Article

Seeing the Forest and the Trees: Wood in Stream Restoration in the Colorado Front Range, United States

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Abstract

This chapter reviews the processes that influence wood dynamics in mountain streams: recruitment, storage, and transport. These processes are incorporated within the numerical value of wood load or volume of wood per area of stream. Spatial variations in wood load within a stream can be substantial as a result of spatial and temporal variations in the processes that influence wood dynamics. Such variation makes it challenging to define either the historical range of variation in wood loads or targets for restoring in-stream wood loads. Considering wood dynamics in the context of geomorphic setting, as delineated in process domains, helps to constrain the relative importance of individual processes influencing wood dynamics, as well as spatial variations in wood load. Taking the Colorado Front Range as a case study, information from reference sites, regional data, and mechanistic models is used to illustrate how partial information from multiple sources can be assembled to estimate historical range of variation in wood loads and to develop targets for restoration of in-stream wood. Although the details will vary between regions, this approach should be applicable to any mountain stream network.

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... Numerous studies now document the important influences exerted by instream wood on hydraulics (MacFarlane and Wohl, 2003), storage of sediment and organic matter (Faustini and Jones, 2003;Beckman and Wohl, 2014a), channel geometry (Montgomery et al., 1995), river and floodplain habitat (Fausch and Northcote, 1992;Schowalter et al., 1998;Benke, 2001;Braccia and Batzer, 2001), and channel-floodplain connectivity O'Connor et al., 2003;Sear et al., 2010;Wohl, 2011;Collins et al., 2012) in mountain streams. Logjams, defined as three or more pieces in contact with each other, exert a particularly strong influence on bed and bank scour, sediment storage, and riverine habitat (Keller and Swanson, 1979;Bilby and Likens, 1980;Richmond and Fausch, 1995;Beckman and Wohl, 2014b). ...
... These studies indicate that valley geometry exerts a stronger influence on wood loads than forest stand age when considered at channel lengths of several kilometers (Wohl and Cadol, 2011), but forest age significantly influences wood loads within stream reaches of similar geometry (Beckman and Wohl, 2014b). Old-growth forests, which require 200 years to develop (Veblen, 1986), are associated with streams containing greater wood loads, larger and more closely spaced logjams (Wohl, 2011), and greater instream storage of organic carbon in the form of logs and coarse particulate organic matter (Beckman and Wohl, 2014a) than streams flowing through younger forests. Individual wood pieces are quite mobile on an interannual basis, although pieces within jams are less mobile (Wohl and Goode, 2008). ...
... Laterally confined valleys (ratio of valley bottom width to bankfull channel width ≤ 2) limit floodplain area and contribute to greater flow depth and velocity, thus limiting wood storage in the channel (Braudrick and Grant, 2000;Bocchiola et al., 2008;Wohl and Cadol, 2011) and across the floodplain. Unconfined valleys (Table 1) have the potential for greater floodplain width and greater wood retention within channels and on the floodplain, in part because overbank flows and the presence of secondary channels limit flow depth and velocity within the main channel, thus limiting transport capacity for wood (Wohl, 2011). ...
... Windthrow can be a dominant mechanism of LW recruitment even when it is spread over decades rather than concentrated in a single catastrophic event (e.g., May and Gresswell, 2003a), but catastrophic windstorms in the form of tornadoes (Peterson, 2007), hurricanes or cyclones (Hilton et al., 2008;Phillips and Park, 2009), and microbursts (Wohl, 2013c) can favor formation of wood rafts and numerous smaller jams by introducing enormous quantities of LW nearly instantaneously. Stand-killing fires can result in increased LW recruitment for a period of years to decades (Bendix and Cowell, 2010;King et al., 2013), followed by a period of decreased recruitment as trees regrow (Jones and Daniels, 2008;Wohl, 2011a). Mass mortality can also be associated with ice storms (Hooper et al., 2001;Kraft and Warren, 2003) in cold climates and with volcanic eruptions in volcanically active regions (e.g., Lisle, 1995;Nakamura and Swanson, 2003). ...
... (1) and (2) can be used to predict at least relative importance of different variables in the equations or the relative magnitude of either S or individual variables in Eqs. (1) and (2) within a river or river network ( Fig. 6; Lawrence et al., 2013), although the latter has been attempted for relatively few rivers (Marcus et al., 2002(Marcus et al., , 2011Wohl, 2011a). ...
... As with spatial variations in LW load, field studies and numerical simulations over diverse time spans suggest that LW load within a river segment can vary substantially from year to year (Wohl and Goode, 2008;Curran, 2010;Marcus et al., 2011;Dixon and Sear, 2014) and over periods of decades to centuries (Bragg, 2000;Lancaster et al., 2003;Wohl, 2011a). Even in river segments for which LW load remains relatively constant over the time period under consideration, substantial exchange can occur because individual wood pieces are highly mobile and constantly being removed and replaced by other pieces (Piégay et al., 1998;Wohl and Goode, 2008;Curran, 2010). ...
Article
Nearly 50 years of research focused on large wood (LW) in rivers provide a basis for understanding how wood enters rivers; how wood decays, breaks, and is transported downstream; and how at least temporarily stable wood influences channel geometry, fluxes of water, sediment, and organic matter, and the abundance and diversity of aquatic and riparian organisms. Field-based studies have led to qualitative conceptual models and to numerical stimulations of river processes involving wood. Numerous important gaps remain, however, in our understanding of wood dynamics. The majority of research on wood in rivers focuses on small- to medium-sized rivers, defined using the ratio of wood piece size to channel width as channels narrower than the locally typical wood-piece length (small) and slightly narrower than the longer wood pieces present (medium). Although diverse geographic regions and biomes are represented by one or a few studies in each region, the majority of research comes from perennial rivers draining temperate conifer forests. Regional syntheses most commonly focus on the Pacific Northwest region of North America where most of these studies originate. Consequently, significant gaps in our understanding include lack of knowledge of wood-related processes in large rivers, dryland rivers, and rivers of the high and low latitudes. Using a wood budget as an organizing framework, this paper identifies other gaps related to wood recruitment, transport, storage, and how beavers influence LW dynamics. With respect to wood recruitment, we lack information on the relative importance of mass tree mortality and transport of buried or surficial downed wood from the floodplain into the channel in diverse settings. Knowledge gaps related to wood transport include transport distances of LW and thresholds for LW mobility in small to medium rivers. With respect to wood storage, we have limited data on longitudinal trends in LW loads within unaltered large and great rivers and on fluctuations in LW load over time intervals greater than a few years. Other knowledge gaps relate to physical and ecological effects of wood, including the magnitude of flow resistance caused by LW; patterns of wood-related sediment storage for diverse river sizes and channel geometry; quantification of channel-floodplain-LW interactions; and potential threshold effects of LW in relation to physical processes and biotic communities. Finally, knowledge gaps are related to management of large wood and river corridors, including understanding the consequences of enormous historical reductions in LW load in rivers through the forested portions of the temperate zone; and how to effectively reintroduce and manage existing LW in river corridors, which includes enhancing public understanding of the importance of LW. Addressing these knowledge gaps requires more case studies from diverse rivers, as well as more syntheses and metadata analyses.
... Similar diversity exists within temperate coniferous forests. These forests include the relatively small species of semiarid forests in the U.S. Southern Rockies (Pinus, Abies, Pseudotsuga, Picea spp.; mostly b30 m tall and 10 to 50 cm diameter) (Wohl, 2011b(Wohl, , 2011c and the much larger trees of the humid temperate rainforest in the northwestern U.S. and southwestern Canada (Tsuga, Picea, Pseudotsuga, Abies spp.; 50-100 m tall and 60-600 cm diameter) (Hassan et al., 2005). ...
... Multithread channels occur only in wide valley segments with forests that are old-growth (>200 years in age) or have experienced only natural disturbances such as wildfire. Slow rates of wood decay in the dry climate (Wohl, 2011b), limited wood transport relative to steep, narrow valley segments , and high levels of instream debris roughness (Braudrick and Grant, 2001) caused by protruding clasts, ramped wood pieces and logjams, create a wood-rich channel able to trap wood in transport and maintain stable, persistent jams. Where instream wood is anthropogenically removed, the wood-poor channel is less able to trap and retain subsequently recruited wood, leading to a single-thread channel planform with lower channel-floodplain connectivity and floodplain storage of sediment and organic matter (Wohl et al., 2012b). ...
... Subsequent research may also reveal thresholds of floodplain-wood interaction with respect to temporal or spatial scales, so that such interactions are more appropriately considered as distinct populations rather than as a continuum. (1) England (12-25 km 2 (Gregory et al., 1985;Jeffries et al., 2003;Sear et al., 2010)); South Carolina (130-270 km 2 ); Germany (John and Klein, 2004); Canada (72 km 2 (Green and Westbrook, 2009)); Ireland (6 km 2 (Harwood and Brown, 1993)) (2) Colorado (20 km 2 (Wohl, 2011a(Wohl, , 2011b(Wohl, , 2011c; 100 km 2 ); Wyoming (30 km 2 (Oswald and Wohl, 2008)) (3) Pacific Northwest (1200-1900 km 2 (Collins et al., 2012)); Midwestern US (1300-1700 km 2 (Guyette et al., 2008)); Australia (200-650 km 2 (Brooks et al., 2003)); California (3000 km 2 (Florsheim and Mount, 2002)) (4) Willamette River, Oregon (29,000 km 2 (Sedell and Froggatt, 1984)); Red and Atchafalaya Rivers, Louisiana (236,000 km 2 (Triska, 1984; Phillips and Park, 2009)). ...
