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Hydraulic Properties of the Riffle Crest and Applications for Stream Ecosystem Management
Abstract
Riffle crests are important hydraulic controls in riffle‐pool‐dominated streams, influencing pool hydraulics and water quality that collectively control lotic habitat for many organisms. We define a simple stream depth measurement, the riffle crest thalweg (RCT), describe measurement methods and utility, and suggest that RCT depth is a better independent variable than streamflow (Q) for many instream flow and habitat assessment applications. Using RCT depth as an independent variable, rather than streamflow, reduces the need for gauging or streamflow measurements in many management applications. Unlike streamflow, RCT depth varies directly with fundamental elements of riverine habitat such as channel morphology and bed roughness. We also suggest that relationships between RCT depth and streamflow (RCT‐Q curves) can be used to evaluate the risk of streamflow alteration at ungauged sites. We describe three case studies to demonstrate the utility of RCT depth and RCT‐Q rating curves in stream ecosystem management: (1) evaluating the effect of a top‐down flow prescription on modeled salmonid habitat, (2) estimating the risk from the incremental reduction of RCT depth on habitat and ecological responses, and (3) identifying relationships between RCT depth and dissolved oxygen in ungauged streams. An easy‐to‐measure, inexpensive, and ecologically sensitive metric, RCT depth holds promise as a useful tool in stream ecosystem management.
... The movements we observed, however, were strongly associated with declining ri e crest thalweg depths. This metric is used to identify hydraulic conditions that in uence sh passage between pools (Rossi et al. 2023, Kastl 2023) and habitat quality within passage thresholds for migration between contiguous habitat pools for salmonids (Rossi et al. 2021a(Rossi et al. & 2021b. Although the ri e crest depth may not be the causal factor of movement, it likely serves as an indicator of the changing ecological and physical habitat conditions that lotic organisms respond to (Rossi et al. 2021a). ...
... This metric is used to identify hydraulic conditions that in uence sh passage between pools (Rossi et al. 2023, Kastl 2023) and habitat quality within passage thresholds for migration between contiguous habitat pools for salmonids (Rossi et al. 2021a(Rossi et al. & 2021b. Although the ri e crest depth may not be the causal factor of movement, it likely serves as an indicator of the changing ecological and physical habitat conditions that lotic organisms respond to (Rossi et al. 2021a). Additionally, increasing stream temperatures had a weaker but still signi cant in uence on stream-resident LCT movement. ...
... Across all variables analyzed as potential drivers of LCT movement, change in RCT depth was the strongest predictor across all models ( Table 2). Our study did not examine RCT depths at exact sh location sites, but rather at nearby sites with similar channel geometry and bed roughness; however, RCT depths within stream reaches of similar channel geometry and roughness tend to have similar relationships with stream ow (Rossi et al. 2021a). We observed an associated minimum RCT depth of 4 cm across all sh emigration observations (n=46), suggesting a minimum passage threshold for movement between habitat units. ...
Background: Understanding the movement of organisms is critical for species conservation in the context of changing landscapes and climate. As climatic extremes impact the United States Great Basin, quantifying the movements of native fishes like Lahontan cutthroat trout (Oncorhynchus clarkii henshawi) is vital for facilitating their persistence. These climatic extremes are projected to alter flow regimes, specifically, reducing hydrologic connectivity needed to maintain populations. By studying fish movement patterns during streamflow recession and baseflow conditions, we can identify the factors responsible for movement and habitat selection to better manage these factors in a changing world.
Methods: We tagged 57 Lahontan cutthroat trout from early summer to fall in 2021 and 2022 in the Summit Lake watershed (NV, USA). The location of each fish was associated with local hydraulic, physical habitat, invertebrate drift concentration, and water quality data to assess which factors impact habitat selection, abandonment, and overall movement. Multiple linear regression models were used to assess which factors were associated with trout movement, and a two-sample permutation test was used to identify factors associated with habitat selection or abandonment.
Results: Stream-resident trout displayed little movement during streamflow recession and baseflow conditions, with median daily movements of 0.3 m/day and a median home range of 10.2 m; these results suggest even less movement than those reported in previous studies. Abrupt declines in riffle crest thalweg (RCT) depth were the primary factor associated with increases in distance traveled, yet there were only four observed movements below RCT depths of 5 cm and no observations below 4 cm. The only factor that impacted trout habitat selection or
abandonment was fork length and weight, with smaller individuals abandoning habitat more often than larger, dominant individuals.
Conclusions: The findings from this study suggest that trout movement occurs when absolutely necessary, such as escaping drying reaches or being displaced by larger or more aggressive individuals. We suggest that watershed managers implement low-flow hydrologic monitoring to identify vulnerable stream reaches, with an emphasis on preserving streamflow connectivity for stream-rearing salmonids. Additionally, this emphasizes the importance of tracking movements for species of interest as a strategy to identify factors potentially reducing population fitness.
... During the outmigration season in 2018 and 2019, we measured the depth of water within the channel across a range of flow conditions using a stadia rod. Specifically, we measured water depths at the riffle crest thalweg (RCT), which is the deepest point of the channel (thalweg) at the riffle crest (Rossi et al., 2021). The riffle crest occurs immediately downstream of the pool tail and is the shallowest depth in the longitudinal profile of riffle-pool sequences. ...
... Specifically, we used power law regression analysis to develop mathematical relationships between median RCT depth and discharge (i.e., the RCT rating curve). Relationships between discharge at a given channel cross-section and the associated average water depth generally have been shown to follow power-law relationships (Leopold & Maddock, 1953) and have been specifically used for describing the relationship between discharge and RCT depth (Mierau et al., 2017;Rossi et al., 2021): ...
... (b) Example of riffle crest thalweg (RCT) field measurement location, marked by rebar. (c) Pool-riffle sequence (vertical exaggeration) and RCTs (adapted fromRossi et al., 2021). ...
Local human impacts and climate change are presenting freshwater species around the world with conditions that are more extreme than those to which they evolved. Human water use and increasing drought severity are causing unprecedented low stream flow and warm water. Consequently, cold-water, migratory fish, such as salmon and steelhead trout (Oncorhynchus spp.), face challenges in completing their life cycles. As Pacific salmon navigate extensive river networks during their migration, individual fish often cannot avoid heavily impaired reaches of streams and rivers, particularly in California, USA. Salmon in the southern extent of their range already experience conditions that approach or exceed their tolerance limits. Yet, we have an incomplete understanding of flow and temperature conditions required to complete their life cycles and sustain healthy populations. Are salmon approaching thresholds in low flows, minimum depths, and maximum temperatures? How do streamflow and temperature influence the critical timing of downstream migration (outmigration) of juveniles? Do minimum thresholds exist in stream water depths that salmon can navigate? How do additional environmental and ecological stressors interact with water temperature to affect physiological thermal limits of salmon? In this dissertation, I explore how an endangered population of coho salmon (Oncorhynchus kisutch) near the southern extent of the species range in the agricultural Russian River watershed is affected by receding streams and warming waters. Together, the chapters of this dissertation offer insights to better understand, manage, and mitigate the adverse impacts of low flow, shallow water, and warm water in salmon-bearing streams in coastal California. Our findings suggest that maintaining adequate streamflows from March through June is critical to preserving an outmigration window that is sufficiently long in duration to reduce risks of phenological mismatches when juvenile salmon reach the ocean to feed. Policies that set minimum water depth limits are likely needed in many of California’s impaired streams to protect the salmon movement during the outmigration period.
