on a large scale result in lowering the groundwater table (Brunke and Gonser, 1997) or a decline in the baseflow of rivers. Last but not least, colmation can even transform a perennial river into an ephemeral one (Webb and Leake, 2006). Finally, colmation in the beds of reservoirs reduces groundwater recharge. ...
... Eighty percent of the riparian area of North America and Europe has disappeared in the last 200 years [3]. The ecological functioning and sustainability of these ecosystems depends on the hydrological processes driven by groundwater and surface water interactions [4]. Occasional overbank flooding and reliable access to groundwater regenerate and sustain native riparian vegetation [5,6], as the lifecycle of the riparian vegetation is adapted to the magnitude, timing, and frequency of floods [7]. ...
Article
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Climate change affects many elements of the natural environment and strongly influences the hydrology of rivers. In this study, we investigated trends in temperature, precipitation, and the water level characteristics in the small lowland river Lebiedzianka in northeastern Poland for the 50 year long period of observations (1970–2019). We recorded significant increase in air temperature and potential evapotranspiration, but the annual sum of precipitation did not change. We found significant downward trends for annual runoff. The results show a steady decrease in the number of days with high water levels. These changes caused by global warming will have a strong impact on forest habitats associated with high water levels and periodic inundations. In Europe, many of these precious habitats are protected under the Natura 2000 network as sites of high heritage value; nevertheless, their sustainability will be at risk due to the ongoing changes in their hydrological regime.
... Drought on the Colorado Plateau can be an excursion from the normal precipitation (median precipitation 300 mm/yr (11.8 in/ yr)) in the summer, the winter, or both (Dettinger et al., 1998;Hereford et al., 2002). Winter snowpack is important to support the establishment of perennial woody plant communities whose roots can reach deeper supplies of water (Ni et al., 2002;Weiss and Leake, 2006). ...
Article
A 3300 year-long reconstruction of paleoenvironmental moisture conditions was constructed from a sediment core from North Gate Bog (NGB) in the northern section of Range Creek Canyon within the Colorado Plateau. The methods used to analyze the record include loss on ignition (LOI), magnetic susceptibility (MS), elemental analysis with X-ray fluorescence (XRF), charcoal influx, isotopic analysis, elemental ratios and pollen percentages, influx, and ratios. This study adds two new insights to the paleoenvironmental record of the northern section of the Colorado Plateau. First, four climatic zones were established. Zone 1 (3300–2750 cal yr BP) had 100-year wet to dry variations with droughts recorded from 3300–3200, 3000–2900, and 2800–2700 cal yr BP. Zone 2 (2750–1600 cal yr BP) had an overall dry period with an 800-year transition to increased warmth and winter moisture. Zone 3 (1600–850 cal yr BP) had an overall warm, wet, summer precipitation climate conducive to the establishment of Zea Mays and Pinus edulis, two staple foods of the Fremont culture. The Medieval Climate Anomaly (MCA) registered warm and wet in this part of the Colorado Plateau. Zone 4 (850–0 cal yr BP) had a sharp transition to a drier climate from 850 to 400 cal yr BP. During the Little Ice Age (LIA), wetter climate taxon increases such as Artemisia, Cyperaceae, and Pinus edulis. The second overall finding in this study was that NGB was a place of human activity including Fremont farming. The identification of a Zea mays pollen grain confirms the archeological presumptions that this higher elevation site was used to farm corn along with other sites in Range Creek Canyon (RCC). The post Fremont occupation period was marked by a sharp increase in organic material and a return of pinyon-juniper woodlands.
... Curry Draw empties into a historically wet reach of the San Pedro River (Leenhouts et al. 2006;Pool and Dickinson 2007). The riparian corridor consists of cottonwood-willow galleries often bordered by grassy floodplains or wetlands (Stromberg et al. 2005;Webb and Leake 2006). Sacaton grasslands dominate the floodplain in the upper San Pedro Basin (Stromberg et al. 1996). ...
... En cuanto a la cuenca, cabe destacar que se trata de un espacio poco alterado por la acción humana; aun así, la superficie forestal ha aumentado desde 1950, cuando se impulsaron planes de reforestación en la Comunidad de Madrid(García & de Desarrollo, 2011), aumentando la estabilidad de los suelos, disminuyendo los procesos erosivos en laderas y perdiendo parte de la conectividad entre las vertientes y el cauce(Boix-Fayos et al., 2007).En este tramo de estudio, el aumento de barras de gravas y la pérdida de FCC inducidos por la eliminación del azud y, posiblemente, la ocurrencia de caudales suficientemente altos propiciaron la aparición local de nuevas zonas potenciales para la colonización vegetal.La dinámica de la vegetación riparia tiende hacia un desarrollo funcional y estructural más complejo, un corredor maduro(Corenblit et al., 2014). En las últimas décadas, la reducción de la magnitud de las avenidas ha propiciado la estabilización de la vegetación de ribera, generando un estrechamiento del cauce y limitando el reclutamiento ante la poca disponibilidad de espacios abiertos para especies pioneras(Webb & Leake, 2006;Rivaes et al., 2014). La demolición del azud de Pinilla ha permitido aumentar de manera localizada el dinamismo, produciéndose una apertura de espacios expuestos a la luz, y un aumento de la disponibilidad de barras de gravas colonizables. ...
Article
Las metodologías y herramientas que permitan conocer la evolución del corredor ripario tras actuaciones de restauración ecológica, son imprescindibles para monitorizar la eficiencia de las acciones desarrolladas, y el cumplimiento de los objetivos del proyecto. El uso de la tecnología LiDAR, la clasificación de imágenes y el uso de herramientas SIG, facilitan el seguimiento de las acciones de restauración. En este artículo se expone una metodología de seguimiento de la evolución del corredor ripario en el marco de un proyecto de restauración para un caso de estudio que consistió en la eliminación de un azud en el río Lozoya. Este seguimiento se ha basado en cuantificar los cambios en las variables dendrométricas (altura, diámetro y biomasa) y dasométricas (Fracción de Cabida Cubierta) a partir dos coberturas LiDAR PNOA, realizadas en los años 2010 y 2016, y de la clasificación de imágenes RGB PNOA. Los resultados muestran cambios en el corredor ripario asociados al crecimiento natural, aumentando el número de pies de clases medias de altura y diámetro. Por el contrario, la fracción de cabida cubierta ha disminuido pasando de valores medios en torno al 60-70% a valores entre 40-50%, como resultado de la apertura de espacios abiertos generando un ensanchamiento notable de la zona de cauce activo. Los cambios más relevantes se observaron en un cauce secundario situado en la margen derecha y en las zonas limítrofes con la lámina de agua más próxima a la antigua ubicación del azud. Finalmente, se analizaron las respuestas del corredor a la recuperación de la conectividad, así como las limitaciones y ventajas de emplear esta metodología para analizar la evolución del corredor ripario tras actuaciones de restauración ecológica.
... Estimates regard ing the amount of riparian vegetation lost in the Southwest range from 70% to 95% (Brinson et al. 1981, Johnson andHaight 1984). The most common estimate used in reference to Arizona and New Mexico is 90% of habitat lost (Ohmart andAnderson 1986, Webb andLeake 2006); however, this amount might be arguable. According to Johnson and Jones (1977), drainages such as the Gila and San Pedro Rivers once supported healthy riparian communities, but anthropological redirecting of water resources has resulted in many areas that are now dry and devoid of native riparian vegetation. ...