Article
Instream wood affects floodplain form and process by altering flow resistance, conveyance and channel–floodplain connectivity, and influencing lateral and vertical accretion of floodplains. Instream wood reflects floodplain form and process as the floodplain influences wood recruitment via bank erosion and overbank flow, and wood transport and storage via floodplain effects on stage-discharge relations and flow resistance. Examining turnover times for instream wood at the reach scale in the context of a wood budget, floodplain characteristics influence fluvial transport and dynamics (wood recruitment), valley geometry (wood transport and storage), and hydraulics and river biota (wood decay and breakage).
... Regarding the bridge and flow characteristics, De Cicco et al. (2015 showed that the shape of a bridge pier and the flow froude number can change the probability of impact. Moreover, about the bridges without pier, Schmocker & Hager (2010, 2011Schmocker & Weitbrecht (2013) studied the effect of free board, flow Froude number and the bridge shape on bridge clogging. However, there is the remaining question one step before logjam, about the parameters driving impact between wood and bridge pier. ...
... Therefore, it is important to know different scales for studying the amount of wood and the function of wood at different scales. A single piece of large wood in a large channel, for example, will likely have only local effects, whereas a large jam that spans a channel can influence processes and forms along an entire stream reach (Gurnell et al., 2002;Wohl, 2011). Therefore, the effects of variation of pieces of wood along a river can be studied at large spatial scale (10 3 − 10 5 m), known as the segment scale (Fausch et al., 2002), at medium spatial scale (10 1 − 10 3 m) known as the reach scale and at low spatial scale (10 0 − 10 1 m) known as the river unit scale (Wohl et al., 2016). ...
... In addition, in the field, there is the transition from single wood pieces to multiple wood pieces and to log jams. In this context, some works are quite promising to understand the combined effect of flow, bridge and wood characteristics (Schalko, 2017;Schalko et al., 2019;De Cicco et al., 2015Schmocker & Hager, 2010, 2011Schmocker & Weitbrecht, 2013) which still constitute however a challenging question for future works. ...
Thesis
Driftwood is an integral part of river corridors where it plays an important role both in river ecology and morphology. During the last decades, the amount of large wood transported in some of the European rivers has increased, notably due to modifications in the human pressure and management of riparian forest buffers along rivers. This causes an increase of potential hazards for hydraulic structures and urban areas. In this context, the aim of this thesis is to study the driftwood dynamics in rivers in order to provide elements for hazard assessment. This is carried out in two ways: (i) using in-situ streamside videography to measure the amount of wood transported by the river during floods and (ii) analyzing the dynamics of individual pieces of wood both on the field and in a well-controlled experimental environment combined with theoretical models. The present work provides several scientific and technical contributions. First by studying the link between wood discharge and flood characteristics, such as flood magnitude, hydrograph and inter-flood time, we consolidate and extend the present knowledge about the link between flow and wood discharges. Second, our studies show that when a piece of wood is recruited into the river, it is accelerated on a limited distance, which scales as the wood length in the flow direction. Once the wood piece reaches the flow velocity, it behaves as a flow tracer. In terms of technical contributions, by comparing the video monitoring technique in two different sites, we provide some recommendations that are useful for practitioners for installing new monitoring stations. This work will be part of the driftwood hazard and risk assessments, for which accurate wood dynamics quantities are required.
... Large wood (LW) in steams draining forested watersheds is recognized as having significant geomorphic and ecological roles (Gurnell et al., 2002;Montgomery et al., 2003;Hassan et al., 2005). Wood loads are often highly variable between stream reaches, being influenced by position with watershed, adjacent forest type, disturbance history, and relative mobility (Benda and Sias, 2003;Wohl, 2011;Wohl and Cadol, 2011). Wood loads are often greater in small, low-order, forested channelsboth in terms of number of pieces and size of individual logs (Keller and Swanson, 1979;Wohl et al., 2006;Wohl and Jaeger, 2009;Wohl, 2011). ...
... Wood loads are often highly variable between stream reaches, being influenced by position with watershed, adjacent forest type, disturbance history, and relative mobility (Benda and Sias, 2003;Wohl, 2011;Wohl and Cadol, 2011). Wood loads are often greater in small, low-order, forested channelsboth in terms of number of pieces and size of individual logs (Keller and Swanson, 1979;Wohl et al., 2006;Wohl and Jaeger, 2009;Wohl, 2011). Individual trunks and limbs, some similar in size to the dimensions of the channel, can exert considerable influence over flow hydraulics, sediment transport and storage in these narrow systems (Bilby and Ward, 1989;Bilby and Bisson, 1998). ...
Article
Large fallen wood can have a significant impact on channel form and process in forested mountain streams. In this study, four small channels on the Fraser Experimental Forest near Fraser, Colorado, USA, were surveyed for channel geometries and large wood loading, including the size, source, and characteristics of individual pieces. The study is part of a larger effort to understand the impact of mountain pine beetle infestation on a suite of watershed properties. Here, we present baseline data collected at the onset of widespread tree mortality. Channels range from 1.5 to 2 m in width, with slopes ranging from 3 to > 10%. Median (D50) streambed particle sizes range from gravel to very coarse gravel. Channels are characterized as cascade, step-pool, and plane bed over varying scales. Large wood loads ranged from about 0.4 to 1.0 piece per meter length of channel, which is comparable to values reported for other Colorado sites. Much of the wood showed indications of being in place for long periods of time (decayed/rotten, broken into ramps, and partially buried in beds and banks). Nearly all surveyed reaches contained steps formed from small boulders and/or logs. Significant portions of the elevation drop in some of the reaches were made up by log steps, though the percentages varied (0 to 60%). Individual log steps trap a portion of the coarse sediment moved as bedload, forming wedge-shaped accumulations upstream of the logs. The particle size distributions for measured bedload and step accumulations largely overlapped, but more so for the coarse ends of the distributions, suggesting a trapping inefficiency for the finer component of bedload. Estimates of the total volume of sediment stored behind log steps were approximately an order of magnitude greater than the mean sediment volume exported on an annual basis, as determined from measured accumulations in weir ponds. The particle size distribution of sediment in the ponds – ranging from sand to medium gravel – is considerably finer than sediment stored in steps. The series of comparisons between storage volumes, particle size distributions, and sediment export suggests that log steps effectively trap coarse sediment moved in these small streams and act as a series of check dams that inhibit channel erosion, but may be less effective at trapping finer sediment (sand and small gravel).
... Because of the wide riparian area, high stem density and larger diameter trees, this substantial pool of stored carbon continues to accumulate over time. Large diameter trees decay very slowly: complete decay for the dominant conifer species in this environment requires 300-900 years 28 . Fourth, in unconfined valley segments in the subalpine forest zone at higher elevations of the study area, annual peak flows result from snowmelt. ...
Article
Full-text available
Published research emphasizes rapid downstream export of terrestrial carbon from mountainous headwater rivers, but little work focuses on mechanisms that create carbon storage along these rivers, or on the volume of carbon storage. Here we estimate organic carbon stored in diverse valley types of headwater rivers in Rocky Mountain National Park, CO, USA. We show that low-gradient, broad valley bottoms with old-growth forest or active beaver colonies store the great majority of above- and below-ground carbon. These laterally unconfined valley segments constitute <25% of total river length, but store ∼75% of the carbon. Floodplain sediment and coarse wood dominate carbon storage. Our estimates of riverine carbon storage represent a previously undocumented but important carbon sink. Our results indicate that: not all mountainous rivers rapidly export carbon; not all valley segments are equally important in carbon storage; and historical changes in riverine complexity have likely reduced carbon storage.
... Consistent with prior assessments (e.g., Booth, 2005;Bernhardt and Palmer, 2007;Palmer et al., 2010), this research points to the importance of managing/restoring a more natural flow and disturbance regime, if managers desire systemwide geomorphic and biotic recovery. It also points to the benefits and importance of simple restoration activities, such as reintroduction of LWD (Abbe and Brooks, 2011;Wohl, 2011), beavers (Pollock et al., 2007), and native (or slightly coarsened) bed material in an effort to resupply otherwise denuded reaches (sensu Kondolf, 1997). ...
Article
Stream systems naturally respond to watershed land use dynamics, particularly in urban developments with unmanaged impervious areas. Such urban-provoked alterations to channel morphology cause water quality impairments, have adverse effects on aquatic biota, and pose risks to adjacent public infrastructure. Over the past four years we have collected detailed hydrogeomorphic data at 40 unique stream locations throughout northern Kentucky, with at least two rounds of annually repeated surveys at 70% of the sites and three rounds of surveys at 50% of the sites. Analysis of this time-series data encompassed measured rates of instability across three distinct dimensions including (1) channel cross sections, (2) longitudinal profiles, and (3) bed material particle composition. Regression analyses between geomorphic change and 2011 watershed imperviousness indicated stream cross sections in urban/suburban watersheds tend to be getting larger—their overall shape is both deepening and widening. Additionally, stream riffle lengths are shrinking and their pools are becoming both longer and deeper; and finally, their bed material composition is coarsening, particularly in streams in the early stages of watershed development. By documenting fluvial geomorphologic dynamics in such detail, this study highlights the process by which unmitigated urbanization homogenizes stream habitat and degrades aquatic ecosystems. This improved, process-based understanding of the urban-induced channel response sequence has clear implications to both stormwater management and stream/ecosystem restoration, particularly in stream systems where headcut migration is a primary driver of channel instability.