... Each study reach contained four riffle-pool habitat units which served as replicates, resulting in eight "control" units and eight "impact" units ( Figure 1, bottom). The riffle-pool unit is a ubiquitous geomorphic feature in alluvial streams with slopes between 0.5% and 2% (Leopold and Wolman 1957) and provides a discrete habitat unit for evaluating juvenile salmonid rearing and foraging during the low-flow period (Rossi et al. 2021). During the dry season period, pools approach or reach disconnectivity and are sufficiently isolated to be considered independent units. ...
... Onset HOBO U20 water level loggers were deployed in all pools within each study reach to measure continuous changes in pool depth. Water level loggers were mounted to rebar and installed near the pool maximum depth (Figure 1 (Rossi et al. 2021). RCT depth served a proxy for hydraulic connectivity (between pools); although it has also been shown as a predictor of poor dissolved oxygen concentrations and changes in salmonid foraging behavior (Rossi et al. 2021;Rossi et al. 2021b). ...
... Water level loggers were mounted to rebar and installed near the pool maximum depth (Figure 1 (Rossi et al. 2021). RCT depth served a proxy for hydraulic connectivity (between pools); although it has also been shown as a predictor of poor dissolved oxygen concentrations and changes in salmonid foraging behavior (Rossi et al. 2021;Rossi et al. 2021b). ...
Objective
In the western United States, juvenile salmon Oncorhynchus spp. and steelhead O. mykiss are especially vulnerable to streamflow depletion in the dry season. Releasing water from off‐channel storage into small streams is a novel restoration strategy to offset impacts from anthropogenic flow alteration on salmonid fishes. To date, no studies have evaluated the ecological effects of small‐scale flow augmentations. Here, we quantify the effects of one such augmentation project on habitat connectivity, water quality, invertebrate drift, and juvenile salmonid movement and survival.
Methods
Our study took place in a northern California stream and included an unusually wet summer (2019) and a more typical dry summer (2020). We used categorical and time‐series analyses in a before–after, control–impact (BACI) design, along with capture–mark–recapture methods to evaluate the ecological impacts of a 13.9‐L/s flow augmentation.
Result
We found that differences in ambient streamflows between the two years mediated the physical and ecological effects of the flow augmentation treatment. In the dry year, habitat connectivity and dissolved oxygen markedly increased at sites over 1.5 km downstream from the point of augmentation, whereas during the wet year, the effects on those variables were negligible. In both years, invertebrate drift marginally increased after augmentation. Interpool movement of wild juvenile steelhead and stocked Coho Salmon O. kisutch increased after augmentation during the dry summer but not during the wet summer. Flow augmentation increased the survival probability for salmonids, with a larger effect during the dry summer (24% higher survival for Coho Salmon and 20% higher survival for steelhead) than during the wet summer (no effect was observed for steelhead survival, and Coho Salmon survival increased by 11%).
Conclusion
This study indicates that appropriately designed small‐scale flow augmentations can improve conditions for rearing salmonids in small streams, particularly during dry years. More broadly, it provides empirical evidence that efforts to restore summer streamflow in small, salmon‐bearing streams can yield significant ecological benefits.
... These holistic methods still require managers to determine an acceptable level of departure from the unimpaired hydrograph that balances ecological and societal risks (Tharme, 2003). To date, few methods have been developed to estimate protective diversion rates for holistic management (but see Richter et al., 2012;Rossi, Mierau, et al., 2021), and none have done so using process-based and fish-oriented flow-ecology relationships. ...
... After summarizing energetic conditions for the four initial POF-diverison flow scenarios, we asked what would be the highest allowable POF diversion that would maintain NREI within unimpaired levels. This is a risk-based approach that evaluates the ecological risk of incremental degrees of departure from unimpaired habitat values (Rossi, Mierau, et al., 2021). We followed the same set of steps as above to calculate NREI for every POF diversion, from 1% to 50%, at 1% increments. ...
Bioenergetics models produce quantitative flow‐ecology relationships that summarize changes in habitat and food resources from altered flows. We used a drift‐foraging bioenergetics model to quantify the net rate of energetic intake (NREI) for trout above and below a water diversion. NREI is reduced by >95% below the water diversion in July–September, when up to 98% of unimpaired flows are diverted. We then used a risk‐based approach to estimate the maximum diversion rate, expressed as a percentage of unimpaired flow, that would produce NREI values that are not significantly lower than values under unimpaired flows throughout a 62‐year period. NREI decreased with increased precent‐of‐flow diversion rates in low‐flow months (July–September). Diversion rates of 16% in July and 9% in August and September would maintain NREI within the range of unimpaired flow conditions. In higher flow months, May–June, increasing diversions brought estimated instream flows closer to the peak NREI flow, leading to the assessment that increased diversions would increase NREI. Bioenergetic models can be used to develop protective flow rates at times of the year when fish growth and production would be high under unimpaired flows, which often coincides with when water is diverted. Our study is the first to develop protective percent‐of‐flow diversion rates for holistic flow management using a quantitative process‐based and fish‐centric ecological metric.
... Given that channels may have large local variations in slope (e.g., between riffles and pools), they can be easily biased by a small number of geomorphic units (e.g., deep pools). Using the gradient measured between riffle crests has the advantage of being more relevant for hydraulic calculations and models, as the riffle crest gradient approximates the water surface slope at high flows and provides a consistent measure in the field as it is easy to identify (Rossi et al., 2021). Care should also be taken to measure the gradient over a sufficiently long distance, typically advised over at least 20 channel widths to obtain a representative average reach gradient (Rosgen, 1994). ...
... It is recommended that gauges are installed in relatively stable cross-sections at riffle crests to ensure consistent relationships between measured flow depth and discharge. Additionally, taking measurements of the thalweg depth at riffle crests can be easily related to discharge estimates (Rossi et al., 2021). ...
Fluvial geomorphology, which describes the form and processes of rivers, is increasingly being incorporated into river assessment procedures. However, the complexity of geomorphic processes makes a single universal and standardized assessment protocol a challenging and possibly impractical task. In this paper, we present a set of recommendations for choosing appropriate river assessment procedures and measuring geomorphic indicators to effectively capture important geomorphic processes required to support river management goals. We outline steps for building a river assessment procedure based on an adaptive approach rather than a one‐size‐fits‐all approach, where the geomorphic indicators, spatial and temporal scale, and methodologies used are carefully chosen based on the goals of the management project; the assessment aims to support. Guidance for choosing the appropriate geomorphic indicators is based on their significance (usefulness in characterizing the system), ease of measurement, and temporal scale needs. We also present recommendations on measurement techniques for each indicator while highlighting recent technological and methodological advancements that help overcome resource challenges often faced in river assessment. Given the wealth of scientific and technological developments in the field of geomorphology, it is possible to improve how geomorphic form and function are measured and incorporated into river assessments that support watershed management goals.
... The riffle-pool unit is a dominant geomorphic feature in most alluvial streams. During low flow, they can be partially or completely isolated from each other, and so can provide discrete habitats for evaluating juvenile salmonid rearing and foraging (Naman et al., 2018;Rossi, Mierau, & Carah, 2021). We selected riffle-pool units that supported multiple age classes of foraging salmonids and were separated from each other by at least two pools. ...