... The increase in soil salinity has been concomitant with the reduction of the native willow-cottonwood forests (Salix spp. and Populus spp.) that had been the dominant vegetation in western rivers prior to European settlement (Webb & Leake, 2006), both of which are glycophytes and intolerant of salinity levels exceeding 8 ppt Nagler et al., 2011). Previous work has shown that Tamarix spp. ...
Article
The form and function of terrestrial plants is largely governed by the availability of water, with plants in water limited environments expressing traits minimizing water loss and tissue damage during drought. Areas with high salinity are analogous to those with low water availability, even where water is abundant. We evaluated variation in salinity tolerance and water use strategies in an introduced halophytic shrub, Tamarix spp by measuring gas exchange rates, biomass accumulation, and turgor loss points. We hypothesized that salinity tolerance is not uniformly expressed, with high salinity ecotypes expressing more conservative water use traits that maximize hydraulic safety over high conductance rates. We studied two populations of Tamarix spp. along the lower Colorado River where groundwater salinity differed by nearly six‐fold. Cuttings collected from the sites were grown in a greenhouse with five salinity levels from 0 to 32 parts per thousand (ppt), imposing increasing water limitation. In situ measurements of leaf water potentials (Ψ) and branch xylem anatomy were taken on ecotypes sourced from high‐ and low‐salinity sites. In the greenhouse, biomass and gas exchange rates were evaluated over the 0‐32 ppt salinity gradient, and a separate dry down experiment was performed to determine turgor loss points and stomatal responses to drying soils. In the greenhouse, the low salinity population accumulated 72% more biomass when grown at 4 ppt compared to 16 ppt, while the high salinity population produced 50% more biomass when grown at 16 ppt. Net carbon assimilation was greater at lower salinities in the low salinity population but independent of salinity in the high salinity population. The high salinity population had a lower turgor loss point and exhibited greater stomatal control relative to the low salinity population. Synthesis. Results provide evidence for divergence of traits related to plant water use across salinity gradients in this recently introduced halophyte. Local adaptation to increased salinity has implications in aridland riparian ecosystems where water management or drought may lead to altered soil salinities. The interaction of trait variation within Tamarix spp. and increasing salinity is likely to favor its continued dominance.
... In desert areas, rainfall is scarce, and evapotranspiration rates are high. Groundwater and surface water are the main water sources that ensure the existence, vitality, and regeneration of desert plants [14,15]. ...
Article
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Surface water is an important factor affecting vegetation change in desert areas. However, little research has been conducted on the effects of surface water on vegetation expansion. In this study, the annual spatial distribution range of vegetation and surface water in the Daliyabuyi Oasis from 1990 to 2020 was extracted using Landsat time-series images. Based on multi-temporal and multi-scale remote sensing images, several plots were selected to demonstrate the process of landform change and vegetation expansion, and the influence of surface water on vegetation expansion was analyzed. The results show that the vegetation distribution and surface water coverage have increased from 1990 to 2020; and surface water is a critical factor that drives the expansion of vegetation. On the one hand, surface water in the study area was essential for reshaping the riparian landform, driving the transformation of dunes into floodplains, and increasing the potential colonization sites for vegetation. However, landform changes ultimately changed the redistribution of surface water, ensuring that enough water and nutrients provided by sediment were available for plant growth. Our study provides a critical reference for the restoration of desert vegetation and the sustainable development of oases.
Article
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In Northern Baja California, environmental conservation may not be fully accomplished in the absence of a clear understanding of key factors controlling and protecting biodiversity. Groundwater is an environmental agent triggering the presence of a wide range of arid ecosystems; therefore, any sustainable development in the region has to maintain a balance between environmental, social and economical benefits. The present study has placed a great emphasis on the understanding and distribution of groundwater flow systems, and their environmental interactions in the protected area of the Valle de Los Círios, using physical-chemical and isotope methods. Results show the presence of three groundwater groups portraying contrasting flow systems conditions. Analyzed samples have been classified as fresh-brackish, and thermal alkaline groundwater as suggested by pH, temperature and TDS. The Br/Cl ratio in most samples was close to the ocean molar value (1.5x10-3) indicating the importance of marine aerosol fraction in the recharged waters; however, Mg/Cl,Na/Cl,Ca/Cl,SO4/Cl and Cl/HCO3+Cl ratios were significantly different to those of seawater mixing conditions. Dominant ions (Na,Ca and Cl), chemical age proxies (Li), and δ18O suggest waters with long residence time, with the possibility of been recharge during the late glacial period. Thermal ground water shows low minor and trace element content and exhibits the dominance of Na-plagioclase hydrolysis; their low levels of Sr and Ba suggest negligible contact with calcareous rocks and their estimated temperature at depth (80to100°C) and TDS suggest groundwater has travelled through fractured media. This introduction study suggests that desertic species in Valle de Los Cirios are highly sustained by groundwater of local and intermediate flows.
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Increasing demand for river water now conflicts with increasing desire to maintain riparian ecosystems. Efficiently managing river flows for riparian vegetation requires understanding of the time scale of flow effects, but this information is limited by absence of long‐term studies of vegetation change in response to flow variation. To investigate the influence of short‐ and long‐term flow variability and dam operation on riparian vegetation, we determined the occurrence of 107 plant species in 133 permanent plots of known inundating discharge along the Gunnison River, Colorado, USA, on five different occasions between 1990 and 2013. Individual species moved up and down the gradient of inundating discharge coincident with increases and decreases in mean annual flow, and the correlations between flow and species occurrence were strongest when flows were weighted by time before vegetation sampling with a median half‐life of 1.5 years. Some tall, rhizomatous, perennial species, however, responded to flows on a longer time scale. Logistic regression of species occurrence showed a significant relation with inundation duration for 70 out of 107 species. Plot species richness and total vegetative cover decreased in association with desiccation at low inundation durations and with fluvial disturbance at high inundation durations. Within‐plot similarity in species occurrence between years decreased strongly with increasing inundation duration. Moderate inundation durations were dominated by tall, rhizomatous perennial herbs, including invasive Phalaris arundinacea (reed canary grass). Over the 23‐year study period, species richness declined, and the proportion of rhizomatous perennials increased, consistent with the hypothesis that decreases in flow peaks and increases in low flows caused by flow regulation have decreased establishment opportunities for disturbance‐dependent species. In summary, annual‐scale changes in vegetation were strongly influenced by flow variation, and decadal‐scale changes were influenced by decreases in fluvial disturbance from upstream flow regulation beginning decades prior to the onset of this study.