... The younger forest streams included in the dataset from Rocky Mountain National Park did not experience substantial historical wood removal. The idea of alternative stable states reflects the likelihood that, in unmanaged streams, wood load probably does not drop below some minimum level because of low rates of decay in this semiarid region (Wohl, 2011a). Wood in many managed streams, in contrast, was completely removed during 19th-century timber harvest, placer mining, and log floating (Wohl, 2001), and wood continues to be removed in some managed streams to prevent damage to infrastructure or facilitate recreational boating. ...
Article
Full-text available
Channel-spanning logjams completely span the active channel and create longitudinal discontinuities of the water surface and stream bed across at least two-thirds of the channel width. These jams disproportionately affect channel process and form relative to smaller jams that do not span the entire channel width. We analyze a spatially extensive dataset of 859 channel-spanning jams distributed along 124 km of 16 distinct rivers on the eastern side of Rocky Mountain National Park, Colorado, USA, with drainage areas spanning 2.6 to 258 km2 and diverse valley geometry and forest stand age. We categorized valley geometry in terms of lateral confinement (confined, partly confined, or unconfined), which correlates with gradient. Jams exhibit substantial downstream variability in spacing at channel lengths of 102–103 m. The number of jams within a reach is explained by a statistical model that includes drainage area, valley type (lateral confinement), and channel width. Longitudinal spacing of jams drops substantially at drainage areas greater than ~20 km2, although jam spacing exhibits tremendous variability at smaller drainage areas. We interpret the lack of jams at larger drainage areas to reflect increasing transport capacity for instream wood. We interpret the variability in jam spacing at small drainage areas to reflect local controls of valley geometry and associated wood recruitment and fluvial transport capacity. Our results suggest that management of instream wood designed to facilitate the formation of channel-spanning jams can be most effectively focused on smaller drainage areas where these jams are most abundant in the absence of management that alters instream wood recruitment or retention. Unmanaged streams in the study region with drainage area
... Fig. 8 conceptually illustrates LW loads in the channel and floodplain with time since disturbance in the form of a wildfire. This model is based on wood loads changing over an interval of 100 years (Wohl, 2011a); a bimodal input of wood to the floodplain forest with an initial input from fire-induced mortality and a second similarly-sized peak after 30 years when the snags fall (Bragg, 2000); a bimodal input of wood to the channel with a small peak right after the fire and a large peak 30 years after (Bragg, 2000); and 255 years for a forest to reach steady state after a major, stand-killing disturbance (Stout et al., 2018). Because of the lack of data for California forests and the fact that wood loads can differ greatly depending on the region, species composition, and characteristics of the river corridor, no values or units are given on the y-axis in Fig. 8. ...
Article
We quantified floodplain large wood load (m³ wood/ha) and spatial distribution on the Upper Merced River in Yosemite National Park, California, USA. The upstream portion of the study area includes a recently burned section of the Merced River corridor and the downstream portion contains floodplain with undisturbed forest, facilitating investigation of the effects of wildfire on floodplain large wood. We used measurements of wood load and spatial distribution to test hypotheses regarding floodplain wood dynamics in the channel versus the floodplain and in burned versus unburned portions of the study area. The median wood load on the Merced River floodplain, as measured along numerous transects, is 259 m³/ha overall, with non-significant differences between burned (median 196 m³/ha) and unburned (median 277 m³/ha) portions of the floodplain. We found that jams can occur across the entire width of the floodplain. Burned wood pieces are present throughout the study area in the channel but are largely absent from unburned portions of the floodplain, despite the occurrence of overbank flows since the wildfire. A greater proportion of large wood is within logjams in burned portions of the floodplain. We infer that wood recruited to the channel via bank erosion moves readily downstream within the channel, whereas wood moving from the channel onto the floodplain concentrates near the margin of the main channel or within secondary channels and depressions on the floodplain, leading to the formation of long, narrow logjams.
... comm., July 2015) or can break apart as individual basal wood pieces break or are dislodged (Wohl and Goode, 2008). Although factors such as proportion of channel cross-sectional area obstructed by the jam, potential for overbank flow and dissipation of hydraulic force (Wohl, 2011), porosity of the jam, and cause of jam formation likely influence the relative stability of individual jams, little is known of the relative importance of diverse processes by which jams become mobile. ...
Article
Full-text available
To effectively manage wood in rivers, we need a better understanding of wood mobility within river networks. Here, we review primarily field-based (and some numerical) studies of wood transport. We distinguish small, medium, large, and great rivers based on wood piece dimensions relative to channel and flow dimensions and dominant controls on wood transport. We suggest further identification and designation of wood transport regimes as a useful way to characterize spatial-temporal network heterogeneity and to conceptualize the primary controls on wood mobility in diverse river segments. We draw analogies between wood and bedload transport, including distinguishing Eulerian and Lagrangian approaches, exploring transport capacity, and quantifying thresholds of wood mobility. We identify mobility envelopes for remobilization of wood with relation to increasing peak discharges, stream size, and dimensionless log lengths. Wood transport in natural channels exhibits high spatial and temporal variability, with discontinuities along the channel network at bankfull flow and when log lengths equal channel widths. Although median mobilization rates increase with increasing channel size, maximum mobilization rates are greatest in medium-sized channels. Most wood is transported during relatively infrequent high flows, but flows under bankfull can transport up to 30% of stored wood. We use conceptual models of dynamic equilibrium of wood in storage and of spiralling wood transport paths through drainage networks, as well as a metaphor of traffic on a road, to explore discontinuous wood movement through a river network. The primary limitations to describing wood transport are inappropriate time scales of observation and lack of sufficient data on mobility from diverse rivers. Improving models of wood flux requires better characterization of average step lengths within the lifetime travel path of a piece of wood. We suggest that future studies focus on: (i) continuous or high-frequency monitoring of wood mobility; (ii) monitoring changes in wood storage; (iii) using wood characteristics to fingerprint wood sources; (iv) quantifying volumes of wood buried within river corridors; (v) obtaining existing or new data from unconventional sources, such as citizen science initiatives, and (vi) creating online interactive data platforms to facilitate data synthesis.
... By having at least a foundational understanding of the historic landscape, restoration practitioners may more readily incorporate actions (both large and small) that are restorative of ecosystem processes into conventional habitat restoration programs. For example, the natural flow (Poff et al. 1997), sediment (Wohl et al. 2015), and wood (Wohl 2011) regimes are likely to be essential components of ecosystem restoration. By focusing on restoring ecosystem processes as opposed to forms (Beechie et al. 2010), actions that one might not conventionally classify as restoration may actually present the greatest restorative potential for the lowest cost. ...
Article
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Despite large advances in the state of the science of stream ecology and river mechanics, the practitioner-driven field of stream restoration remains plagued by narrowly focused projects that sometimes even fail to improve aquatic habitat or geomorphic stability—two nearly universal project goals. The intent of this article is to provide an accessible framework that bridges that gap between the current state of practice and a more geomorphically robust and ecologically holistic foundation that also provides better accounting of socioeconomic factors in support of more sustainable stream restoration outcomes. It points to several more comprehensive design references and presents some simple strategies that could be used to protect against common failure mechanisms of ubiquitous design approaches (i.e., regional curves, Rosgen planform, and grade control). From the simple structure design to the watershed-scale restoration program, this may be a first step toward a more geomorphically principled, ecologically holistic, and socioeconomically sustainable field.
... The potential magnitude of error from inbuilt ages of old wood are dependent upon decomposition rates of specific species in a given climate, which regulate the turnover time of dead wood. Turnover time of softwood tree species in our study area are on the order of1 50-900 years 43 , meaning that radiocarbon ages may overestimate the timing of a fire by up to 900 of years. The potential for inbuilt ages that could produce errors in radiocarbon ages on the order of 500 years 31 , creates less separation between high and low-elevation sites. ...
Article
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High-elevation mountain streams are commonly viewed as erosive environments, but they can retain sediment along river corridors for thousands of years. In 2013, an extreme flood evacuated floodplain sediment in the Colorado Front Range, USA. We use fifty-two ¹⁴C ages collected along four streams prior to the flood to estimate mean residence time of floodplain sediment. Here we show that mountain streams above the elevation of the Pleistocene terminal moraine retain floodplain sediment for longer durations than those at lower elevation, but that wildfires may decrease floodplain sediment residence time at high elevations. Comparison of field sites and differencing of pre- and post-flood lidar show that valley confinement is a significant predictor of residence time, sediment flux, and floodplains disturbed by the 2013 flood. Elevational trends in floodplain disturbance regime also reflect differences in forest type, precipitation pattern, and wildfire regime, which are expected to shift under a changing climate.
Technical Report
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Stream restoration practitioners and researchers have devoted a great deal of effort in recent decades to developing extensive guidance for stream restoration. The available resources are diverse, reflecting the wide ranging approaches used and expertise required to develop effective stream restoration projects. To help practitioners in sorting through the extensive amount of available information, this technical note has been developed to provide a guide to the available guidance. The document structure is primarily a series of short literature reviews followed by a hyperlinked reference list for readers to find more information on each topic. The primary topics incorporated into this guidance include general methods, an overview of stream processes and restoration, case studies, data compilation, preliminary assessments, and field data collection. Analysis methods and tools, and planning and design guidance for specific restoration features are also provided. This technical note is a bibliographic repository of information available to assist professionals with the process of planning, analyzing, and designing stream restoration projects. It is updated periodically.