The growth of any organism depends on habitat conditions, food availability, and their seasonal interactions. Yet in the vast literature on Pacific salmon (Oncorhynchus), the seasonal interaction between habitat conditions and food availability has received relatively little attention. We examined juvenile Oncorhynchus mykiss rearing, physical habitat, and resource phenologies in two Mediterranean coastal streams—one perennial, cool, and shaded and the other intermittent, seasonally warm, and sunny. We used a bioenergetic model to investigate the timing and magnitude of growth potential for drift‐foraging O. mykiss during the spring and summer in both systems. Growth potential peaked at least 2 months earlier in the intermittent stream than in the perennial stream. By early summer (June), growth potential had declined in the intermittent stream, whereas growth rates were peaking in the perennial stream. However, the mid‐July lipid content of juvenile O. mykiss in the intermittent stream was nearly twice that of fish in the perennial stream. By late summer (August), foraging profitability declined in both streams, as abiotic conditions in the intermittent stream approached lethal. In contrast, the perennial stream maintained suitable abiotic conditions even though the growth rate was low. We suggest that the divergent resource phenologies and seasonal mortality risks experienced by anadromous O. mykiss rearing in these streams could drive diversification of traits governing size, age, and timing of outmigration.
Salmonids frequently adapt their feeding and movement strategies to cope with seasonally fluctuating stream environments. Oncorhynchus mykiss tend to drift-forage in higher velocity habitat than other salmonids, yet their presence in streams with seasonally low velocity and drift suggests behavioral flexibility. We combined 3D videogrammetry with measurements of invertebrate drift and stream hydraulics to investigate the drivers of O. mykiss foraging mode and movement during the seasonal recession in a California stream. From May to July (2016), foraging movement rate increased as prey concentration and velocity declined; however, movement decreased in August as pools became low and still. In May, 80% of O. mykiss were drift-foraging, while by July, over 70% used search or benthic-foraging modes. Velocity and riffle crest depth were significant predictors of foraging mode, while drift concentration was a poor univariate predictor. However, top-ranked additive models included both hydraulic variables and drift concentration. A drift-foraging bioenergetic model was a poor predictor of foraging mode. We suggest that infall and benthic prey, as well as risk aversion, may influence late-summer foraging decisions.
The science and practice of environmental flows have advanced significantly over the last several decades. Most environmental flow approaches require quantifying the relationships between hydrologic change and biologic response, but this can be challenging to determine and implement due to high data requirements, limited transferability, and the abundance of hydrologic metrics available for evaluation. We suggest that a functional flows approach, focusing on elements of the natural flow regime known to sustain important ecosystem processes, offers a pathway for linking understanding of ecosystem processes with discrete, quantifiable measures of the flow regime for a broad range of native taxa and assemblages. Functional flow components can be identified as distinct aspects of the annual hydrograph that support key biophysical processes, such as wet season flood flows or spring recession flows, and then quantified by flow metrics, such as 5% exceedance flow or daily percent decrease in flow, respectively. By selecting a discrete set of flow metrics that measure key functional flow components, the spatial and temporal complexity of flow regimes can be managed in a holistic manner supportive of multiple ecological processes and native aquatic species requirements. We provide an overview of the functional flows approach to selecting a defined set of flow metrics and illustrate its application in two seasonally variable stream systems. We further discuss how a functional flows approach can be utilized as a conceptual model both within and outside of existing environmental flow frameworks to guide consideration of ecological processes when designing prescribed flow regimes.
Quantitative habitat suitability models (HSMs) are frequently used to inform the conservation and management of lotic organisms, often in the context of instream flow management. Correlative statistical models relating hydraulic variables to habitat preferences (habitat suitability curves based on use:availability ratios) are the most common form of HSM, but face significant criticism on the grounds that habitat preference may not reflect the fitness consequences of habitat use. Consequently, there has been a drive to develop mechanistic approaches that link habitat to direct correlates of fitness.
Bioenergetic foraging models relating hydraulic conditions to energy balance are particularly well‐developed for drift‐feeding fishes (e.g. salmonids) and show promise as a more mechanistic approach to modelling suitability. However, these models are rarely validated empirically or quantitatively compared with correlative HSMs. We addressed these gaps by comparing the ability of a bioenergetics‐based HSM and two correlative HSMs (a traditional suitability index and a resource selection function) to predict density and growth of stream salmonids (juvenile steelhead, Oncorhynchus mykiss , and coastal cutthroat trout, Oncorhynchus clarki ).
Suitability estimates differed between the approaches, with both correlative models predicting higher suitability relative to the bioenergetic model at shallow depths and low to intermediate velocities, but lower suitability as depth increased. The bioenergetic model explained over 90% of variation in trout growth, compared to c . 50% for the correlative model. The bioenergetic model was also better at predicting fish density; however, the improvement was less striking and a high proportion of variation remained unexplained by either method.
Differences in suitability estimates between approaches probably reflect biotic interactions (e.g. territorial displacement or predation risk) that decouple realised habitat use from energetics‐based estimates of habitat quality. Results highlight fundamental differences between correlative HSMs, based on observed habitat use, and mechanistic HSMs, based on the physiology and behaviour of the focal taxa. They also suggest that mechanistic bioenergetics‐based models provide more rigorous estimates of habitat suitability for drift‐feeding stream fishes. The bioenergetics approach is readily accessible to instream flow practitioners because model predictions are expressed in terms of traditional habitat suitability curves.
Increasing habitat availability (i.e. habitat suitable for occupancy) is often assumed to elevate the abundance or production of mobile consumers; however, this relationship is often nonlinear (threshold or unimodal). Identifying the mechanisms underlying these nonlinearities is essential for predicting the ecological impacts of habitat change, yet the functional forms and ultimate causation of consumer‐habitat relationships are often poorly understood.
Nonlinear effects of habitat on animal abundance may manifest through physical constraints on foraging that restrict consumers from accessing their resources. Subsequent spatial incongruence between consumers and resources should lead to unimodal or saturating effects of habitat availability on consumer production if increasing the area of habitat suitable for consumer occupancy comes at the expense of habitats that generate resources. However, the shape of this relationship could be sensitive to cross‐ecosystem prey subsidies, which may be unrelated to recipient habitat structure and result in more linear habitat effects on consumer production.
We investigated habitat‐production relationships for juveniles of stream‐rearing Pacific salmon and trout ( Oncorhynchus spp.), which typically forage in low‐velocity pool habitats, while their prey (drifting benthic invertebrates) are produced upstream in high‐velocity riffles. However, juvenile salmonids also consume subsidies of terrestrial invertebrates that may be independent of pool‐riffle structure.
We measured salmonid biomass production in 13 experimental enclosures each containing a downstream pool and upstream riffle, spanning a gradient of relative pool area (14%–80% pool). Increasing pool relative to riffle habitat area decreased prey abundance, leading to a nonlinear saturating effect on fish production. We then used bioenergetics model simulations to examine how the relationship between pool area and salmonid biomass is affected by varying levels of terrestrial subsidy. Simulations indicated that increasing terrestrial prey inputs linearized the effect of habitat availability on salmonid biomass, while decreasing terrestrial inputs exaggerated a “hump‐shaped” effect.
Our results imply that nonlinear effects of habitat availability on consumer production can arise from trade‐offs between habitat suitable for consumer occupancy and habitat that generates prey. However, cross‐ecosystem prey subsidies can effectively decouple this trade‐off and modify consumer‐habitat relationships in recipient systems.