Chapter
Hydrological and geomorphological processes are influenced by beaver (Castor canadensis and C. fiber) activities in aquatic and semi-aquatic environments throughout much of North America, Eurasia, and the austral archipelago of Chile and Argentina. The main hydrologic signature of beaver activities varies with hydrogeomorphic setting—along confined streams it is the pond formed upstream of dams, along unconfined streams it is downstream flooding on floodplains and terraces, and in preexisting wetlands it is the formation of open-water bodies. A review of the existing literature shows that it is rich with descriptions of how beaver activities influence specific hydrologic and geomorphic processes. The main findings are that beaver dams moderate stream flows, increase surface water and riparian groundwater storage, regulate hyporheic flows, and enhance evapotranspiration rates. Beavers also excavate canals on the margins of beaver ponds and create extensive burrow systems in riverbanks where damming is not possible. Bank burrows are also common in beaver ponds. Missing in the beaver hydrogeomorphological literature, however, are clear linkages between affected hydrological processes and ecosystem functioning, especially at larger spatial and temporal scales. In addition, knowledge of effects of beaver activities on the form and function of the expansive peatlands that span northern latitudes is lacking.
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The critical zone—from treetops to groundwater—is an increasingly studied part of the earth system, where scientists study interactions between water, air, rock, soil, and life. Groundwater is both a boundary and an essential store in this integrated system, but is often not well considered in part because of the difficulty in accessing it and its slow movement relative to other parts of the system. Here, we describe some fundamental areas where groundwater hydrology is of fundamental importance to critical zone science, including sustaining streamflow and vegetation, reacting with minerals to produce dissolved solutes and regolith, and influencing energy fluxes across the land‐atmosphere interface. As the timing and type of precipitation changes with climate, groundwater may play an even more important role in CZ processes as a sustainable water source for plants and streamflow. Many open questions also exist about the role of CZ processes on groundwater. Many data streams are needed and important to quantifying the integrated response of the critical zone to groundwater and vice versa, but long‐term data records are often incomplete or discontinued due to limited funding. We argue that the long timescales of processes that involve groundwater necessitate data collection efforts beyond typical federal funding timespans. Sustaining monitoring networks and developing new ones aimed at testing hypotheses related to slow‐moving, groundwater‐controlled critical zone processes should be a scientific priority, and here we outline some open questions that we hope will motivate groundwater scientists to get involved in CZ science. This article is protected by copyright. All rights reserved.
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Global climate change and in particular sea level rise have resulted in water table level rise in the coastal wetland, which may alter the magnitude and direction of carbon flux. However, the degree to which different water table level affects soil CO2 and CH4 emissions remains uncertain in coastal wetland. Here, a soil microcosm experiment with five water table levels (−40, −30, −20, −10, 0 cm) was conducted in the Yellow River Delta, China. The water table level was controlled by manual. The soil CO2 and CH4 emissions of each water table levels were measured during 150-days incubation in 2018. Our results showed that water table level rise decreased soil CO2 emissions, while increased soil CH4 emissions. However, there was no significant difference in soil CO2 and CH4 emissions from −20 to −40 water table levels, respectively. In addition, water table level rise significant alter soil physical and chemical properties in the uppermost soil layer (0–10 cm) in coastal wetland, in particular soil moisture and salinity, which probably jointly affected soil CO2 and CH4 emissions. Furthermore, cumulative soil CH4 emission was positively significantly correlated to soil organic carbon and total carbon, suggesting that carbon component can supply energy and nutrients and benefit for soil CH4 production. Additionally, there was a significant relationship between cumulative soil CO2 emission and dissolved organic carbon, which indicated that CO2 was mainly contributed from dissolved organic carbon. Cumulative soil CO2 emission was significantly correlated with soil microbial biomass carbon, suggesting that microbial activity played an important role in CO2 emissions in coastal wetlands. Our results also indicate that water table level rise caused by sea level rise may contribute to the storage of soil organic carbon and produces a lower global warming potentials of CH4 and CO2 in the further climate change. Therefore, it is necessary to estimate the effect of hydrological, especially water table level on carbon cycles in coastal wetland when evaluating the climate–carbon feedback scenarios.
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This review tries to explain the effects of river infiltration and the interaction between groundwater and surface water on nitrogen (N) and phosphorus (P) distribution in riparian zones. On the basis of previous studies, this review summarized the distribution characteristics of N and P of 7 different riparian zones in China, compared the water quality of water bodies adjoining these riparian zones, proposed the distribution regularity of N and P in diverse riparian zones. The concentrations of N and P in the riparian zones with better water quality have increasing trend from riverside to upland. And those rivers with poorer water quality have decreasing trend. If the infiltrated concentration of runoff pollutant is more than river pollutant infiltration, the upland soil may obtain higher contaminants. On the contrary, if the concentration of river infiltration is more than that of the runoff, the riverside soil will show higher concentration. Similarly, if those two effects are equivalent, the concentration of N and P in the riparian zone will be low on riverside and upland, and somewhere in the middle will be higher than elsewhere. Clean rivers and polluted rivers taken with different quantity of contaminants will infiltrate diverse concentrations of N and P. It is expected that this review can provide a new theoretical basis for revealing the environmental effects of riparian zones and managing, protecting and restoring the riparian ecosystems.
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Lowland riparian ecosystems, defined as those occurring at elevations at or below 5,000 feet (1,564 meters), constitute a small fraction of total land area in the southwestern United States and northern Mexico, yet they are extremely important to human livelihoods and biotic communities. In the hotter and drier conditions projected under ongoing climate change, riparian ecosystems are increasingly critical to the well-being of humans and wildlife. Riparian areas have been modified in various ways and to a large extent through human endeavor to utilize resources more predictably. These alterations often interfere with multiple and complex ecological processes, making riparian areas vulnerable to disturbance and change. Few naturally functioning riparian areas remain in the region, and those that do are imperiled by climate change, groundwater pumping, land use, and altered disturbance regimes. Some evidence suggests that fire regimes are changing in southwestern riparian zones; wildfires may be increasing in frequency and severity. This literature review summarizes and synthesizes the state of the knowledge of wildfire and prescribed fire effects on physical processes and vegetation, and postfire rehabilitation.Changing fire regimes are likely to have drastic and potentially irreversible effects on regional biodiversity and ecosystem function. However, options are available for managing riparian ecosystems that could make them more resilient to fire and climate change. This study is intended to inform management and identify gaps in systematically reviewed literature.
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The western distinct population segment of yellow‐billed cuckoo (Coccyzus americanus; western cuckoo) has been extirpated from most of its former breeding range in the United States because of widespread loss and degradation of riparian cottonwood (Populus spp.)‐willow (Salix spp.) forests. Restoration and management of breeding habitat is important to the recovery of this federally threatened species, and identification of high‐quality breeding habitat can help improve the success of recovery. In 2005, the Lower Colorado River Multi‐Species Conservation Program, a long‐term, multi‐agency effort, was initiated to maintain and create wildlife habitat within the historical floodplain of the lower Colorado River (LCR) for federally endangered and threatened species, including western cuckoos. We conducted an empirical, multi‐scale field investigation from 2008–2012 to identify habitat characteristics selected by nesting western cuckoos along the LCR. Multiple logistic regression models revealed that western cuckoos selected nest sites characterized by increased densities of small, native, early successional trees measuring 8–23 cm diameter at breast height, and lower diurnal temperature compared to available habitat in restoration and natural forests. Nesting cuckoos selected sites with increased percent canopy closure, which was also important for nest success in restoration sites along the LCR. Our results show habitat components selected by nesting western cuckoos in restoration and natural riparian forests and can help guide the creation, enhancement, and management of riparian forests with habitat conditions necessary to promote nesting of western cuckoos. © 2021 The Wildlife Society. Our results can help guide the creation, enhancement, and management of riparian forests with desired habitat conditions necessary to promote nesting of the federally threatened western yellow‐billed cuckoo. The strategic placement of small native trees at high densities in a specific site or the management of riparian forests for high density patches of early successional native trees could significantly improve or increase optimal nesting habitat for western cuckoos in restoration and natural riparian forests.