Conference Paper
Background/Question/Methods Headwater streams are known to be important in the global carbon cycle, yet few studies have investigated carbon (C) storage along mountainous stream-riparian corridors. To better understand the magnitude and spatial distribution of C stocks along headwater fluvial networks, we estimated above- and below-ground C pools in 100-m-long reaches in six different valley types in Rocky Mountain National Park, Colorado. Valley types were distinguished based on downstream gradient and valley-bottom width relative to active channel width (valley geometry) and the presence of biotic drivers, notably beaver dams and channel-spanning logjams associated with old-growth forest that contribute to development of multi-thread channel patterns. Three valley types were associated with old-growth riparian forest: laterally confined; laterally unconfined with multi-thread channels; and laterally unconfined with single-thread channels. A laterally confined valley type occurring in younger forest was also sampled. Two valley types were distinguished as low-gradient, laterally unconfined stream segments: beaver-meadow complexes with multi-thread channels and mixed riparian vegetation (conifer, willow, and herbaceous); and abandoned beaver-meadow complexes with single-thread channels, dominated by herbaceous vegetation. From field measurements, we estimated carbon stored in the riparian vegetation (live and dead), forest floor, instream and floodplain large wood, and floodplain sediment for each 100-m-long valley segment. Results/Conclusions In the forested reaches, total C pools that summed vegetation biomass (live and dead), forest floor, and downed large wood ranged from 190 Mg/ha to 450 Mg/ha. Highest biomass C stocks were found in the laterally unconfined valley segment with multi-thread channels and dominated by riparian old-growth forest. C pools for forest floor components and floodplain large wood were consistently higher than upland forests with similar species composition. Belowground C pools were highest in the beaver-meadow complexes (approximately 1000 to 3080 Mg/ha), reflecting the substantial storage potential of wide, low-gradient valleys and the high C content of floodplain sediments. Results indicate that large quantities of terrestrial carbon are concentrated in predictable segments of the stream-riparian corridor. Multi-thread channel segments influenced by beaver or logjams cover less than 25 percent of the total length of stream networks in the study area, yet they may account for more than three-quarters of the stored C. Historical removal of beavers and old-growth forests has likely resulted in a reduction of total ecosystem C pools and potential for C storage along headwater river networks in the Colorado Front Range.
Article
Among the dominant twentieth century conceptual models of geomorphology that rely on insights resulting from field-based research are Stanley A. Schumm's formulations of complex response, intrinsic thresholds, river metamorphosis, and spatial zonation of drainage basins. Schumm's research focused primarily on finer grained alluvial channels in lower relief environments. As a result of his work, most investigators now approach river process and form within a framework based on three fundamental assumptions. First, channel changes are abrupt and driven by crossing external and internal thresholds. Second, channel change is likely to be asynchronous, resulting in different portions of a river or a river network behaving in very different manners at a given point in time. Third, different portions of a river network are dominated by distinct disturbance regimes and resulting suites of geomorphic processes and forms. More recent research on resistant-boundary mountain channels illustrates how field evidence demonstrates that river process and form are inherently nonlinear, with spatial and temporal thresholds. Multithread channels can form within unconfined valley segments in mountainous river networks of the Colorado Front Range, but only in the presence of biotic drivers in the form of (i) old-growth forest that facilitates the formation of closely spaced, channel-spanning logjams or (ii) beavers that build dams. Thresholds of channel and valley geometry govern the occurrence and persistence of jams and dams, and these channel obstructions initiate specific nonlinear responses in valley and channel form. When the biotic drivers are removed, river metamorphosis occurs. Alluvial channels, which are typically regarded as being relatively responsive to changes in water and sediment yield and substrate composition, and channels with more resistant boundaries that typically respond to external changes over longer timespans exhibit nonlinear complex behavior. In both cases, the nonlinear behavior of rivers with numerous interdependent variables, multiple internal and external thresholds, and complex responses would be difficult to conceptualize and quantify in the absence of extensive field data. One of the management implications of complex, nonlinear behavior is that a one-size-fits-all approach to managing rivers is inadequate. Field research, initially focused on understanding specific examples of river process and form, revealed underlying patterns that give rise to conceptual models broadly applicable within fluvial geomorphology.
Article
We measured wood piece characteristics and particulate organic matter (POM) in stored sediments in 30 channel-spanning logjams along headwater streams in the Colorado Front Range, USA. Logjams are on streams flowing through old-growth (> 200 years), disturbed (< 200 years, natural disturbance), or altered (< 200 years, logged) subalpine conifer forest. We examined how channel-spanning logjams influence riverine carbon storage (measured as the total volatile carbon fraction of stored sediment and instream wood). Details of carbon storage associated with logjams reflect age and disturbance history of the adjacent riparian forest. A majority of the carbon within jams is stored as wood. Wood volume is significantly larger in old-growth and disturbed reaches than in altered reaches. Carbon storage also differs in relation to forest characteristics. Sediment from old-growth streams has significantly higher carbon content than altered streams. Volume of carbon stored in jam sediment correlates with jam wood volume in old-growth and disturbed forests, but not in altered forests. Forest stand age and wood volume within a jam explain 43% of the variation of carbon stored in jam sediment. First-order estimates of the amount of carbon stored within a stream reach show an order of magnitude difference between disturbed and altered reaches. Our first-order estimates of reach-scale riverine carbon storage suggest that the carbon per hectare stored in streams is on the same order of magnitude as the carbon stored as dead biomass in terrestrial subalpine forests of the region. Of particular importance, old-growth forest correlates with more carbon storage in rivers.
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The natural wood regime forms the third leg of a tripod of physical processes that supports river science and management, along with the natural flow and sediment regimes. The wood regime consists of wood recruitment, transport, and storage in river corridors. Each of these components can be characterized in terms of magnitude, frequency, rate, timing, duration, and mode. We distinguish the natural wood regime, which occurs where human activities do not significantly alter the wood regime, and a target wood regime, in which management emphasizes wood recruitment, transport, and storage that balance desired geomorphic and ecological characteristics with mitigation of wood-related hazards. Wood regimes vary across space and through time but can be inferred and quantified via direct measurements, reference sites, historical information, and numerical modeling. Classifying wood regimes with respect to wood process domains and quantifying the wood budget are valuable tools for assessing and managing rivers
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provides insight into both the longer-term range of riv-erine forms and processes under a similar hydroclimatic regime and the underlying landscape template for resto-ration. Along the continuum of restoration from purely process-based modeling to restoring to a reference condi-tion, analysis of the historical range of variability of channel planform bridges these extremes by to reconstruct the past without requiring all biotic and physical processes and their interactions to be fully understood, a requirement that can be very difficult to meet in many systems. Biotic influences on stream planform Stream planform is typically characterized as a single-thread channel or as a multithread channel, with secondary channels that branch and rejoin downstream. Single-thread channels are further distinguished as straight or meandering on the basis of sinuosity, which is the ratio of a channel's length to its straight-line distance; a meandering channel has a sinuosity greater than 1.5. Multithread channels can be dif-ferentiated as braided channels, in which flow is separated by bars within a defined channel, or as anabranching channels, in which individual channels are separated by vegetated or otherwise stable bars and islands that are broad and long relative to the width of the channels and that divide flows at P rocess-based restoration of fluvial systems is intended
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We draw on published studies of floodplain organic carbon storage, wildfire‐related effects on floodplains in temperate and high latitudes, and case studies to propose a conceptual model of the effects of wildfire on floodplain organic carbon storage in relation to climate and valley geometry. Soil organic carbon typically constitutes the largest carbon stock in floodplains in fire‐prone regions, although downed wood can contain significant organic carbon. We focus on the influence of wildfire on soil organic carbon and downed wood as opposed to standing vegetation to emphasize the geomorphic influences resulting from wildfire on floodplain organic carbon stocks. The net effect of wildfire varies depending on site‐specific characteristics including climate and valley geometry. Wildfire is likely to reduce carbon stock in steep, confined valley segments because increased water and sediment yields following fire create net floodplain erosion. The net effect of fire in partly confined valleys depends on site‐specific interactions among floodplain aggradation and erosion, and, in high‐latitude regions, permafrost degradation. In unconfined valleys in temperate latitudes, wildfire is likely to slightly increase floodplain organic carbon stock as a result of floodplain aggradation and wood deposition. In unconfined valleys in high latitudes underlain by permafrost, wildfire is likely in the short‐term to significantly decrease floodplain organic carbon via permafrost degradation and reduced organic‐layer thickness. Permafrost degradation reduces floodplain erosional resistance, leading to enhanced stream bank erosion and greater carbon fluxes into channels. The implications of warming climate and increased wildfires for floodplain organic carbon stock thus vary. Increasing wildfire extent, frequency, and severity may result in significant redistribution of organic carbon from floodplains to the atmosphere via combustion in all environments examined here, as well as redistribution from upper to lower portions of watersheds in the temperate zone and from floodplains to the oceans via riverine transport in the high‐latitudes.
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Historical range of variability (HRV) describes the range of temporal and spatial variations in river variables such as flow regime or channel planform prior to intensive human alteration of the ecosystem. In mountainous river networks, HRV is most usefully applied to spatially differentiated geomorphic process domains with distinctive form and process. Using the Colorado Front Range as an example, three examples of how knowledge of HRV can assist river management and restoration are discussed. The examples involve instream wood load and channel morphology, beaver colonies and valley-bottom form and process, and flow thresholds in regulated rivers. The question of what a river should look like – that is, what range of process and form the river included prior to intensive human alteration – can be addressed by (i) placing the river within a process domain, (ii) establishing correlations between form parameters that can be remotely sensed and reach-scale process and form, so that the spatial extent, connectivity, and rarity of process domains within a river network or a region can be quickly assessed, (iii) inferring characteristics of the river prior to intensive alteration by documenting characteristics of the least altered reference rivers and by using proxy indicators of pre-alteration conditions, and (iv) establishing process thresholds that must be exceeded to maintain form (e.g. flow thresholds to mobilize bed sediment). Once this context has been established, resource managers can better evaluate the options for restoring altered riverine form and function. Copyright © 2011 John Wiley & Sons, Ltd.