As extinctions continue across the globe, conservation biologists are turning to species reintroduction programs as one optimistic tool for addressing the biodiversity crisis. For repatriation to become a viable strategy, fundamental prerequisites include determining the causes of declines and assessing whether the causes persist in the environment. Invasive species—especially pathogens—are an increasingly significant factor contributing to biodiversity loss. We hypothesized that Batrachochytrium dendrobatidis (Bd), the causative agent of the deadly amphibian disease chytridiomycosis, was important in the rapid (<10 years) localized extirpation of a North American frog (Rana boylii) and that Bd remains widespread among extant amphibians in the region of extirpation. We used an interdisciplinary approach, combining interviews with herpetological experts, analysis of archived field notes and museum specimen collections, and field sampling of the extant amphibian assemblage to examine (1) historical relative abundance of R. boylii; (2) potential causes of R. boylii declines; and (3) historical and contemporary prevalence of Bd. We found that R. boylii were relatively abundant prior to their rapid extirpation, and an increase in Bd prevalence coincided with R. boylii declines during a time of rapid change in the region, wherein backcountry recreation, urban development, and the amphibian pet trade were all on the rise. In addition, extreme flooding during the winter of 1969 coincided with localized extirpations in R. boylii populations observed by interview respondents. We conclude that Bd likely played an important role in the rapid extirpation of R. boylii from southern California and that multiple natural and anthropogenic factors may have worked in concert to make this possible in a relatively short period of time. This study emphasizes the importance of recognizing historical ecological contexts in making future management and reintroduction decisions.
In Mediterranean‐type river systems, naturally low seasonal stream flows are often overexploited, which has implications for managing flows for environmental as well as human needs.
Traditional approaches to instream flow management are not well suited to unregulated systems with strong seasonal patterns of water availability and many water diverters, and are challenging to implement in such systems. They often do not protect the full range of variability in the annual hydrograph, require extensive site‐specific data, expensive modelling or both.
In contrast, holistic flow management strategies, such as percent‐of‐flow ( POF ) strategies are designed to protect multiple ecological processes and preserve inter‐annual flow variability. However, POF approaches typically require real‐time streamflow gauging, and often lack a robust metric relating a diversion rate to ecological processes in the stream.
To address these challenges, we present a modified percent‐of‐flow ( MPOF ) diversion approach where diversions are allocated from a streamflow baseline which is derived from a regional relationship between a conservative streamflow‐exceedance and date. The streamflow baseline remains the same from year to year, and is independent of water‐year type. This approach protects inter‐annual flow variability and provides a predictable daily allowable volume of diversion at any diversion point—supporting efficient water management planning.
The allowable diversion rate in the MPOF approach is based not on a fixed percentage of the ambient streamflow, but rather on a maximum allowable percentage change in riffle crest thalweg depth, an ecologically meaningful, common hydraulic measurement.
In this paper, we demonstrate that the MPOF approach is a holistic approach well suited to manage diversions in unregulated streams typical of California's Mediterranean‐type coastal drainages.
Intermittent streams lose surface flow during part of the year but can provide important habitat for imperiled fishes in residual pools. However, extended intermittency can drive high mortality as pool contraction decreases pool quality, and some pools dry completely. We evaluated the influence of a suite of abiotic habitat characteristics on the over-summer survival of two imperiled salmonid fishes (coho salmon Oncorhynchus kisutch; steelhead trout Oncorhynchus mykiss) at four study sites on two tributaries of Salmon Creek (Sonoma County, California, USA) from 2012 to 2014, during deepening drought conditions. Study sites spanned an intermittency gradient from continuous flow to near-dry conditions, and included alluvial and bedrock stream reaches. We estimated over-summer survival at the pool scale from fish presence–absence data based on paired early-late summer snorkel surveys. We measured pool dimensions and water quality parameters monthly (more frequently during summer dry down) and, in 2013 and 2014, recorded water quality with continuous loggers in selected pools. We performed: (1) logistic regression in a generalized linear modeling framework to identify factors limiting over-summer survival and (2) classification trees using the random forests ensemble learning method to identify abiotic thresholds for sustaining salmonids. Results suggested that different factors governed mortality of the two species. Coho salmon, which tended to survive in large, deep pools, were limited by minimum dissolved oxygen (DO) concentrations. In contrast, steelhead trout, which tended to survive in pools with large surface area, were sensitive to pool geometry and temperature. Both species persisted for weeks in large pools with low DO levels, including in pools where at least part of the water column reached sublethal or lethal levels. Our results suggest that shallow, lateral hyporheic flow may be important for maintaining DO and temperatures suitable for sustaining salmonids in isolated pools, whereas groundwater discharge originating from deeper flow paths may generate low-DO conditions that inhibit salmonid persistence. Geomorphically complex watersheds with a variety of pool geometries and high rates of lateral hyporheic exchange are those most likely to serve as “sanctuary reaches” for imperiled Pacific salmonid populations in semi-arid regions in the context of a changing climate.
Instream flow models link a physical habitat model that predicts flow-related changes in hydraulics to a biological model that predicts the response of fish to altered velocity and depth. Habitat suitability curves (HSCs) based on frequency of habitat use (fish occurrence relative to available habitat) remain the most widely used biological models in habitat simulations. However, in some contexts fish density may be a poor indicator of habitat quality, leading to biased predictions of optimal flow. We explore the use of bioenergetics to derive mechanistic HSCs based on the fundamental energetics of habitat use. Using flow-related changes in production of Coho Salmon Oncorhynchus kisutch smolt as reference data to evaluate model predictions, we found that bioenergetic-based HSCs matched the validation data better than frequency-based HSCs, which systematically underestimated optimal flows. However, biases remained using bioenergetic HSCs, suggesting that habitat suitability may not be independent of discharge as is often assumed. Declining invertebrate drift concentration, increasing temperature, and density dependence of growth at low flows are potential mechanisms of flow-related declines in habitat suitability; measuring these effects and incorporating them into flow models is an important step in further improving model predictions, particularly at low flows.
Received December 3, 2015; accepted May 11, 2016
Hatfield and Paul (2015) note the potential for hidden value trade-offs in desktop models used for establishing environmental flows, which highlights the need to separate the contributions of science vs. values when developing flow regulations. This is best achieved with regulatory frameworks that clarify underlying values and objectives, identify any trade-offs that may be entertained between instream and out-of-stream uses, and provide clear guidance on the ecological and socio-economic contexts where different trade-offs and corresponding regulations may be implemented. Accurate flow-ecological response relationships are key to credible assessment of trade-offs or protection of instream values; when quantitative relationships are unavailable, model predictions and performance are highly sensitive to the assumed shape of the flow-ecology relationship. For example, by assuming that habitat capacity asymptotes at 20% of mean annual discharge, Hatfield and Paul (2015) implicitly discount the ecological value of higher base flows. Although cross-calibration of desktop models has value, performance of models for predicting environmental flows is best evaluated against independent measures of biological response to varying discharge (e.g. observed flow effects on fish biomass, production or other ecological attributes).
The pebble count procedure, widely used among geomorphol-ogists, can be adapted for use in fishery studies to determine surface bed material size distributions. At least 100 randomly selected bed particles are measured and a size distribution curve is developed for each distinct population on the streambed, The characteristics of that population are then applied to all obser-vation points that fall within the area underlain by that population. This ap-proach constitutes an improvement over visual estimates of surface bed material size, which rely entirely on observer judgment and estimation. Because the pebble count technique has been shown to be reproducible, bed material size data collected in different studies can be compared with some confidence. We performed pebble counts on sites on Rush Creek, California, for which visual estimates were available. Size distribution curves developed from the two tech-niques had similar shapes, but visual estimates were finer than pebble counts overall and tended to exaggerate differences among populations, KEY WORDS: Instream flow, pebble count, substrate.