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Achieving a climate-resilient future requires rapid, sustained and far-reaching transformations in energy, land-use, infrastructure and industrial systems. Large-scale expansion of renewable energy can play a critical role in meeting the world’s growing energy demands and in the fight against climate change. However, even ‘clean’ energy sources can have significant unintended impacts on the environment. The guidelines aim to provide practical support for solar and wind energy developments by effectively managing risks and improving overall outcomes related to biodiversity and ecosystem services. They are industry-focused and can be applied across the whole project development life cycle, from early planning through to decommissioning and repowering, using the mitigation hierarchy as a clear framework for planning and implementation. The mitigation hierarchy is applied to direct, indirect and cumulative impacts.
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Dense impenetrable thickets of invasive trees and shrubs compete with other water users and thus disrupt ecosystem functioning and services. This study assessed water use by the evergreen Prosopis juliflora, one of the dominant invasive tree species in semi-arid and arid ecosystems in the tropical regions of Eastern Africa. The objectives of the study were to (1) analyze the seasonal water use patterns of P. juliflora in various locations in Afar Region, Ethiopia, (2) up-scale the water use from individual tree transpiration and stand evapotranspiration (ET) to the entire invaded area, and 3) estimate the monetary value of water lost due to the invasion. The sap flow rates of individual P. juliflora trees were measured using the heat ratio method while stand ET was quantified using the eddy covariance method. Transpiration by individual trees ranged from 1–36 L/day, with an average of 7 L of water per tree per day. The daily average transpiration of a Prosopis tree was about 3.4 (± 0.5) mm and the daily average ET of a dense Prosopis stand was about 3.7 (± 1.6) mm. Using a fractional cover map of P. juliflora (over an area of 1.18 million ha), water use of P. juliflora in Afar Region was estimated to be approximately 3.1–3.3 billion m ³ /yr. This volume of water would be sufficient to irrigate about 460,000 ha of cotton or 330,000 ha of sugar cane, the main crops in the area, which would generate an estimated net benefit of approximately US$ 320 million and US$ 470 million per growing season from cotton and sugarcane, respectively. Hence, P. juliflora invasion in the Afar Region has serious impacts on water availability and on the provision of other ecosystem services and ultimately on rural livelihoods.
Preprint
Beavers (castor fiber, castor canadensis) are the most influential mammalian ecosystem engineer, heavily modifying river corridors and influencing hydrology, geomorphology, nutrient cycling, and ecosystems. As an agent of disturbance, they achieve this first and foremost through dam construction, which impounds flow and increases the extent of open water, and from which all other landscape and ecosystem impacts follow. After a long period of local and regional eradication, beaver populations have been recovering and expanding throughout Europe and North America, as well as an introduced species in South America, prompting a need to comprehensively review the current state of knowledge on how beavers influence the structure and function of river corridors. Here, we synthesize the overall impacts on hydrology, geomorphology, biogeochemistry, and aquatic and terrestrial ecosystems. Our key findings are that a complex of beaver dams can increase surface and subsurface water storage, modify the reach scale partitioning of water budgets, allow site specific flood attenuation, alter low flow hydrology, increase evaporation, increase water and nutrient residence times, increase geomorphic heterogeneity, delay sediment transport, increase carbon, nutrient and sediment storage, expand the extent of anaerobic conditions and interfaces, increase the downstream export of dissolved organic carbon and ammonium, decrease the downstream export of nitrate, increase lotic to lentic habitat transitions and aquatic primary production, induce ‘reverse’ succession in riparian vegetation assemblages, and increase habitat complexity and biodiversity on reach scales. We then examine the key feedbacks and overlaps between these changes caused by beavers, where the decrease in longitudinal hydrologic connectivity create ponds and wetlands, transitions between lentic to lotic ecosystems, increase vertical hydraulic exchange gradients, and biogeochemical cycling per unit stream length, while increased lateral connectivity will determine the extent of open water area and wetland and littoral zone habitats, and induce changed in aquatic and terrestrial ecosystem assemblages. However, the extent of these impacts depends firstly on the hydro-geomorphic landscape context, which determines the extent of floodplain inundation, a key driver of subsequent changes to hydrologic, geomorphic, biogeochemical, and ecosystem dynamics. Secondly, it depends on the length of time beavers can sustain disturbance at a given site, which is constrained by top down (e.g. predation) and bottom up (e.g. competition) feedbacks, and ultimately determines the pathways of river corridor landscape and ecosystem succession following beaver abandonment. This outsized influence of beavers on river corridor processes and feedbacks is also fundamentally distinct from what occurs in their absence. Current river management and restoration practices are therefore open to re-examination in order to account for the impacts of beavers, both positive and negative, such that they can potentially accommodate and enhance the ecosystem engineering services they provide. It is hoped that our synthesis and holistic framework for evaluating beaver impacts can be used in this endeavor by river scientists and managers into the future as beaver populations continue to expand in both numbers and range.
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Often, regional groundwater research is meant to have some tens or a few hundreds of water samples for chemical and stable isotopes analyses. However, in remote arid areas as "Los Cirios" in Baja California Sur, Mex. it became a challenge to carry out such study due to the scarce general development. The regional flow systems theory allowed to have an acceptable hypothesis from available water samples (in springs and wells). The fresh and thermal water provided a sensitive relation with ecosystems through the three defined types of groundwater.
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This report contains reviews and syntheses of scientific literature for the purpose of informing the development of policies related to management of riparian areas and watersheds of Washington State.