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We describe the distinguishing physical characteristics of old ponderosa pine trees in the Front Range of Colorado and the ecological processes that tend to preserve them. Photographs illustrate identifying features of old ponderosa pines and show how to differentiate them from mature and young trees. The publication includes a photographic gallery of old ponderosa pine trees growing on poor, moderate, and good sites. We illustrate trees growing under various forest conditions and with different injuries and histories. The companion general technical report includes a more detailed description of ponderosa pine ecology and the role of old trees in the historical and modern landscapes.
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Fish communities in high-elevation, Rocky Mountain streams consist of only one or a few trout species, so these streams are ideal for quantifying how physical habitat manipulation influences population biology. Managers often alter habitat structure in hopes of increasing the number of size of fish in a population, but this practice has not been rigorously evaluated, and the mechanisms involved are not well understood. We measured fish abundance and habitat conditions in each half of 500-m study reaches in six streams for 2 yr before and 6 yr after installing 10 low log weirs in a randomly designated half (treatment section). Mean depth, pool volume, total cover, and the proportion of fine substrate particles in the stream bed increased in treatment sections within 1 to 2 years, whereas habitat in adjacent controls remained unchanged. Abundance and biomass of adult fish, but not juveniles, increased in treatments relative to controls in all streams. Recaptures of trout that were individually tagged and others that were batch marked revealed that immigration was primarily responsible for increased adult abundance and biomass, whereas no biologically significant differences occurred for recruitment, survival, or growth. Few (
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River restoration is at the forefront of applied hydrologic science. However, many river restoration projects are conducted with minimal scientific context. We propose two themes around which a research agenda to advance the scientific basis for river restoration can be built. First, because natural variability is an inherent feature of all river systems, we hypothesize that restoration of process is more likely to succeed than restoration aimed at a fixed end point. Second, because physical, chemical, and biological processes are interconnected in complex ways across watersheds and across timescales, we hypothesize that restoration projects are more likely to be successful in achieving goals if undertaken in the context of entire watersheds. To achieve restoration objectives, the science of river restoration must include (1) an explicit recognition of the known complexities and uncertainties, (2) continued development of a theoretical framework that enables us to identify generalities among river systems and to ask relevant questions, (3) enhancing the science and use of restoration monitoring by measuring the most effective set of variables at the correct scales of measurement, (4) linking science and implementation, and (5) developing methods of restoration that are effective within existing constraints. Key limitations to river restoration include a lack of scientific knowledge of watershed-scale process dynamics, institutional structures that are poorly suited to large-scale adaptive management, and a lack of political support to reestablish delivery of the ecosystem amenities lost through river degradation. This paper outlines an approach for addressing these shortcomings.
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Debris dam structure and retention of coarse particulate organic matter were examined during a 17-mo period in Powdermill Run, a 3rd-order Appalachian Mountain stream. Through the use of detailed feature maps, changes in debris dam morphology were recorded, including the complete ''life-cycle'' (i.e., initial formation to destruction) of a dam. Stream sections in which dams were naturally destroyed became markedly less retentive. Leaves were used as tracers in retention experiments that varied in duration from 3 h to 4 wk. Results implied that migration over time occurred by a simple mechanism of leaves falling off rocks and settling into debris dams. A series of releases over 12 d showed increasing retentiveness as discharge decreased: Seasonal differences in retention potential were evaluated using 3-h releases conducted during winter, summer, and autumn Summer was the most retentive season due to base-flow conditions. Debris dams were most retentive in autumn,less so in winter, and least retentive in summer. Cobbles showed the opposite pattern Leaf retention ranged from 1.8 to 23.2% retained/m (-k: 0.02 to 0.26), depending on season A significant negative relationship was found between mean depth and % retained/m, but the relationship of % retained/m to discharge was not significant. A consideration of season is necessary when comparing retentive abilities between streams.
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Ecological restoration, conservation, and land management are often based on comparisons with reference sites or time periods, which are assumed to represent “natural” or “properly functioning” conditions. Such reference conditions can provide a vision of the conservation or management goal and a means to measure progress toward that vision. Although historical ecology has been used successfully to guide resource management in many parts of the world, the continuing relevance of history is now being questioned. Some scientists doubt that lessons from the past can inform management in what may be a dramatically different future, given profound climate change, accelerated land use, and an onslaught of plant and animal invasions.
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Previous work on bedrock canyons has suggested that joint distributions may affect the evolution of canyon form. To this end, spacings between steeply dipping joints were measured in two canyons in the Cache la Poudre river system in the Silver Plume Granite in Colorado. Spacings normal to the rivers were measured on vertical canyon walls at multiple locations in both wide and narrow reaches. Spacing distributions, means, and medians for each reach type were compared statistically. Differences between wide and narrow reaches in one canyon are statistically significant, but although spacing is wider in the narrow reach and closer in the wide reach of the canyon, the differences were not significant. Fractal analysis of all field data and of lineations over two sites suggest that these differences are in fact real. These results suggest that joint distribution does affect canyon evolution and that the type of reach is at least partly dependent on joint spacing. This difference may be due to increased weathering, plucking, abrasion, and rock fall where joint spacing is closer.
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Field surveys of stream channels in forested mountain drainage basins in southeast Alaska and Washington reveal that pool spacing depends on large woody debris (LWD) loading and channel type, slope, and width. Mean pool spacing in pool-riffle, plane-bed, and forced pool-riffle channels systematically decreases from greater than 13 channel widths per pool to less than 1 channel width with increasing LWD loading, whereas pool spacing in generally steeper, step-pool channels is independent of LWD loading. Although plane-bed and pool-riffle channels occur at similar low LWD loading, they exhibit typical pool spacings of greater than 9 and 2-4 channels widths, respectively. Forced pool-riffle channels have high LWD loading, typical pool spacing of
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broad alluvial valley during the summers of 2002-2005. We studied a 1.5 km reach of the fourth-order Colorado River in Rocky Mountain National Park (RMNP), Colorado, USA. The beaver dams and ponds greatly enhanced the depth, extent, and duration of inundation associated with floods; they also elevate the water table during both high and low flows. Unlike previous studies we found the main effects of beaver on hydrologic processes occurred downstream of the dam rather than being confined to the near-pond area. Beaver dams on the Colorado River caused river water to move around them as surface runoff and groundwater seepage during both high- and low-flow periods. The beaver dams attenuated the expected water table decline in the drier summer months for 9 and 12 ha of the 58 ha study area. Thus we provide empirical evidence that beaver can influence hydrologic processes during the peak flow and low-flow periods on some streams, suggesting that beaver can create and maintain hydrologic regimes suitable for the formation and persistence of wetlands.
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The number of years since tree death for wind-thrown logs of lodgepole pine (Pinus contorta var. latifolia Engelm.) and Engelmann spruce (Picea engelmannii Parry) was used to examine the longevity of this component of coarse woody debris in an old-growth subalpine forest in the central Rocky Mountains. Death dates of downed logs were determined by dendrochronological cross-dating methods. We were able to determine death dates for 73 logs from. both species, the oldest being a lodgepole pine dead 139 years ago. Sound lodgepole pine and Engelmann spruce logs lying on the ground persisted for many decades with a majority of their volume intact. No difference was seen in decay classes of logs collected from two primary study sites on opposite (north and south) exposures. There was also no significant difference in decay classes between the two species, although lodgepole pine logs were in general older than Engelmann spruce logs within any decay class. There was little decrease in the specific gravity of wood remaining in logs with time, although there was a corresponding greater loss of wood volume.
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Coarse woody debris (CWD) is an important component of temperate stream and forest ecosystems. This chapter reviews the rates at which CWD is added and removed from ecosystems, the biomass found in streams and forests, and many functions that CWD serves. CWD is added to ecosystems by numerous mechanisms, including wind, fire, insect attack, pathogens, competition, and geomorphic processes. Despite the many long-term studies on tree mortality, there are few published rates of CWD input on mass-area-1 time-1 basis. CWD is significantly transformed physically and chemically. Movement of CWD, especially in streams, is also an important but poorly documented mechanism whereby CWD is lost from ecosystems. Many factors control the rate at which CWD decomposes, including temperature, moisture, internal gas composition of CWD, substrate quality, size of CWD, and types of organisms involved. However, the importance of many of these factors has yet to be established in field experiments. CWD performs many functions in ecosystems, serving as autotrophic and heterotrophic habitat and strongly influencing geomorphic processes, especially in streams. It is also a major component of nutrient cycles in many ecosystems and is an important functional component of stream and forest ecosystems.