This paper explores the role of riffle-pool bedforms in functional eco-hydraulic river rehabilitation designs and addresses the need for quantitative means of generalizing flow behaviour. Riffle hydraulic performance is modelled using a cross-sectionally disaggregated form of broad crested weir equation. For a given channel reach, geostatistical analysis of mapped flow velocity at contrasting flow stages is then used to identify coherent flow behaviour which might be related to channel bedform morphology and hydraulic function. Finally, observed and inferred flow characteristics are reviewed against the data requirements of habitat simulation models and with respect to the application flow simulation modelling. By coupling morphology, flow characterization and habitat modelling, the aim is to provide a simple, but scientifically sound approach for assessing the eco-hydraulic performance of channel bedforms to improve both project design and post-project audit of rehabilitation schemes.
The techniques and standards for making discharge measurements at streamflow gaging stations are described in this publication. The vertical axis rotating-element current meter, principally the Price current meter, has been traditionally used for most measurements of discharge; however, advancements in acoustic technology have led to important developments in the use of acoustic Doppler current profilers, acoustic Doppler velocimeters, and other emerging technologies for the measurement of discharge. These new instruments, based on acoustic Doppler theory, have the advantage of no moving parts, and in the case of the acoustic Doppler current profiler, quickly and easily provide three-dimensional stream-velocity profile data through much of the vertical water column. For much of the discussion of acoustic Doppler current profiler moving-boat methodology, the reader is referred to U.S. Geological Survey Techniques and Methods 3–A22 (Mueller and Wagner, 2009).
Personal digital assistants (PDAs), electronic field notebooks, and other personal computers provide fast and efficient data-collection methods that are more error-free than traditional hand methods. The use of portable weirs and flumes, floats, volumetric tanks, indirect methods, and tracers in measuring discharge are briefly described.
Organisms that live in highly variable environments, such as rivers, rely on adaptations to withstand and recover from disturbance. These adaptations include behavioral traits, such as habitat preference and plasticity of reproductive timing, that minimize the effects of discharge fluctuation. Studies linking hydrologic regime, habitat preference, and population processes, however, are predominantly limited to fish. Information on other sensitive taxa is necessary to facilitate conservation of multispecies assemblages and restoration of biodiversity in degraded river channels. I studied the functional relationship between physical habitat and reproduction of the foothills yellow-legged frog (Rana boylii), a California State Species of Special Concern. From 1992 to 1994, I mapped breeding sites along 5.3 km of the South Fork Eel River in northern California and monitored egg survival to hatching. Frogs selected sites over a range of spatial scales and timed their egg-laying to avoid fluctuations in river stage and current velocity associated with changes in discharge. The main sources of mortality were desiccation and subsequent predation of eggs in a dry year and scour from substrate in wet years, both caused by changes in stage and velocity. At the finest spatial scale, frogs attached eggs to cobbles and boulders at lower than ambient flow velocities. At larger scales, breeding sites were near confluences of tributary drainages and were located in wide, shallow reaches. Clutches laid in relatively narrower and deeper channels had poor survival in rainy as well as dry springs. Most breeding sites were used repeatedly, despite between- and within-year variation in spring stage of the river. This pattern of site selection suggests that conservation of Rana boylii may be enhanced by maintaining or restoring channels with shapes that provide stable habitat over a range of river stages.
Transport of coarse, heterogeneous debris in a natural stream under a wide range of flows usually results in a remarkably stable, undulatory bed profile, which manifests an in transit sorting process of the bed material. In general, finer material representative of the bulk of the normal bed load resides in the deep sections, or pools, below flood stages. At high flows, pools may scour to immovable boulders or bedrock. Coarser material transported at more infrequent flows forms the shallow sections, or riffles. Above a flow threshold, the pool fill material is often scoured and deposited in part over the riffles (Lane and Borland, 1954; Andrews, 1977). Interactions among coarse particles in motion in turbulent flow tend to concentrate them in groups with the coarsest particles at the surface (Langbein and Leopold, 1968). Incipient accumulations of coarse particles may be perpetuated by altering the flow conditions which influence bed load transport. As flow increases, submergence of the riffle-pool bed topography modifies its effect on the local hydraulic conditions. At low flow, mean volocity and water surface slope are greater, and mean depth is less at a riffle than at a pool. Competence is greater at the riffle. As the stage increases, the water-surface profile tends to even out as the hydraulic gradient over a riffle decreases and that over a pool increases (Leopold and others, 1964). Corresponding values of velocity and depth tend to converge, although depth often less so (Richards, 1976; Lisle, 1976; Andrews, 1977). The convergence of respective values of water-surface slope over a riffle and pool and the greater depth of the pool cause mean shear stress, τ = γRSE (where γ is the specific gravity of water, R is the hydraulic radius or approximately the mean depth, and SE is the energy gradient), to increase more rapidly at the pool (Leopold and others, 1964). As a result, competence as measured by velocity or bottom shear stress should become more evenly distributed over a riffle and pool (Richards, 1976) or even reversed in. hierarchy at high flow (Gilbert, 1914; Keller, 1971).
Dams and water diversions can dramatically alter the hydraulic habitats of stream ecosystems. Predicting how water depth and velocity respond to flow alteration is possible using hydraulic models, such as Physical Habitat Simulation (PHABSIM); however, such models are expensive to implement and typically describe only a short length of stream (102 m). If science is to keep pace with development, then more rapid and cost-effective models are needed. We developed a generalized habitat model (GHM) for brown and rainbow trout that makes similar predictions to PHABSIM models but offers a demonstrated reduction in survey effort for Colorado Rocky Mountain streams. This model combines the best features of GHMs developed elsewhere, including the options of desktop (no-survey) or rapid-survey models. Habitat–flow curves produced by PHABSIM were simplified to just two site-specific components: (i) Q95h (flow at 95% of maximum habitat) and (ii) Shape. The Shape component describes the habitat–flow curves made dimensionless by dividing flow increments by Q95h and dividing habitat (weighted usable area) increments by maximum habitat. Both components were predicted from desktop variables, including mean annual flow, using linear regression. The rapid-survey GHM produced better predictions of observed habitat than the desktop GHM (rapid-survey model explained 82–89% variance for independent validation sites; desktop 68–85%). The predictive success of these GHMs was similar to other published models, but survey effort to achieve that success was substantially reduced. Habitat predicted by the desktop GHM (using geographic information system data) was significantly correlated with the abundance of large brown trout (p < 0.01) but not smaller trout. Copyright © 2013 John Wiley & Sons, Ltd.
After the flood of December 1964, 12 gaging sections in northern
California widened as much as 100% and aggraded as much as 4 m, and then
degraded to stable levels during a period of 5 years or more. As
channels aggraded, bed material became finer, and low to moderate flow
through gaging sections in pools became shallower, faster, and steeper.
Comparisons of longitudinal profiles also show the diminishment of pools
as well as a decrease in bar relief accompanying the excessive sediment
load. As gaging sections degraded, hydraulic geometries recovered to a
limited degree; full recovery probably depends on channel narrowing and
further depletion of sediment supply. The hydraulic changes with
aggradation indicate an increase in the effectiveness of moderate
discharges (less than 1- to 2-year recurrence interval, annual flood
series) to transport bed load and shape the bed. Bars become smaller,
pools preferentially fill, and riffles armored with relatively small
gravel tend to erode headward during falling stages and form a gentler
gradient. Excess sediment can thus be more readily transported out of
channels when additional contributions from watersheds are usuall
slight.