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Abstract JUNK, W. J., P. B. BAYLEY, AND R. E. SPARKS, 1989. The flood pulse concept in river-floodplain systems, p. 110-127. In D. P. Dodge [ed.] Proceedings of the International Large River Symposium. Can. Spec. Publ. Fish. Aquat. Sci. 106. The principal driving force responsible for the existence, productivity, and interactions of the major biota in river—floodplain systems is the flood pulse. A spectrum of geomorphological and hydrological conditions produces flood pulses, which range from unpredictable to predictable and from short to long duration. Short and generally unpredictable pulses occur in low-order streams or heavily modified systems with floodplains that have been leveed and drained by man. Because low-order stream pulses are brief and unpredictable, organisms have limited adaptations for directly utilizing the aquatic/terrestrial transition zone (ATTZ), although aquatic organisms benefit indirectly from transport of resources into the lotic environment. Conversely, a predictable pulse of long duration engenders organismic • adaptations and strategies that efficiently utilize attributes of the ATTZ. This pulse is coupled with a dynamic edge effect, which extends a "moving littoral" throughout the ATTZ. The moving littoral prevents prolonged stagnation and allows rapid recycling of organic matter and nutrients, thereby resulting in high productivity. Primary production associated with the ATTZ is much higher than that of permanent water bodies in unmodified systems. Fish yields and production are strongly related to the extent of accessible floodplain, whereas the main river is used as a migration route by most of the fishes. In temperate regions, light and/or temperature variations may modify the effects of the pulse, and anthropogenic influences on the flood pulse or floodplain frequently limit production. A local floodplain, however, can develop by sedimentation in a river stretch modified by a low head dam. Borders of slowly flowing rivers turn into floodplain habitats, becoming separated from the main channel by levées. The flood pulse is a "batch" process and is distinct from concepts that emphasize the continuous processes in flowing water environments, such as the river continuum concept. Flooclplains are distinct because they do not depend on upstream processing inefficiencies of organic matter, although their nutrient pool is influenced by periodic lateral exchange of water and sediments with the main channel. The pulse concept is distinct because the position of a floodplain within the river network is not a primary determinant of the processes that occur. The pulse concept requires an approach other than the traditional limnological paradigms used in lotic or lentic systems. Résumé JUNK, W. J., P. B. BAYLEY, AND R. E. SPARKS. 1989. The flood pulse concept in river-floodplain systems, p. 110-127. In D. P. Dodge [cd.] Proceedings of the International Large River Symposium. Can. Spec. Publ. Fish. Aquat. Sci . 106. Les inondations occasionnées par la crue des eaux dans les systèmes cours d'eau-plaines inondables constituent le principal facteur qui détermine la nature et la productivité du biote dominant de même que les interactions existant entre les organismes biotiques et entre ceux-ci et leur environnement. Ces crues passagères, dont la durée et la prévisibilité sont variables, sont produites par un ensemble de facteurs géomorphologiques et hydrologiques. Les crues de courte durée, généralement imprévisibles, surviennent dans les réseaux hydrographiques peu ramifiées ou dans les réseaux qui ont connu des transformations importantes suite à l'endiguement et au drainage des plaines inondables par l'homme. Comme les crues survenant dans les réseaux hydrographiques d'ordre inférieur sont brèves et imprévisibles, les adaptations des organismes vivants sont limitées en ce qui a trait à l'exploitation des ressources de la zone de transition existant entre le milieu aquatique et le milieu terrestre (ATTZ), bien que les organismes aquatiques profitent indirectement des éléments transportés dans le milieu lotique. Inversement, une crue prévisible de longue durée favorise le développement d'adaptations et de stratégies qui permettent aux organismes d'exploiter efficacement 1 'ATTZ. Une telle crue s'accompagne d'un effet de bordure dynamique qui fait en sorte que l'ATTZ devient un « littoral mobile'<. Dans ces circonstances, il n'y a pas de stagnation prolongée et le recyclage de la matière organique et des substances nutritives se fait rapidement, ce qui donne lieu à une productivité élevée. La production primaire dans l'ATTZ est beaucoup plus élevée que celle des masses d'eau permanentes dans les réseaux hydrographiques non modifiés. Le rendement et la production de poissons sont étroitement reliés à l'étendue de la plaine inondable, tandis que le cours normal de la rivière est utilisé comme voie de migration par la plupart des poissons.
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Woody riparian vegetation in western North American riparian ecosystems is commonly dependent on alluvial groundwater. Various natural and anthropogenic mechanisms can cause groundwater declines that stress riparian vegetation, but little quantitative information exists on the nature of plant response to different magnitudes, rates, and durations of groundwater decline. We observed groundwater dynamics and the response of Populus fremontii, Salix gooddingii, and Tamarix ramosissima saplings at 3 sites between 1995 and 1997 along the Bill Williams River, Arizona. At a site where the lowest observed groundwater level in 1996 (-1.97 m) was 1.11 m lower than that in 1995 (-0.86 m), 97-100% of Populus and Salix saplings died, whereas 0-13% of Tamarix stems died. A site with greater absolute water table depths in 1996 (-2.55 m), but less change from the 1995 condition (0.55 m), showed less Populus and Salix mortality and increased basal area. Excavations of sapling roots suggest that root distribution is related to groundwater history. Therefore, a decline in water table relative to the condition under which roots developed may strand plant roots where they cannot obtain sufficient moisture. Plant response is likely mediated by other factors such as soil texture and stratigraphy, availability of precipitation-derived soil moisture, physiological and morphological adaptations to water stress, and tree age. An understanding of the relationships between water table declines and plant response may enable land and water managers to avoid activities that are likely to stress desirable riparian vegetation.
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Turner, R.M., Webb, R.H., Bowers, J.E., and Hastings, J.R., 2003, The Changing Mile Revisited: Tucson, University of Arizona Press, 334 p.
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Webb, R.H., Smith, S.S., and McCord, V.A.S., 1991, Historic channel change of Kanab Creek, southern Utah and northern Arizona: Grand Canyon Natural History Association Monograph Number 9, 91 p.
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The Flood Pulse Concept in River—Floodplain Systems Wolfgang J. Junk Max Planck Institut für Limnologie, August Thienemann Strasse 2, Post fach 165, D-2320 Pion, West Germany Peter B. Bayley and Richard E. Sparks Illinois Natural History Survey, 607 E. Peabody Dr., Champaign, IL 61820, USA Abstract JUNK, W. J., P. B. BAYLEY, AND R. E. SPARKS, 1989. The flood pulse concept in river-floodplain systems, p. 110-127. In D. P. Dodge [ed.] Proceedings of the International Large River Symposium. Can. Spec. Publ. Fish. Aquat. Sci. 106. The principal driving force responsible for the existence, productivity, and interactions of the major biota in river—floodplain systems is the flood pulse. A spectrum of geomorphological and hydrological conditions produces flood pulses, which range from unpredictable to predictable and from short to long duration. Short and generally unpredictable pulses occur in low-order streams or heavily modified systems with floodplains that have been leveed and drained by man. Because low-order stream pulses are brief and unpredictable, organisms have limited adaptations for directly utilizing the aquatic/terrestrial transition zone (ATTZ), although aquatic organisms benefit indirectly from transport of resources into the lotic environment. Conversely, a predictable pulse of long duration engenders organismic • adaptations and strategies that efficiently utilize attributes of the ATTZ. This pulse is coupled with a dynamic edge effect, which extends a "moving littoral" throughout the ATTZ. The moving littoral prevents prolonged stagnation and allows rapid recycling of organic matter and nutrients, thereby resulting in high productivity. Primary production associated with the ATTZ is much higher than that of permanent water bodies in unmodified systems. Fish yields and production are strongly related to the extent of accessible floodplain, whereas the main river is used as a migration route by most of the fishes. In temperate regions, light and/or temperature variations may modify the effects of the pulse, and anthropogenic influences on the flood pulse or floodplain frequently limit production. A local floodplain, however, can develop by sedimentation in a river stretch modified by a low head dam. Borders of slowly flowing rivers turn into floodplain habitats, becoming separated from the main channel by levées. The flood pulse is a "batch" process and is distinct from concepts that emphasize the continuous processes in flowing water environments, such as the river continuum concept. Flooclplains are distinct because they do not depend on upstream processing inefficiencies of organic matter, although their nutrient pool is influenced by periodic lateral exchange of water and sediments with the main channel. The pulse concept is distinct because the position of a floodplain within the river network is not a primary determinant of the processes that occur. The pulse concept requires an approach other than the traditional limnological paradigms used in lotic or lentic systems. Résumé JUNK, W. J., P. B. BAYLEY, AND R. E. SPARKS. 