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In the northern Colorado Front Range, fire suppression during the 20th cen- tury is believed to have created a high hazard of catastrophic fire in ponderosa pine (Pinus ponderosa) forests. Since the early 1990s, resource managers have increased the use of prescribed fires to re-create fire regimes and forest structures similar to those of the pre- Euro-American settlement period in order both to reduce fire hazard and to improve forest health. To improve understanding of historical fire regimes, we conducted a study of fire history along an elevational gradient from ;1830 to 2800 m in ponderosa pine forests in the northern Front Range. Fire-scar dates were determined from 525 partial cross sections from living and dead trees at 41 sample sites. Fire frequencies and fire intervals were analyzed in relation to changes in human activities and interannual climatic variability as recorded in instrumental climatic records and tree-ring proxy records. Prior to modern fire suppression, the low elevation, open ponderosa pine forests of the northern Front Range were characterized by frequent surface fires, similar in frequency to many other ponderosa pine ecosystems in the West. In contrast, in higher elevation forests (above ;2400 m) where ponderosa pine is mixed with Douglas-fir (Pseudotsuga menziesii) and lodgepole pine (Pinus contorta), the fire regime was characterized by a much lower fire frequency and included extensive stand-replacing fires as well as surface fires. In the mid-1800s there was a marked increase in fire occurrence that can be related both to Euro- American settlement and increased climatic variability. This episode of increased fire left a legacy of dense, even-aged stands in higher elevation ponderosa pine forests, whereas increased stand densities in low elevation forests are attributed mainly to fire exclusion during the 20th century. Warmer and drier spring-summers, indicated in instrumental climatic records (1873- 1995) and in tree-ring proxy records of climate (1600-1983), are strongly associated with years of widespread fire. Years of widespread fire also tend to be preceded two to four years by wetter than average springs that increase the production of fine fuels. Alternation of wet and dry periods over time periods of 2-5 years is conducive to fire spread and is strongly linked to El Nino-Southern Oscillation (ENSO) events. The warm (El Nino) phase of ENSO is associated with greater moisture availability during spring that results in a peak of fire occurrence several years following El Nino events. Conversely, dry springs associated with La Nina events were followed by more widespread fire during the same year. The 1600-1920 fire-scar record indicates that individual years during which high per- centages of the 41 sample sites synchronously recorded fire have occurred at least several times per century. The association of these years of widespread fire with very strong ENSO events demonstrates the importance of ENSO-related climatic variabililty in creating ex- treme fire hazard at a landscape scale.
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In western North America, snow provides crucial storage of winter precipitation, effectively transferring water from the relatively wet winter season to the typically dry summers. Manual and telemetered measurements of spring snow-pack, corroborated by a physically based hydrologic model, are examined here for climate-driven fluctuations and trends during the period of 1916-2002. Much of the mountain West has experienced declines in spring snowpack, especially since midcentury, despite increases in winter precipitation in many places. Analysis and modeling show that climatic trends are the dominant factor, not changes in land use, forest canopy, or other factors. The largest decreases have occurred where winter temperatures are mild, especially in the Cascade Mountains and northern California. In most mountain ranges, relative declines grow from minimal at ridgetop to substantial at snow line. Taken together, these results emphasize that the West's snow resources are already declining as earth's climate warms. Joint Institute for the Study of the Atmosphere and the Ocean Contribution Number 1073.
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Tree-ring records spanning the past seven centuries from the central and southern Rocky Mountains were studied using wavelet analysis to examine multidecadal (>30-70 yr) patterns of drought variation. Fifteen tree-ring series were grouped into five regional composite chronologies based on shared low-frequency behavior. Strong multidecadal phasing of moisture variation was present in all regions during the late 16th century megadrought. Oscillatory modes in the 30-70 yr domain persisted until the mid-19th century in two regions, and wet-dry cycles were apparently synchronous at some sites until the 1950s drought. The 16th/17th century pattern of severe multidecadal drought followed by decades of wet conditions resembles the 1950s drought and post-1976 wet period. The 16th century megadrought, which may have resulted from coupling of a decadal (similar to20-30 yr) Pacific cool phase with a multidecadal warm phase in the North Atlantic, marked a substantial reorganization of climate in the Rocky Mountain region.
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Total flow resistance, measured as Darcy–Weisbach f, in 20 step-pool channels with large woody debris (LWD) in Washington, ranged from 5 to 380 during summer low flows. Step risers in the study streams consist of either (1) large and relatively immobile woody debris, bedrock, or roots that form fixed, or “forced,” steps, or (2) smaller and relatively mobile wood or clasts, or a mixture of both, arranged across the channel by the stream. Flow resistance in step-pool channels may be partitioned into grain, form, and spill resistance. Grain resistance is calculated as a function of particle size, and form resistance is calculated as large woody debris drag. Combined, grain and form resistance account for less than 10% of the total flow resistance. We initially assumed that the substantial remaining portion is spill resistance attributable to steps. However, measured step characteristics could not explain between-reach variations in flow resistance. This suggests that other factors may be significant; the coefficient of variation of the hydraulic radius explained 43% of the variation in friction factors between streams, for example. Large woody debris generates form resistance on step treads and spill resistance at step risers. Because the form resistance of step-pool channels is relatively minor compared to spill resistance and because wood in steps accentuates spill resistance by increasing step height, we suggest that wood in step risers influences channel hydraulics more than wood elsewhere in the channel. Hence, the distribution and function, not just abundance, of large woody debris is critical in steep, step-pool channels.
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A multidisciplinary study of streamflow, precipitation, and paleohydrology was conducted to improve the understanding of flood hydrometeorology in Colorado. Conventional flood- frequency analyses do not adequately characterize the flood hydrology in the foothills and mountains of Colorado. Annual peak flows are caused by snowmelt at higher elevations in the mountains and by rainfall at lower elevations. Above 2300 meters (this elevation is lower in some river basins), snowmelt rather than rainfall contributes to the flood potential. Below 2300 m, large rainfall-generated floods are common. Regional flood-frequency methods, supported by paleoflood information, were developed that indicate the 1976 Big Thompson River flood has a recurrence interval of approximately 10000 years. The approach and results may be useful in decreasing the uncertainty in the design of hydraulic structures as described for the spillway of Olympus Dam in the Big Thompson River basin or for other hydrologic studies. (A)
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The paper discusses the role of large organic debris on channel form, fluvial processes, and development and maintenance of anadromous fish habitat for streams flowing through an old-growth redwood forest. Specifically considered are: how the large organic debris affects channel width, depth, and slope; diversity of anadromous fish habitats such as pools, riffles and bars; areal sorting of bedload material; erosion-deposition patterns; and the instream residence time for large organic debris.
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Wood has been falling into rivers for millions of years, resulting in both local effects on channel processes and integrated influences on channel form and dynamics over a wide range of spatial and temporal scales. Effects of stable pieces of wood on local channel hydraulics and sediment transport can influence rates of bank erosion, create pools, or initiate sediment deposition and bar formation. At larger spatial scales, changes in the supply of large wood can trigger changes in both river-reach morphology and the interaction between a river and its floodplain. Over long time scales, wood-rich rivers may retain more sediment and have lower sediment transport rates and steeper slopes than comparable wood-poor channels. Most geomorphic effects of wood in rivers arise from large, stable logs that catalyze changes in the routing and storage of both smaller wood and sediment. The size of a log relative to the channel provides a reasonable gauge of the potential stability of in-channel wood. Channels with a high supply of large, potentially stable wood may experience substantial vertical variability in bed elevation independent from external forcing (e.g., climate variability, temporal variations in sediment supply, or tectonic activity). In some river systems, changes in the wood regime, as described by the size and amount of wood supplied to a river, can result in effects as great as those arising from changes in the sediment supply or the discharge regimes. Consequently, an understanding of the geomorphic effects of wood is crucial for assessing the condition and potential response of forest channels.
Chapter
Thirty to forty m.y. of post-Laramide degradation of the southern Rocky Mountains likely produced relatively low-relief topography within the crystalline cores of the ranges, and capped the adjacent sedimentary basins with easily eroded sediments. We focus on the modern, more dissected topography of these ranges, reflecting late Cenozoic evolution driven by fluvial and glacial exhumation, each of which affects different portions of the landscape in characteristic ways. Ongoing exhumation of the adjacent basins, in places by more than 1 km, is effectively lowering base level of streams draining the crystalline range cores. The streams have incised deep bedrock canyons that now cut the flanks of the range. Over the same time scales, glaciation of the headwaters of the major streams has modified the range crests. We utilize the topography of the northern Front Range of Colorado to explore the response of a Laramide range both to the exhumation of the adjacent basin and to glaciation in the high elevations. We break the problem of whole landscape evolution into three related, one-dimensional problems: evolution of the high smooth summit surfaces; evolution of the longitudinal profiles of adjacent glacial troughs; and evolution of the fluvial profiles downstream of the glacial limit. We review work on the high summit surfaces, showing quantitatively that they are steady-state features lowering at rates on the order of 5 μm/yr, and are entirely decoupled from the adjacent glacial troughs. Glaciers not only truncate these high surfaces, but greatly alter the longitudinal profiles of the major streams: major steps occur at tributary junctions, and profiles above the glacial limit are significantly flattened from their original fluvial slopes. We extend existing models of glacial valley evolution by including processes that allow headwall retreat. This serves to enhance the headward retreat of east-facing valleys, and explains the asymmetric truncation of the high smooth surfaces that form the spine of the range. Fluvial profiles downstream of the glacial limit commonly display a prominent convexity inboard of the range edge. Stream-power-based numerical models of profile evolution of specific rivers demonstrate that this reflects a transient response of the streams to base-level lowering. This response varies significantly with drainage basin area. We explore the degree to which this differential response controls the location of major remnants of pediments on the edge of the Great Plains, such as the prominent Rocky Flats and adjacent surfaces.