This document is intended to update the concepts and ideas first presented in Information Paper 12, the first attempt to describe the Instream Flow Incremental Methodology in its entirety in 1982. This also is to serve as a comprehensive introductory textbook on IFIM for training courses as it contains the most complete and comprehensive description of IFIM in existence today. This should also serve as an official guide to IFIM in publication to counteract the misconceptions about the methodology that have pervaded the professional literature since the mid-1980's as this describes IFIM as it is envisioned by its developers The document is aimed at the decisionmakers of management and allocation of natural resources in providing them an overview; and to those who design and implement studies to inform the decisionmakers. There should be enough background on model concepts, data requirements, calibration techniques, and quality assurance to help the technical user design and implement a cost-effective application of IFIM that will provide policy-relevant information. Some of the chapters deal with basic organization of IFIM, procedural sequence of applying IFIM starting with problem identification, study planning and implementation, and problem resolution.
Temporary streams are those water courses that undergo the recurrent cessation of flow or the complete dry-ing of their channel. The structure and composition of biolog-ical communities in temporary stream reaches are strongly dependent on the temporal changes of the aquatic habitats determined by the hydrological conditions. Therefore, the structural and functional characteristics of aquatic fauna to assess the ecological quality of a temporary stream reach cannot be used without taking into account the controls im-posed by the hydrological regime. This paper develops meth-ods for analysing temporary streams' aquatic regimes, based on the definition of six aquatic states that summarize the transient sets of mesohabitats occurring on a given reach at a particular moment, depending on the hydrological condi-tions: Hyperrheic, Eurheic, Oligorheic, Arheic, Hyporheic and Edaphic. When the hydrological conditions lead to a change in the aquatic state, the structure and composition of the aquatic community changes according to the new set of available habitats. We used the water discharge records from gauging stations or simulations with rainfall-runoff models to infer the temporal patterns of occurrence of these states in the Aquatic States Frequency Graph we developed. The visual analysis of this graph is complemented by the devel-opment of two metrics which describe the permanence of flow and the seasonal predictability of zero flow periods. Fi-nally, a classification of temporary streams in four aquatic regimes in terms of their influence over the development of aquatic life is updated from the existing classifications, with stream aquatic regimes defined as Permanent, Temporary-pools, Temporary-dry and Episodic. While aquatic regimes describe the long-term overall variability of the hydrologi-cal conditions of the river section and have been used for many years by hydrologists and ecologists, aquatic states de-scribe the availability of mesohabitats in given periods that Published by Copernicus Publications on behalf of the European Geosciences Union. 3166 F. Gallart et al.: A novel approach to analysing the regimes of temporary streams determine the presence of different biotic assemblages. This novel concept links hydrological and ecological conditions in a unique way. All these methods were implemented with data from eight temporary streams around the Mediterranean within the MIRAGE project. Their application was a precon-dition to assessing the ecological quality of these streams.
The stability of the pool–riffle sequence is one of the most fundamental features of alluvial streams. For several decades, the process of velocity, or shear stress, reversal has been proposed as an explanation for an increase in the amplitude of pool–riffle sequence bars during high flows, offsetting gradual scour of riffles and deposition in pools during low flows. Despite several attempts, reversal has rarely been recorded in field measurements. We propose that, instead of being reversed, maxima and minima in shear stress are phase-shifted with respect to the pool–riffle sequence bedform profile, so that maximum shear stress occurs upstream of riffle crests at high flow, and downstream at low flow. Such phase-shifts produce gradients of shear stress that explain riffle deposition, and pool scour, at high flow, in accord with sediment continuity. The proposal is supported by results of a one-dimensional hydraulic model applied to the surveyed bathymetry of a pool–riffle sequence in a straight reach of a gravel-bed river. In the sequence studied, the upstream phase-shift in shear stress at high flow was associated with variations in channel width, with width minima occurring upstream of riffle crests, approximately coincident with shear stress maxima, and width maxima occurring downstream of riffle crests. Assuming that the width variation is itself the result of flow deflection by riffle crests at low flow, and associated bank-toe scour downstream, low and high flow can be seen to have complementary roles in maintaining alluvial pool–riffle sequences. Copyright © 2004 John Wiley & Sons, Ltd.
1] We report the first laboratory simulations of hyporheic exchange in gravel pool-riffle channels, which are characterized by coarse sediment, steep slopes, and three-dimensional bed forms that strongly influence surface flow. These channels are particularly important habitat for salmonids, many of which are currently at risk worldwide and which incubate their offspring within the hyporheic zone. Here we perform a set of laboratory experiments examining the effects of discharge and bed form amplitude on hyporheic exchange, with surface-subsurface mixing measured directly from the concentration decay of a conservative tracer (fluorescein) injected into the surface flow. Near-bed pressure measurements were also used to predict hyporheic exchange from a three-dimensional pumping transport model. Comparison of the predicted and observed hyporheic exchange shows good agreement, indicating that the major mechanism for exchange is bed form–induced advection. However, the effect of bed forms is modulated by discharge and the degree of topographic submergence. We also tested the performance of the hydrostatic pressure as a proxy for the observed near-bed pressure in driving hyporheic exchange, which would facilitate field measurement and analysis of hyporheic flow in natural rivers. We found agreement with measured hyporheic exchange only for low bed form amplitudes and high flows. Citation: Tonina, D., and J. M. Buffington (2007), Hyporheic exchange in gravel bed rivers with pool-riffle morphology: Laboratory experiments and three-dimensional modeling, Water Resour. Res., 43, W01421, doi:10.1029/2005WR004328.
Streamflow observations can be used to understand, predict, and contextualize hydrologic, ecological, and biogeochemical processes and conditions in streams. Stream gages are point measurements along rivers where streamflow is measured, and are often used to infer upstream watershed‐scale processes. When stream gages read zero, this may indicate that the stream has dried at this location; however, zero‐flow readings can also be caused by a wide range of other factors. Our ability to identify whether or not a zero‐flow gage reading indicates a dry fluvial system has far reaching environmental implications. Incorrect identification and interpretation by the data user can lead to inaccurate hydrologic, ecological, and/or biogeochemical predictions from models and analyses. Here, we describe several causes of zero‐flow gage readings: frozen surface water, flow reversals, instrument error, and natural or human‐driven upstream source losses or bypass flow. For these examples, we discuss the implications of zero‐flow interpretations. We also highlight additional methods for determining flow presence, including direct observations, statistical methods, and hydrologic models, which can be applied to interpret causes of zero‐flow gage readings and implications for reach‐ and watershed‐scale dynamics. Such efforts are necessary to improve our ability to understand and predict surface flow activation, cessation, and connectivity across river networks. Developing this integrated understanding of the wide range of possible meanings of zero‐flows will only attain greater importance in a more variable and changing hydrologic climate.
This article is categorized under:
• Science of Water > Methods
• Science of Water > Hydrological Processes
• Water and Life > Conservation, Management, and Awareness
Traditional methods for the calculation of rating curves from measurements of water level and discharge are criticised as being limited and complicated to implement, such that manual methods are still often used. Two methods for automatic computation are developed using least-squares approximation, one based on polynomials and the other on piecewise-continuous splines. Computational problems are investigated and procedures recommended to overcome them. Both methods are found to work well and once the parameters for a gauging station have been determined, rating data can be processed automatically. For some streams, ephemeral changes of resistance may be important, evidenced by scattered or loopy data. For such cases, the approximation methods can be used to generate a rating envelope as well, allowing the routine calculation also of maximum and minimum expected flows. Criticism is made of current shift curve practices. Finally, the approximation methods allow the specification of weights for the data points, enabling the filtering of data, especially decreasing the importance of points with age and allowing the computation of a rating curve for any time in the past or present.