1989. The flood pulse concept in river-floodplain systems, p. 110-127. In D. P. Dodge [cd.] Proceedings of the International Large River Symposium. Can. Spec. Publ. Fish. Aquat. Sci . 106. Les inondations occasionnées par la crue des eaux dans les systèmes cours d'eau-plaines inondables constituent le principal facteur qui détermine la nature et la productivité du biote dominant de même que les interactions existant entre les organismes biotiques et entre ceux-ci et leur environnement. Ces crues passagères, dont la durée et la prévisibilité sont variables, sont produites par un ensemble de facteurs géomorphologiques et hydrologiques. Les crues de courte durée, généralement imprévisibles, surviennent dans les réseaux hydrographiques peu ramifiées ou dans les réseaux qui ont connu des transformations importantes suite à l'endiguement et au drainage des plaines inondables par l'homme. Comme les crues survenant dans les réseaux hydrographiques d'ordre inférieur sont brèves et imprévisibles, les adaptations des organismes vivants sont limitées en ce qui a trait à l'exploitation des ressources de la zone de transition existant entre le milieu aquatique et le milieu terrestre (ATTZ), bien que les organismes aquatiques profitent indirectement des éléments transportés dans le milieu lotique. Inversement, une crue prévisible de longue durée favorise le développement d'adaptations et de stratégies qui permettent aux organismes d'exploiter efficacement 1 'ATTZ. Une telle crue s'accompagne d'un effet de bordure dynamique qui fait en sorte que l'ATTZ devient un « littoral mobile'<. Dans ces circonstances, il n'y a pas de stagnation prolongée et le recyclage de la matière organique et des substances nutritives se fait rapidement, ce qui donne lieu à une productivité élevée. La production primaire dans l'ATTZ est beaucoup plus élevée que celle des masses d'eau permanentes dans les réseaux hydrographiques non modifiés. Le rendement et la production de poissons sont étroitement reliés à l'étendue de la plaine inondable, tandis que le cours normal de la rivière est utilisé comme voie de migration par la plupart des poissons. 110
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The effects of river damming on geomorphic processes and riparian vegetation were evaluated through field studies along the regulated Green River and the free-flowing Yampa River in northwestern Colorado, USA. GIS analysis of historical photographs, hydrologic and sediment records, and measurement of channel planform indicate that fluvial processes and riparian vegetation of the two meandering stream reaches examined were similar prior to regulation which began in 1962. Riparian plant species composition and canopy coverage were measured during 1994 in 36, 0.01 ha plots along each the Green River in Browns Park and the Yampa River in Deerlodge Park. Detrended correspondence analysis (DCA) of the vegetation data indicates distinctive vegetation differences between Browns Park and Deerlodge Park. Canonical correspondence analysis (CCA) indicates that plant community composition is controlled largely by fluvial processes at Deerlodge Park, but that soil chemical rather than flow related factors play a more important role in structuring plant communities in Browns Park.
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Groundwater depletion threatens many riparian ecosystems in arid and semiarid regions of the world. The aquifer that sustains Arizona's San Pedro River riparian ecosystem, for example, is threatened by regional groundwater declines and localized pumping from the alluvial aquifer. This paper demonstrates the important role of shallow groundwater in structuring the San Pedro River plant community, portions of which function as reference areas that indicate site potential for a globally rare forest type (Sonoran riparian Populus-Salix forests). Several ecological indicators varied with depth to groundwater, including a weighted average wetland indicator score calculated for herbaceous and woody plant species, cover of plants within wetland indicator groups, and frequency of indicator plant species. These relationships can be used in a space-for-time substitution to predict consequences of groundwater decline. For example, the wetland indicator score changed sharply as depth to groundwater ranged from 0 to 4 m, and abundance of obligate wetland herbs (the group most sensitive to groundwater changes) declined sharply at groundwater depths below approximate to 0.25 m. Such sequential ''desertification'' of the riparian flora (i.e., loss or reduction in cover of species based on their probability of occurrence in wetlands) is one predicted response to groundwater decline. Other predicted impacts of groundwater decline include reduced establishment of Populus fremontii-Salix gooddingii forests, and reduced cover of herbaceous species associated with the fine-textured soils and shady conditions of floodplain terraces stabilized by these early seral tree species. High floodplain terraces (depth to groundwater of 5-8 m) had wetland indicator scores below those of upland sites and were vegetated by species (e.g., Prosopis velutina and Sporabolus wiightii) with low sensitivity to groundwater changes.
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Vegetation change in the American West has been a subject of concern throughout the twentieth century. Although many of the changes have been recorded qualitatively through the use of comparative photography and historical reports, little quantitative information has been available on the regional or watershed scale. It is currently possible to measure change over large areas and determine trends in ecological and hydrological condition using advanced space-based technologies. Specifically, this process is being tested in a community-based watershed in southeast Arizona and northeast Sonora, Mexico using a system of landscape pattern measurements derived from satellite remote sensing, spatial statistics, process modeling, and geographic information systems technology. These technologies provide the basis for developing landscape composition and pattern indicators as sensitive measures of large-scale environmental change and thus may provide an effective and economical method for evaluating watershed condition related to disturbance from human and natural stresses. The project utilizes the database from the North American Landscape Characterization (NALC) project which incorporates triplicate Landsat Multi-Spectral Scanner (MSS) imagery from the early 1970s, mid 1980s, and the 1990s. Landscape composition and pattern metrics have been generated from digital land cover maps derived from the NALC images and compared across a nearly 20-year period. Results about changes in land cover for the study period indicate that extensive, highly connected grassland and desertscrub areas are the most vulnerable ecosystems to fragmentation and actual loss due to encroachment of xerophytic mesquite woodland. In the study period, grasslands and desertscrub not only decreased in extent but also became more fragmented. That is, the number of grassland and desertscrub patches increased and their average patch sizes decreased. In stark contrast, the mesquite woodland patches increased in size, number, and connectivity. These changes have important impact for the hydrology of the region, since the energy and water balance characteristics for these cover types are significantly different. The process demonstrates a simple procedure to document changes and determine ecosystem vulnerabilities through the use of change detection and indicator development, especially in regard to traditional degradation processes that have occurred throughout the western rangelands involving changes of vegetative cover and acceleration of water and wind erosion.