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Cumulative age distributions were statistically different for each species. Engelmann spruce Picea engelmannii and ponderosa pine Pinus ponderosa age distributions each exhibited a strong inflection point at approx 210 and 125 yr, respectively, while neither lodgepole pine Pinus contorta nor limber pine Pinus flexilis showed such a sharp inflection point. This inflection point may be indicative of 'climax' type species in mature stands. The age distributions of these species may be interpreted as reflective of their climax, colonizing and fugitive ecological patterns, respectively. Diameter distribution curves exhibited patterns markedly different from the age patterns. Ponderosa pine and Engelmann spruce showed almost identical distribution despite widely disparate age structure. The complex of forces which influence size distributions in forest trees is thus very similar in ponderosa pine and spruce although they form very different ecological communities. Lodgepole and limber pine diameter distributions were quite different. The presence of a sharp inflection point in cumulative age distributions indicates a 'climax' species will often have coincident diameter distributions.-from Authors
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Removal of all organic debris dams from a 175-m stretch of second-order stream at the Hubbard Brook Experimental Forest in New Hampshire led to a dramatic increase in the export of organic carbon from this ecosystem. Output of dissolved organic carbon (<0.50 @mm) increased 18%. Fine particulate organic carbon (0.50 @mm-1 mm) export increased 632% and coarse particulate organic matter (>1 mm) export increased 138%. Measurement of the standing stock of coarse particulate organic matter on streambeds of the Hubbard Brook Valley revealed that organic debris dams were very important in accumulating this material. In first-order streams, debris dams contain nearly 75% of the standing stock of organic matter. The proportion of organic matter held by dams drops to 58% in second-order streams and to 20% in third-order streams. Organic debris dams, therefore, are extremely important components of the small stream ecosystem. They retain organic matter within the system, thereby allowing it to be processed into finer size fractions in headwater tributaries rather than transported downstream in a coarse particulate form.
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Through dam building and feeding activities, beaver act as a keystone species to alter hydrology, channel geomorphology, biogeochemical pathways and community productivity. In Quebec, density of dams on the small streams (= or <4th order) studied averages 10.6 dams/km; the streams retain up to 6500 m3 of sediment per dam, and the wetted surface area of the channel is increased up to several hundredfold. Beaver are also active in larger order streams (= or >5th order), but their effects are most noticeable along riverbanks and in floodplains. Comparative carbon budgets per unit area for a riffle on 2nd order Beaver Creek and a beaver pond downstream show the pond receives only 42% of the carbon acquired by the riffle annually, but because the pond has a surface area 7 times greater than the riffle, it receives nearly twice as much carbon as the riffle per unit of channel length. Carbon in the pond has an estimated turnover time of 161 yr compared to 24 yr for the riffle. Beaver ponds are important sites for organic matter processing. -from Authors
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Abstract.-To preserve and recover evolutionarily significant units (ESUs) of anadromous salmonids Oncorhvnchus spp. in the Pacific Northwest. long-term and short-term ecological processes that create and maintain freshwater habitats must be restored and protected. ...
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Human activities in the last 150 years greatly altered the riverine landscape and salmonid habitats of the Puget Lowland. Archival investigations together with field studies of relatively undisturbed rivers make it possible to describe the landscape prior to settlement by Euro-Americans. Landforms, dynamics, and habitats in lowland river valleys and estuaries varied broadly with differ-ences in regional geologic history. Rivers that incised a Holocene valley through Pleistocene glacial sediments typically had an anastomosing pattern with multiple channels, floodplain sloughs, and frequent channel-switching avulsions, due in large part to wood jams. In contrast, rivers in broader, lower-gradient valleys created by runoff below Pleistocene glaciers generally had a single-channel meandering pattern, with oxbow lakes, infrequent meander-cut-off avulsions, and vast floodplain wetlands. Because wood appears to have strongly influenced riverine dynamics at a wide range of scales, flood-plain forests are central to river restoration. Archival sources can characterize species and diameters of trees in historical forests and the geomorphic, hydro-logic, and geographic variables influencing them; process studies indicate conditions and wood characteristics necessary for jam formation. Regional differences in channel morphologies, processes, suites of valley-bottom land-forms, and forests, combined with different land-use histories, have important implications for the rationale, approach, and land area needed in restoring lowland river and forest ecosystems.
Book
This book outlines a generic set of procedures, termed the River Styles Framework, which provides a set of tools for interpreting river character, behavior, condition, and recovery potential. Applications of the framework generate a coherent package of geomorphic information, providing a physical template for river rehabilitation activities. management and restoration of rivers is a rapidly growing topic for environmental scientists, geologists and ecologists - this book provides a learning tool with which to approach geomorphic applications to river management describes the essential geomorphological principles underlying river behaviour and evolution demonstrates how the River Styles Framework can turn geomorphic theory into practice, to develop workable strategies for restoration and management based on real case studies and authors extensive experience applicable to river systems worldwide synthesises fluvial geomorphology, ecology and management.
Article
Channel morphology along the Middle Fork of the South Platte River near Fairplay, Colorado has changed from 1859 to the present in association with placer gold mining. Based on the premise that mining destabilized channel-bed and bank sediments, we hypothesize that there will be a relation between mining and channel morphologic and sedimentologic characteristics. To quantify differences in channel morphology between mined and unmined reaches, four primary channel characteristics (shape, bed material, migration, and sinuosity) were examined. A discriminant function analysis showed no statistical difference in channel shape between mined and unmined reaches. Bed material in reaches where mining occurred in the channel can be distinguished from unmined reaches on the basis of mean grain-size, sorting, and skewness. Mean grain-size and sorting increase and the skewness of the distribution is nearly symmetrical in mined areas. Statistical differences in planimetric form also were observed. The channel has been more mobile and has a lower sinuosity in mined areas than in unmined areas. The loss of fines during mining decreases bank cohesion because vegetation is removed and cannot reestablish on the coarse tailing piles. The channel still has not recovered from the mining disturbance in 67–82 years. To be reclaimed, the tailing piles may have to be stabilized and the channel bed may need to reflect that of an undisturbed area.
Article
Natural rates of input and depletion of large woody debris (LWD) in southeast Alaska streams were studied to provide a basis for managing streamside zones to maintain LWD for fish habitat after timber harvest. Debris was inventoried in a variety of stream types in undisturbed old-growth forest; 252 pieces of LWD were dated from the age of trees growing on them. Longevity of LWD was directly related to bole diameter: small LWD (10–30 cm in diameter) was less than 110 years old, whereas large LWD (>60 cm in diameter) was up to 226 years old. Assuming equilibrium between input and depletion of LWD in streams in old-growth forests and exponential decay of LWD, we calculated input and depletion rates from mean age of LWD. Input and depletion rates were inversely proportional to LWD diameter and ranged from 1%/year for large LWD in all stream types to 3%/year for small LWD in large, high-energy, bedrock-controlled streams. A model of changes in LWD after timber harvest (which accounted for depletion of LWD and input from second-growth forest) indicated that 90 years after clear-cut logging without a stream-side buffer strip large LWD would be reduced by 70% and recovery to prelogging levels would take more than 250 years. Because nearly all LWD is derived from within 30 m of the stream, the use of a 30-m wide, unlogged buffer strip along both sides of the stream during timber harvest should maintain LWD.
Article
We modeled large woody debris (LWD) recruitment and pool formation in northwestern Washington streams after simulated stand-clearing disturbance using two computer models: Forest Vegetation Simulator for stand development and Riparian-in-a-Box for LWD recruitment, depletion, and pool formation. We evaluated differences in LWD recruitment and pool formation among different combinations of channel size, successional pathway, and stand management scenario. The models predict that time to first recruitment of pool-forming LWD is about 50% shorter for red alder Alnus rubra than for Douglas-fir Pseudotsuga menziesii at all channel widths. Total LWD abundance increases faster in red alder stands than in Douglas-fir stands but declines rapidly after 70 years as the stand dies and pieces decompose. Initial recovery is slower for Douglas-fir stands, but LWD recruitment is sustained longer. Total LWD abundance increases faster with decreasing channel size, and pool abundance increases faster with decreasing channel width and increasing channel slope. The models predict that thinning of the riparian forest does not increase recruitment of pool-forming LWD where the trees are already large enough to form pools in the adjacent channel and that thinning reduces the availability of adequately sized wood. Thinning increases LWD recruitment where trees are too small to form pools and, because of reduced competition, trees more rapidly attain pool-forming size. On channels less than 20 m wide, thinning of red alder and underplanting shade-tolerant conifers will reduce near-term alder recruitment and increase long-term conifer recruitment. However, the same treatment on channels more than 20 m wide may increase both near-term and long-term recruitment. Compared with the natural fire regime, timber harvest rotations of 40-80 years during the past century have reduced the percentage of riparian stands that can provide LWD of pool-forming size to streams, especially in channels at least 20 m wide.
Article
Large woody debris was measured in 11 undisturbed streams draining subalpine old-growth forests to assess abundance, characteristics, and function. Although large woody debris in Colorado has smaller diameter, length, and volume than in the Pacific Northwest, its abundance and function were similar. Most pools (76%) were plunge and dammed pools formed by large wood debris, most of which spanned the channels perpendicular to stream flow. Smaller streams had a greater proportion of such perpendicular pool-forming pieces than larger streams. Four disturbed streams had significantly less and smaller large woody debris than undisturbed streams. Flows in larger undisturbed streams were capable of moving large woody debris, so pieces were more often located at the stream margins, oriented diagonally, or distributed in clumps than in smaller streams. Individual pools were larger and deeper in larger streams, but their size was not related to the size of large woody debris pieces forming them. The function of large woody debris in forming fish habitat in small Rocky Mountain streams is thus strongly influenced by the stream's location within the watershed. -Authors
Article
Sparsely settled mountain areas of the world, such as Colorado's Front Range, give an impression of wild, untouched, and unchanging nature. Yet in many cases mountain rivers that appear to be pristine natural systems actually have been impaired as a result of human activities. In this timely and accessible book, Ellen Wohl documents two hundred years of land-use patterns on the Front Range and their wide-ranging effects on river ecosystems. If we hope to manage river resources effectively and preserve functioning river ecosystems, the author warns, we must recognize how beaver trapping, placer mining, timber harvesting, flow regulation, road and railroad construction, recreation, cattle grazing, and other human activities have impaired rivers, and continue to do so. The rivers of the Colorado Front Range are representative of mountain rivers throughout the world: land-use patterns affecting their form and function are little-recognized or understood. This book fills an important gap with a clear and comprehensive explanation of how rivers are changed by human activity and includes a generous selection of striking historical and contemporary photographs, maps, and diagrams.