The spatial and temporal scales of hyporheic exchange within the stream corridor are controlled by stream discharge and groundwater inflow interacting with streambed morphology. While decades of study have resulted in a clear understanding of how morphologic form controls hyporheic exchange at the feature scale, we lack comparable predictive power related to stream discharge and the spatial structure of groundwater inflows at the reach scale, where spatial heterogeneity in both geomorphic setting and hydrologic forcing are present. In this study, we simulated vertical hyporheic exchange along a 600 m mountain stream reach under high, medium, and low stream discharge while considering groundwater inflow as negligible, spatially uniform, or proportional to upslope accumulated area. Most changes to hyporheic flow path residence time or length in response to stream discharge were small (< 5%), suggesting that discharge is a secondary control relative to morphologically-driven hyporheic exchange. Groundwater inflow was a primary control and mostly caused decreases in hyporheic flow path residence time and length. This finding generally agrees with expectations from the literature; however, flow path response was not consistent across the study reach. Instead, we found that flow paths driven by large hydraulic gradients coinciding with large morphologic features were less sensitive to changes in groundwater inflow than those driven by hydraulic gradients similar to the valley gradient. Our results indicate that consideration of heterogeneous arrangement of morphologic features is necessary to differentiate between hyporheic flow paths that persist in time and those that are sensitive to changing hydrologic conditions.
Instream flow modeling is one tool historically used by resource managers to assess habitat suitability for aquatic species, including fish and benthic macroinvertebrates. Until recently, these methods had not been used for amphibians. Instream flow modeling requires quantitative habitat suitability criteria across a range of hydraulic conditions. We developed regional habitat suitability criteria (HSC) from data on habitat conditions at Foothill Yellow-legged Frogs (Rana boylii) oviposition and tadpole rearing locations in eight study sites in the northern Sierra Nevada mountains, California. We evaluated both univariate (percentile-based and interval-based) and multivariate logistic regression techniques for creating suitability criteria. The transferability and predictive performance of the HSC were evaluated using validation data gathered from other rivers in the Sierra Nevada and applying the criteria in a two-dimensional (2D) hydrodynamic model. We evaluated conditions under which predictive performance was poor to discern the limitations of each technique. Multivariate logistic regression analyses classified the majority of the river as low suitability, and consequently produced overestimates of suitable area for egg masses and underestimates of highly suitable area for tadpoles. Univariate HSC performed well on rivers that had similar geomorphology to the study rivers. For small rivers and creeks with shallow depths and finer substrates, locally-derived HSC are needed. The percentile-based habitat suitability index (HSI) is recommended for regional habitat suitability criteria when the goal is to assess categorical levels of suitability. The interval-based HSI would be appropriate if further information on population outcomes (e.g., population trajectory, survival rates) could be quantitatively linked to fine scale gradients of suitability in hydraulic conditions. The univariate HSI are easily applied in 2D hydrodynamic models, which can provide information on oviposition and tadpole rearing conditions under various flow regimes. Managers can use HSC to make flow recommendations beneficial to R boylii during the hydropower relicensing process.
The falling limb of the hydrograph – the streamflow recession – is frequently well-approximated by power law functions, in the form dq/dt =− aqb, so that recessions are often characterized in terms of their power law parameters (a, b). The empirical determination and interpretation of the parameter a is typically biased by the presence of a ubiquitous mathematical artifact resulting from the scale-free properties of the power law function. This reduces the information available from recession parameter analysis, and creates several heretofore-unaddressed methodological ‘pitfalls’. This letter outlines the artifact, demonstrates its genesis, and presents an empirical re-scaling method to remove artifact effects from fitted recession parameters. The re-scaling process reveals underlying climatic patterns obscured in the original data, and, we suggest, could maximize the information content of fitted power laws.
We examined the relationship between fluvial geomorphology and hyporheic exchange flows. We use geomorphology as a framework to understand hyporheic process and how these processes change with location within a stream network, and, over time, in response to changes in stream discharge and catchment wetness. We focus primarily on hydrostatic and hydrodynamic processes - the processes where linkages to fluvial geomorphology are most direct. Hydrostatic processes result from morphologic features that create elevational head gradients, whereas hydrodynamic processes result from the interaction between stream flow and channel morphologic features. We provide examples of the specific morphologic features that drive or enable hyporheic exchange and we examine how these processes interact in real stream networks to create complex subsurface flow nets through the hyporheic zone.
Discharge time series in rivers and streams are usually based on simple stage-discharge relations calibrated using a set of direct stage-discharge measurements called gaugings. Bayesian inference recently emerged as a most promising framework to build such hydrometric rating curves accurately and to estimate the associated uncertainty. In addition to providing the rigorous statistical framework necessary to uncertainty analysis, the main advantage of the Bayesian analysis of rating curves arises from the quantitative assessment of (i) the hydraulic controls that govern the stage-discharge relation, and of (ii) the individual uncertainties of available gaugings, which often differ according to the discharge measurement procedure and the flow conditions. In this paper, we introduce the BaRatin method for the Bayesian analysis of stationary rating curves and we apply it to three typical cases of hydrometric stations with contrasted flow conditions and variable abundance of hydraulic knowledge and gauging data. The results exemplify that the thorough analysis of hydraulic controls and the quantification of gauging uncertainties are required to obtain reliable and physically sound results.
Dryland rivers function as strongly linked ecologic-hydrologic systems, including both extended periods of drought and episodic flooding events. However, few studies have combined hydrologic and biogeochemical measurements to better understand the ecology of pools within dryland rivers. We used δ2H and δ18O values of pool water, rainfall, and groundwater combined with pool water measurements of C, N, and P and dissolved organic matter (DOM) fluorescence characteristics to determine (1) the concentration and chemical composition of DOM and (2) the origin of surface water in 16 pools of a dryland river in northern Western Australia. Parallel factor analysis of excitation-emission matrices showed that humic-like components derived mainly from terrestrial plant material dominated total DOM fluorescence for all pools. Evaporation models using δ2H and δ18O showed a variety of pool hydrologic regimes, including pools with moderate to high evaporative water loss that were largely isolated from shallow alluvium water inputs and pools with consistent alluvium water throughflow and low evaporation. Concentrations of C, N, and P as well as total DOM fluorescence were generally greater in pools with high evaporative loss and lower in pools with alluvium water inputs. Pool δ2H and δ18O values were also significantly correlated with DOM fluorescence characteristics and C, N, and P concentrations, providing quantitative evidence of the hydrologic influence on DOM biogeochemistry. Taken together, our findings suggest that individual pools function as distinct ecosystems within the riverine environment.
Resource managers have traditionally had to rely on simple hydrological and habitat-association methods to predict how changes in river flow regimes will affect the viability of instream populations and communities. Yet these systems are characterized by dynamic feedbacks among system components, a high degree of spatial and temporal variability, and connectivity between habitats, none of which can be adequately captured in the commonly employed management methods. We argue that process-oriented ecological models, which consider dynamics across scales and levels of biological organization, are better suited to guide flow regime management. We review how ecological dynamics in streams and rivers are shaped by a combination of the flow regime and internal feedbacks, and proceed to describe ecological modeling tools that have the potential to characterize such dynamics. We conclude with a suggested research agenda to facilitate the inclusion of ecological dynamics into instream flow needs assessments.