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species typically dominate riparian ecosystems throughout arid and semiarid regions of North American and efforts to minimize loss of riparian Populus requires an integrated understanding of the role of surface and groundwater dynamics in the establishment of new, and maintenance of existing, stands. In a controlled, whole-stand field experiment, we quantified responses of Populus morphology, growth, and mortality to water stress resulting from sustained water table decline following in-channel sand mining along an ephemeral sandbed stream in eastern Colorado, USA. We measured live crown volume, radial stem growth, annual branch increment, and mortality of 689 live Populus deltoides subsp. monilifera stems over four years in conjunction with localized water table declines. Measurements began one year prior to mining and included trees in both affected and unaffected areas. Populus demonstrated a threshold response to water table declines in medium alluvial sands; sustained declines ≥1 m produced leaf desiccation and branch dieback within three weeks and significant declines in live crown volume, stem growth, and 88% mortality over a three-year period. Declines in live crown volume proved to be a significant leading indicator of mortality in the following year. A logistic regression of tree survival probability against the prior year's live crown volume was significant (−2 log likelihood = 270, χ2 with 1 df = 232, P < 0.0001) and trees with absolute declines in live crown volume of ≥30 during one year had survival probabilities <0.5 in the following year. In contrast, more gradual water table declines of ~0.5 m had no measurable effect on mortality, stem growth, or live crown volume and produced significant declines only in annual branch growth increments. Developing quantitative information on the timing and extent of morphological responses and mortality of Populus to the rate, depth, and duration of water table declines can assist in the design of management prescriptions to minimize impacts of alluvial groundwater depletion on existing riparian Populus forests.
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Abstract from GoogleBooks: This ambitious book will enthrall armchair naturalists and river runners alike, offering a stunning tour through the natural, environmental, and human history of Cataract Canyon, a seventeen-mile run of free-flowing river above Lake Powell in the canyonlands of southern Utah. Setting the stage with preliminary chapters on geology and hydrology, prehistory and geography, biology, and river-running history the authors take the reader on a "downriver journey," narrating an exploration of the river that is breathtaking in scope. From the plants and animals that live along its banks to the humans who seek out its rapids, from the wind and water that continue to shape the landscape to the government agencies that seek to control it, all of these become stories woven into the larger fabric of a beautiful, fragile, complex ecosystem where change--whether good or bad--is inevitable.
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Vegetation changes in the canyon of the Colorado River between Glen Canyon Dam and Lake Mead were studied by comparing photographs. The matched photographs reveal that changes have occurred in the amount of sand and silt deposited along the banks. The interation of decreased flooding, decreased sediment load, and increased riparian plant coverage makes the future of existing river fans, bars, and terraces uncertain, and a new ecological equilibrium may require many decades. -from Authors
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The San Pedro River of southeast Arizona is a north-flowing tributary of the Gila River. The area of the drainage basin upstream of the 40-km-long study reach is about 3,200 km2. This study traces the historical evolution of the San Pedro River channel- specifically, the deepening, widening, and sediment deposition that have occurred since 1900-and it aims to evaluate the causes of channel widening and deepening, the rate of widening, and the present stability of the channel. Alluvium of the river valley consists of upper Holocene pre- and postentrenchment deposits. The pre-entrenchment alluvium, which forms the principal terrace of the inner valley, accumulated between about A.D. 1450 and 1900 in a relatively sluggish, lowenergy fluvial system with extensive marshy reaches and high water table. In contrast, postentrenchment alluvium, which forms the terrace, floodplain, and channel of the San Pedro River, was deposited in a relatively high-energy, entrenched, and meandering fluvial system. The river flowed in a shallow, narrow channel on the surface of the unentrenched valley before 1890. A series of large floods, perhaps beginning as early as 1881, eventually led to entrenchment of the channel between 1890 and 1908. This deepening placed the channel 1 to 10 m below the former floodplain. The channel has widened substantially since entrenchment through lateral migration and expansion of entrenched meanders; its present size is 5.7 times greater than before entrenchment. The rate of channel expansion, however, has decreased since about 1955, coincident with a decrease of peak-flood discharge. Channel area increased at 0.1 km2 yr-1 from entrenchment until 1955; since then the area increased at only 0.02 km 2 yr-1, suggesting that the channel has stabilized and that further widening will probably be minor under present conditions of land use, discharge, and climate. The reduction of peak-flow rates was related partly to increased channel sinuosity and to development of floodplains and riparian woodlands. The increased sinuosity produced a reservoir effect that attenuated flood waves, and the development of floodplains enabled flood waters to spread laterally, thereby increasing transmission losses. In addition, flow rates were probably affected by improved land use and changes of rainfall intensity and short-term rainfall patterns, which reduced runoff and decreased the time necessary for channel stabilization. Livestock grazing decreased steadily after the turn of the century, and numerous stock ponds and small water-retention structures were constructed in tributaries. The cumulative effect of these structures probably reduced peak-flow rates. Short-term rainfall patterns of the wet season (June 15-October 15) have probably changed from annual alternation of above- and below-average rainfall to a biennial or longer pattern. Moreover, frequency of low-intensity rainfall (daily rainfall less than about 1.27 cm) was consistently above average for the decade 1957-1967. These factors probably improved conditions for growth and establishment of vegetation both in and outside of the channel. The causes of the large floods that resulted in entrenchment are poorly understood, although climate and land use were key factors. Floods followed closely the rapid settlement of the area brought about by mining activity in the late 1870s; population rose from a few hundred to 6,000 in less than 5 yr. Extensive wood cutting for mine timber and fuel, suppression of wildfire, and reintroduction of large cattle herds undoubtedly exacerbated entrenchment. Flood-producing wet-season rainfall in the Southwest, however, was unusually heavy before, during, and shortly after entrenchment.
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Mesquite Prosopis velutina bosques are groundwater-dependent riparian woodlands that were once widespread in the American Southwest. Groundwater was withdrawn from the aquifer below an ephemeral creek in the Sonoran Desert of Arizona. Temporal and spatial varation in plant water potential, leaflet size, leaflet number, canopy height, and live and dead vegetation volume all indicate that the bosque requires a shallow aquifer and that bosque traits change continuously with groundwater depth. The bosque had high water potentials, large leaflets (>7 cm2), tall stature (>12m), and large vegetation volume (>2 m3/m2) only where the water table was <5m below the surface. Summer rains and seasonal surface flow temporarily reduced water stress and increased leaflet size for some trees, but did not offset effects of groundwater decline. Trees in areas of greatest groundwater decline (18-30m) were under sublethal stress, as evidenced by low stem water potentials (<-4MPa), reduced leaflet size (<5.5 cm2), and high levels of canopy mortality (>45%). These deepest groundwater levels are in the range of those lethal to mesquite in other bosque systems. -from Authors
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This book deals with the rapid erosion of deep stream gullies (arroyos) which have become a feature of many valleys of America's South-western states since the middle of the 19th century. The nature and causes of entrenchment have been a matter of interest and practical concern to research workers in the environmental sciences and some fields of terrain management for many years, but the abundant records of environmental change in California and Arizona relevant to the study of historical arroyos have never before been systematically collated and analysed. Those concerned with environmental change or with an interest in fragile, semi-arid environments.