Article
A flume experiment is carried out to explore jamming of Large Woody Debris (LWD) in streams with complex morphology, occurring in mountain streams with in channel boulders or vegetation, in braided rivers or in floodplains during flood events. Non rooted, defoliated LWD is modeled using wood dowels and obstacles to motion are represented by vertical wood rods. Congested transport of LWD is simulated by insertion of a number (100) of dowels. The final position of the dowels is mapped and the observed jams are classified according to their size and position. The key member of each jam is identified and its trapping mechanism evaluated, either by leaning against a single obstacle or by bridging two obstacles. To mimic uncongested transport, the experiment is repeated for single pieces of wood, with subsequent removal. Longer dowels and shallower water result in shorter traveled distance. Wood pieces travel farther when congested transport is observed. The traveled distance of the wood pieces can be modeled using a Gamma distribution, for both congested and uncongested transport. Jams instead display Uniform traveled distance. The number of pieces displays an Exponential distribution. The degree of uniformity in space of jams and wood pieces is evaluated using a neighbor K statistic. Wood pieces show considerable clustering, while jams show sparse distribution. Eventually, the relationship between jams magnitude and position is explored, showing negative correlation. Model application is then discussed and some conclusions and future developments are outlined.
Article
Although coarse woody debris (CWD) is an important component of stream ecosystems in forested areas, the processes of CWD distribution, transport, and retention have not been clarified. In this study the distribution process of CWD pieces shorter than the bankfull width (S-CWD) is discussed using an in situ field experiment of log transport and a field survey of CWD distribution in mountain streams. The transport experiment showed that transport distance has a close relation to flow depth and also implied that the magnitude and sequence of a series of flows were important factors for S-CWD transport and retention in streams. The survey of CWD distribution indicated that in-stream obstructions played an important role in the S-CWD retention in deeper channels where S-CWD pieces were potentially transported distances more than spacing between trapping sites of CWD. Overall, the in situ field experiment and the segment- to reach-scaled analysis using h* (=depth/diameter) helped us understand the actual movement and distribution of CWD.
Article
Publisher Summary This chapter reviews the rates at which Coarse Woody Debris (CWD) is added and removed from ecosystems, the biomass found in streams and forests, and many functions that CWD serves. CWD is an important component of temperate stream and forest ecosystems and is added to the ecosystem by numerous mechanisms, including wind, fire, insect attack, pathogens, competition, and geomorphic processes. Many factors control the rate at which CWD decomposes, including temperature, moisture, the internal gas composition of CWD, substrate quality, the size of the CWD, and the types of organisms involved. The mass of CWD in an ecosystem ideally represents the balance between addition and loss. In reality, slow decomposition rates and erratic variations in input of CWD cause the CWD mass to deviate markedly from steady-state projections. Many differences correspond to forest type, with deciduous-dominated systems having generally lower biomass than conifer-dominated systems. Stream size also influences CWD mass in lotic ecosystems, while successional stage dramatically influences CWD mass in boat aquatic and terrestrial settings. This chapter reviews many of these functions and concludes that CWD is an important functional component of stream and forest ecosystems. Better scientific understanding of these functions and the natural factors influencing CWD dynamics should lead to more enlightened management practices.
Article
During summer of 1994, 328 0.1 ha circular plots were sampled to determine the volume of coarse woody debris (CWD) and standing dead density and basal area in 53 mature or older Pinus ponderosa stands in the Front Range and Southwestern mountains of Colorado. Standing dead volume was estimated using height derived from living trees in the plots. Generally, CWD amounts were lower in these forests than have been reported for Pinus ponderosa forests in Arizona and New Mexico and other forest types in North America. Mean volume for all plots was 15.9 m3/ha ± 20.85 SD. Mass, estimated from published wood density values, was 3.4 Mg/ha ± 5.07. CWD in the Southwestern mountains of Colorado averaged about 7.5 m3/ha > in the Front Range. Sixty-three percent of the CWD was in decay class 4 followed by class 5 with 16%. CWD showed considerable variation across stands and habitat types with generally higher amounts in more mesic conditions. No significant relationships existed between CWD and the various environmental factors measured. CWD volume and variability both increased with stand age. Standing dead density was highly variable with an mean density of 11.9 stems/ha. Most standing dead trees were small with a mean diameter of 34.6 cm and mean basal area of 1.25 m2/ha. Eighty-eight percent of the class 1 CWD was standing dead, but class 1 comprised only 8% of the total CWD. High values of standing dead may be related to disturbance and stand age. In Pinus ponderosa stands, it appears CWD is affected by disturbance, such as fire, timber harvesting and insect infestation > by physical factors of the environment.
Article
Changes in regional temperature and precipitation expected to occur as a result of the accumulation of greenhouse gases may have significant impacts on water resources. We use a conceptual hydrologic model, developed and operated by the National Weather Service, to study the sensitivity of surface runoff in several sub-basins of the Colorado River to these changes. Increases in temperature of 2°C decrease mean annual runoff by 4–12%. A temperature increase of 4°C decreases mean annual runoff by 9–21%. Increases or decreases in annual precipitation of 10–20% result in corresponding changes in mean annual runoff of approximately 10–20%. For the range of scenarios studied, these results suggest that runoff in the basin is somewhat more sensitive to changes in precipitation than to changes in temperature. Seasonal changes were also observed, with peak runoff shifting from June to April or May. Fall and winter flows generally increase, whereas spring and summer flows decrease in most of the scenarios studied. These changes are attributed to an increase of the ratio of rain to snow and to a higher snowline. Although these results suggest that streamflow in the Colorado Basin is less sensitive to climatic changes than previous statistical studies have indicated, the magnitude of possible changes is nonetheless sufficiently great to have significant environmental, economic, and political implications.
Article
The idea of a 'patch' implies a relatively discrete spatial pattern, incorporating relationships both with other patches and with surrounding non-patch areas. 'Disturbance' and 'perturbation', often used synonymously, are here distinguished, the former involving environmental fluctuations and destructive events, even if these are normal to the system, the latter indicating any change in a parameter that defines a system and viewed in terms of that whole system. Endogenous and exogenous causes of disturbance are noted, and implications for changes in the nature of a patch in space and/or time ("patch dynamics') are summarised. Subsequent contributions to this volume are then introduced. -P.J.Jarvis
Article
Step-pool streams dissipate flow energy primarily through spill resistance. We compared the geometry, step characteristics, and flow hydraulics of 20 step-pool reaches without large woody debris (LWD) to 20 step-pool reaches with LWD. Non-LWD streams exhibited significantly shallower flows, lower steps, shorter step spacings, greater percentages of water-surface drop created by steps, larger grain sizes, and smaller Darcy-Weisbach friction factors. Grain resistance was negligible in both stream types. Form resistance created by irregularities in the channel shape associated with steps contributed more to the total flow resistance in LWD reaches. Although both stream types showed poor correlation between step height and flow resistance, the significant positive correlation between flow resistance and step height/length ratio in the non-LWD reaches demonstrates the increasing effect of spill resistance with increasing step height. The lack of such a trend in the LWD-loaded reaches suggests that spill resistance was highly influenced by a few large log steps in these reaches. LWD creates deep pools and increases flow resistance along step-pool streams. It thus stabilizes channels and stores sediment in steep headwater streams recently scoured by debris flows.
Article
Instream large woody debris (LWD) provides several critical functions in riverine ecosystems, including sediment and nutrient retention, salmonid habitat enhancement, and stable colonization sites for incipient floodplain vegetation. In this study, the size and species composition of LWD in the Queets River, Washington, USA, were examined and compared with the size and species composition of forest trees from which they originated, in order to determine a depletion rate for LWD in the active channel. Increment cores from instream LWD were crossdated against cores from riparian conifers to estimate the year each LWD piece was recruited to the river channel. Debris pieces that were decayed or otherwise incompetent to provide cores were dated using standard 14C techniques. Hardwood species (Alnus rubra, Populus trichocarpa,and Acer macrophyllum) were better represented among riparian forests than among instream LWD, and conifers (Picea sitchensis, Tsuga heterophylla, Pseudotsuga menziesii, and Thuja plicata) were better represented among LWD than in the adjacent riparian forest, suggesting that hardwoods were depleted from the channel faster than conifers. The depletion rate of coniferous LWD from the channel followed an exponential decay curve in which 80% of LWD pieces were ,50 yr old, although some pieces have remained for up to 1400 yr. Although most wood is depleted from the channel within 50 yr, some wood is apparently buried in the floodplain and exhumed centuries later by lateral channel migration. The calculated depletion constant of 0.030 is equivalent to a half-life of ;20 yr, meaning that virtually all of the wood will have disappeared within 50 yr. This rapid depletion suggests that harvesting large conifers from the riparian zones of large streams could have adverse impacts within three to five decades.