Piccaninny Creek in northwestern Australia drains approximately 60 km2 of high relief sandstone and conglomerate terrain. The drainage basin lies within the seasonal tropics and is characterized by ephemeral, high-magnitude flows. Flood flows of 50 to 100 m3s-1 are probably primarily responsible for the spectacular erosional features along the bedrock channel segments of the creek. These erosional features occur in two basic forms: as parallel longitudinal grooves, and as a downstream sequence that begins with shallow linear depressions parallel to flow and ends in a deep, narrow, inner channel. The longitudinal grooves are assumed to result from longitudinal vortices, while the erosional sequences, which closely resemble von Karman vortex streets, are interpreted to result from turbulent vortices shed off irregularities of the channel bed. The longitudinal grooves and the inner-channel sequence alternate with each other and with gravel-floored depositional reaches downstream along the channel. The locations of these three types of channel beds do not correlate with channel-substrate characteristics like rock strength, lithology, or structural variability. The locations do not appear to be related to channel gradient, however. The number and length of channel segments with inner channels are greatest at, and immediately upstream of, a 1 km length of steeper channel interpreted as a knickzone. -Author
The use of both linear and generalized linear mixed‐effects models ( LMM s and GLMM s) has become popular not only in social and medical sciences, but also in biological sciences, especially in the field of ecology and evolution. Information criteria, such as Akaike Information Criterion ( AIC ), are usually presented as model comparison tools for mixed‐effects models.
The presentation of ‘variance explained’ ( R ² ) as a relevant summarizing statistic of mixed‐effects models, however, is rare, even though R ² is routinely reported for linear models ( LM s) and also generalized linear models ( GLM s). R ² has the extremely useful property of providing an absolute value for the goodness‐of‐fit of a model, which cannot be given by the information criteria. As a summary statistic that describes the amount of variance explained, R ² can also be a quantity of biological interest.
One reason for the under‐appreciation of R ² for mixed‐effects models lies in the fact that R ² can be defined in a number of ways. Furthermore, most definitions of R ² for mixed‐effects have theoretical problems (e.g. decreased or negative R ² values in larger models) and/or their use is hindered by practical difficulties (e.g. implementation).
Here, we make a case for the importance of reporting R ² for mixed‐effects models. We first provide the common definitions of R ² for LM s and GLM s and discuss the key problems associated with calculating R ² for mixed‐effects models. We then recommend a general and simple method for calculating two types of R ² (marginal and conditional R ² ) for both LMM s and GLMM s, which are less susceptible to common problems.
This method is illustrated by examples and can be widely employed by researchers in any fields of research, regardless of software packages used for fitting mixed‐effects models. The proposed method has the potential to facilitate the presentation of R ² for a wide range of circumstances.
Locally weighted regression, or loess, is a way of estimating a regression surface through a multivariate smoothing procedure, fitting a function of the independent variables locally and in a moving fashion analogous to how a moving average is computed for a time series. With local fitting we can estimate a much wider class of regression surfaces than with the usual classes of parametric functions, such as polynomials. The goal of this article is to show, through applications, how loess can be used for three purposes: data exploration, diagnostic checking of parametric models, and providing a nonparametric regression surface. Along the way, the following methodology is introduced: (a) a multivariate smoothing procedure that is an extension of univariate locally weighted regression; (b) statistical procedures that are analogous to those used in the least-squares fitting of parametric functions; (c) several graphical methods that are useful tools for understanding loess estimates and checking the assumptions on which the estimation procedure is based; and (d) the M plot, an adaptation of Mallow's Csubp/sub procedure, which provides a graphical portrayal of the trade-off between variance and bias, and which can be used to choose the amount of smoothing.
The nature of the flow in most natural streams is gradually varied rather than uniform. This is particularly true of streams with coarse gravel bed material organized into relatively stable riffle and pool features. In spite of this, there are few applications of the gradually varied flow models (e.g. the Bernoulli equation) to such streams. This paper presents some initial results of a simulation of flow patterns in two riffle-pool reaches, using an open channel flow profile computation method based on an equation defining an energy balance between successive cross-sections separated by an incremental distance.
The riffle‐pool sequence has not been subjected to the same level of intensive research as the meandering planform, although riffles and pools may be a fundamental prerequisite for meandering. The pseudo‐cyclic oscillation of the bed in a riffle‐pool stream suggests the application of a variety of techniques of spatial series analysis, which provide objective measures of riffle wavelength, and suggest processes capable of explaining riffles and pools and their relationship with meanders.
The second‐order autoregressive process is suggested as a stochastic process which models the bed‐profile oscillation. Velocity pulsations associated with large scale turbulent eddies are probably responsible for accretions and erosions which interact with the flow to maintain these perturbations, so that sections lagged by distances of 2πw are positively correlated.
The effect of the riffle‐pool sequence on flow geometry is far more significant than the effects of plan geometry or of downstream variations, which supports the view that this feature is a fundamental aspect of channel morphometry. There is a tendency, however, for curved reaches to exhibit reduced variance of roughness, velocity, and water surface slope, which reinforces the minimization hypothesis. The extreme temporal variation between riffle and pool flow characteristics demands that any classificatory scheme uses scale‐free and stable measures, and a discriminant analysis using hydraulic exponents represents a convenient summary of the field data.
Recognition of the escalating hydrological alteration of rivers on a global scale and resultant environmental degradation, has led to the establishment of the science of environmental flow assessment whereby the quantity and quality of water required for ecosystem conservation and resource protection are determined.
A global review of the present status of environmental flow methodologies revealed the existence of some 207 individual methodologies, recorded for 44 countries within six world regions. These could be differentiated into hydrological, hydraulic rating, habitat simulation and holistic methodologies, with a further two categories representing combination‐type and other approaches.
Although historically, the United States has been at the forefront of the development and application of methodologies for prescribing environmental flows, using 37% of the global pool of techniques, parallel initiatives in other parts of the world have increasingly provided the impetus for significant advances in the field.
Application of methodologies is typically at two or more levels. (1) Reconnaissance‐level initiatives relying on hydrological methodologies are the largest group (30% of the global total), applied in all world regions. Commonly, a modified Tennant method or arbitrary low flow indices is adopted, but efforts to enhance the ecological relevance and transferability of techniques across different regions and river types are underway. (2) At more comprehensive scales of assessment, two avenues of application of methodologies exist. In developed countries of the northern hemisphere, particularly, the instream flow incremental methodology (IFIM) or other similarly structured approaches are used. As a group, these methodologies are the second most widely applied worldwide, with emphasis on complex, hydrodynamic habitat modelling. The establishment of holistic methodologies as 8% of the global total within a decade, marks an alternative route by which environmental flow assessment has advanced. Such methodologies, several of which are scenario‐based, address the flow requirements of the entire riverine ecosystem, based on explicit links between changes in flow regime and the consequences for the biophysical environment. Recent advancements include the consideration of ecosystem‐dependent livelihoods and a benchmarking process suitable for evaluating alternative water resource developments at basin scale, in relatively poorly known systems. Although centred in Australia and South Africa, holistic methodologies have stimulated considerable interest elsewhere. They may be especially appropriate in developing world regions, where environmental flow research is in its infancy and water allocations for ecosystems must, for the time being at least, be based on scant data, best professional judgement and risk assessment. Copyright © 2003 John Wiley & Sons, Ltd.