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These papers presented at a symposium at Death Valley National Monument, California, November 1988, provide an appraisal of the status and conservation biology of fishes in arid lands W of the Mississippi Valley, as well as considering ethical and political questions and the progress of management and conservation efforts. The 20 chapters, organized in seven sections, cover discovery and extinction of Western fishes; ichthyological exploration of the American West, 1915-1950; impacts of Green River poisoning on management of native fishes; the Desert Fishes Council; the legacy of the Devils Hole pupfish Cyprinodon diabolis; responsibility for desert fish; ethics, federal legislation and litigation; evolution of a cooperative recovery programme in the upper Colorado River Basin; conservation genetics and genic diversity; design of reserves; history and management of preserves and refuges; conservation of Mexican freshwater fishes; hatcheries; stream renovation as management techniques; conservation and management of short-lived cyprinodontoids; transplanting short-lived fishes; management toward recovery of razorback sucker Xyrauchen texanus; life history of four endangered lacustrine suckers (Catostomidae); ecology and management of Colorado squawfish Ptychocheilus lucius; and an epilogue on the enigma of endangered species. Seventeen of the chapters are abstracted separately (see 92L/10546-10562). -J.W.Cooper
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Prosopis velutina (velvet mesquite) forests are one of many types of aridland riparian ecosystems that are threatened by groundwater pumping and other types of water development. Empirical models developed using both hydrological and vegetational data sets have potential uses in the management of these threatened ecosystems. To this end, we developed models for Prosopis velutina stands across a xeric-to-mesic moisture gradient. The models expressed canopy height, basal area, leaf area index, vegetation volume, and leaflet area as functions of plant water potential, and they expressed plant water potential and riparian stand structure as functions of water table depth. These data indicated that stand structure was strongly related to water availability. Management applications of the models include the ability (1) to identify minimum water-table depths for riparian stand maintenance and (2) to detect stressful hydrological conditions, via water potential measurements, before the onset of structural degradation.
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The reduction in extent of riparian forests in the southwestern United States has been a topic of recent concern. The effect of dams on downstream river flow and the consequent modification of the riparian habitat was studied along the lower Salt River in central Arizona. Dams were found to change the magnitude of river flows and change the seasonal timing of flows in such a way that the habitat appeared less adapted for regeneration of Populus fremontii. Modification of river flow patterns, therefore, appears likely to have been a significant factor causing change in vegetation along the Salt River.
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The rate of invasion of Tamarix pentandra Pall. into the vegetation bordering Utah Lake, Great Salt Lake, Colorado River, and Green River in Utah was determined from historical information and scientific studies. There are no records of tamarix at the above listed lakes and streams prior to 1925. The period from 1925 to 1960 was one of rapid spread and increase in importance of tamarix. The greatest degree of invasion occurred from 1935 to 1955. At the present time, naturalization is occurring along the mountain streams and reservoirs of the Wasatch Mountains.
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Bird species densities were determined for summer and winter on 132 study plots grouped into 25 riparian habitats in or near the Huachuca Mountains of southeastern Arizona. The habitats were defined based on the dominant riparian tree species, the size of the riparian stand, and the type of adjacent upland vegetation. Vegetation characteristics and physical environmental data were collected at each plot. The type of dominant riparian tree species influenced bird species richness and total density during the breeding season. Cottonwood habitats had the greatest richness, and both cottonwood and sycamore habitats had high densities. Upland vegetation was an important factor related to winter species richness and abundance, with plots in open grassland areas having greater richness and density. Riparian stand size was a relatively poor predictor of avian density or richness in either season. Groups of bird species that shared similar density distributions in the summer were associated with specific riparian habitats. The winter pattern of species groups was not as clear, and groups could not he assigned to riparian habitats, but they were related to either wooded or open upland vegetation. Riparian habitats were also clustered based on similar densities of birds. In summer, high-elevation habitats were distinct from low-ele-vation and foothill habitats. In winter, riparian habitats separated into categories of wooded vs. open adjacent vegetation.
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The least that one can say about the report is that it is very enjoyable reading. Every chapter has been carefully written, and the literary merit of some chapters is outstanding (particularly those by Klemes, `Empirical and Causal Models in Hydrology,' and by Baker, `Geology, Determinism, and Risk Assessment'). The best that one can say about the report is that it does meet its stated objectives of (1) evaluation of the adequacy of present hydrologic knowledge and of the appropriateness of present research programs to provide information for decision making and (2) description of the impact of hydrologic knowledge on the planning and management of water resources. The worst that one can say about the report is that it is not particularly original and that there are few really fresh new arguments developed in it. One notable exception is provided in Chapter 11, by Matalas, Landwehr, and Wolman, which challenges the traditional (implicit) assumption that `human activity is an external perturbation of the hydrologic cycle.' Though not the explicit intent of chapter 4, by Bredehoeft, Papadopulos, and Cooper, with the explosion of the water-budget myth in groundwater, this chapter illustrates clearly the profound interaction of man (through wells) in the hydrologic cycle, a situation that cannot be comprehended from a study of the system free from human influence.
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Between 1865 and 1915, arroyos developed in the southwestern United States across diverse hydrological, ecological and cultural settings. That they developed simultaneously has encouraged the search for a common cause --some phenomenon that was equally widespread and synchronous. There are few southwestern streams for which we have even a qualitative understanding of timelines and processes involved in initiation and extension of historic arroyos. Tucson's Santa Cruz River, often cited in the arroyo literature, offers a unique opportunity to chronicle the arroyo legacy and evaluate its causes. The present study reconstructs both the physical and cultural circumstances of channel entrenchment along the Santa Cruz River. Primary data include newspaper accounts, notes and plants of General Land Office surveys, eyewitness accounts, legal depositions, and repeat photography. On the Santa Cruz River, arroyo initiation and extension happened during relatively wet decades associated with frequent warm episodes in the tropical Pacific (El Nino conditions). Intensified El Nino activity during the period 1864-1891 may be symptomatic of long-term climatic change, perhaps indicative of global warming and destabilization of Pacific climate at the end of the Little Ice Age. During this period all but one of the years registering more than three days with rain exceeding 2.54 cm (1 in) in Tucson were El Nino events. The one exception was the summer of 1890, when the central equatorial Pacific was relatively cold but when prevailing low-surface pressures and low -level winds nevertheless steered tropical moisture from the west coast of Mexico into southern Arizona. In the twentieth century, catastrophic channel widening was caused by floods during El Nino events in 1905, 1915, 1977 and 1983. The Santa Cruz River arroyo formed when climatic conditions heightened the probabilities for occurrence of large floods in southern Arizona. Inadequate engineering of ditches that resulted in abrupt changes in the longitudinal profile of the stream further augmented probabilities that any one of these floods would initiate an arroyo. In the future, changing flood probabilities with low-frequency climatic fluctuations and improved flow conveyance due to intensified land use and channel stabilization will further complicate management of the arroyo in an increasingly urbanized floodplain.
Article
ABSTRACT Several plant communities in the Southwest have been drastically altered within a century by the rapid spread of a small number of woody species. The habitats principally affected have been the plateaus and plains at intermediate elevations, which formerly supported grassland and have now been invaded on a massive scale by mesquite and other native shrubs, and the stream courses, which have been extensively occupied by tamarisk, an alien species from Eurasia. Historical evidence and field observations suggest that these invasions have resulted primarily from occupation of the Southwest by American settlers. The development of commercial livestock ranching led to increased seed dispersal, overgrazing, and the suppression of grass fires, the combined effects of which favored the invasion of grassland by woody plants. Short-term climatic fluctuations towards greater aridity have tended to accentuate rather than to initiate the processes of invasion. The spread of tamarisk is owing mainly to changes in stream regime resulting from the creation of reservoirs. The history of recent plant invasions in the Southwest illustrates how man may unintentionally bring about rapid and profound ecological changes in dry areas by the introduction of new systems of land use.