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At-a-station hydraulic geometry (AHG), which describes how channel width, depth, and velocity vary with discharge at a river cross section, has long been used to study fluvial processes. For exampl...

River flows exert dominant controls on in-stream biota. Quantifying linkages between hydrology and biology is important for assessing the effects of flow alteration on ecological functions. Hydrologic models are often used to quantify these flow-ecology relationships and guide management actions. Traditional model calibration techniques typically focus on a best overall fit criterion that may not be suitable for environmental flow applications where certain elements of the flow regime exert a dominant influence on biotic composition. We present an approach for hydrologic model calibration that improves the accuracy of calculated flow metrics known to be significant drivers of ecosystem response. First, we developed regional flow-ecology relationships based on streamflow gage and benthic macroinvertebrate data from southern California to determine which streamflow metrics best explain variability in taxonomic and trait-based biotic indices. Next, we developed and calibrated a series of hydrologic models to minimize error in these important flow metrics. For our study sites, flow flashiness and low flow frequency (indicative of drying) were found to best explain biotic condition. Hydrologic models calibrated specifically to minimize errors in these flow metrics predicted macroinvertebrate indices better than models calibrated to maximize fit to the overall flow regime. This ecological-calibration approach requires some a priori knowledge of flow-ecology relationships, but it produces results that can improve assessment of the impacts of changing flow regimes on biota and guide the development of strategies to mitigate ecological degradation.

Water management and infrastructure design depend on quantifying thresholds in minimum flows. Decreasing trends in low flows have been observed at many stream gauges in the Southeast US; ho...

Excessive river erosion and sedimentation threatens critical infrastructure, degrades aquatic habitat, and impairs water quality. Tools for predicting the magnitude of erosion, sedimentation, and channel evolution processes are needed for effective mitigation and management. We present a new numerical model that simulates coupled river bed and bank erosion at the watershed scale. The model uses modified versions of Bagnold’s sediment transport equation to simulate bed erosion and aggradation, as well as a simplified Bank Stability and Toe Erosion Model (BSTEM) to simulate bank erosion processes. The model is mechanistic and intermediate complexity, accounting for the dominant channel evolution processes while limiting data requirements. We apply the model to a generic test case of channel network response following a disturbance and the results match physical understanding of channel evolution. The model was also tested on two field data sets: below Parker Dam on the lower Colorado River and the North Fork Toutle River (NFTR) which responded dramatically to the 1980 eruption of Mount St. Helens. It accurately predicts observed channel incision and bed material coarsening on the Colorado River, as well as observations for the upstream 18 km of the NFTR watershed. The model does not include algorithms for extensive lateral migration and avulsions and therefore did not perform well in the lower NFTR where the channel migrated across a wide valley bottom. REM is parsimonious and useful for simulating network scale channel change in single thread systems responding to disturbance.

Urbanization alters the delivery of water and sediment to receiving streams, often leading to channel erosion and enlargement, which increases loading of sediment and nutrients, degrades habitat, and harms sensitive biota. Stormwater control measures (SCMs) are constructed in an attempt to mitigate some of these effects. In addition, stream restoration practices such as bank stabilization are increasingly promoted as a means of improving water quality by reducing downstream sediment and pollutant loading. Each unique combination of SCMs and stream restoration practices results in a novel hydrologic regime and set of geomorphic characteristics that interact to determine stream condition, but in practice, implementation is rarely coordinated due to funding and other constraints. In this study, we examine links between watershed-scale implementation of SCMs and stream restoration in Big Dry Creek, a suburban watershed in the Front Range of northern Colorado. We combine continuous hydrologic model simulations of watershed-scale response to SCM design scenarios with channel evolution modeling to examine interactions between stormwater management and stream restoration strategies for reducing loading of sediment and adsorbed phosphorus from channel erosion. Modeling results indicate that integrated design of SCMs and stream restoration interventions can result in synergistic reductions in pollutant loading. Not only do piecemeal and disunited approaches to stormwater management and stream restoration miss these synergistic benefits, they make restoration projects more prone to failure, wasting valuable resources for pollutant reduction. We conclude with a set of recommendations for integrated planning of SCMs and stream restoration to simultaneously achieve water quality and channel protection goals.

Summary The decision to use either daily-averaged or sub-daily streamflow records has the potential to impact the calculation of sediment transport metrics and stream channel design. Using bedload and suspended load sediment transport measurements collected at 138 sites across the United States, we calculated the effective discharge, sediment yield, and half-load discharge using sediment rating curves over long time periods (median record length = 24 years) with both daily-averaged and sub-daily streamflow records. A comparison of sediment transport metrics calculated with both daily-average and sub-daily stream flow data at each site showed that daily-averaged flow data do not adequately represent the magnitude of high stream flows at hydrologically flashy sites. Daily-average stream flow data cause an underestimation of sediment transport and sediment yield (including the half-load discharge) at flashy sites. The degree of underestimation was correlated with the level of flashiness and the exponent of the sediment rating curve. No consistent relationship between the use of either daily-average or sub-daily streamflow data and the resultant effective discharge was found. When used in channel design, computed sediment transport metrics may have errors due to flow data resolution, which can propagate into design slope calculations which, if implemented, could lead to unwanted aggradation or degradation in the design channel. This analysis illustrates the importance of using sub-daily flow data in the calculation of sediment yield in urbanizing or otherwise flashy watersheds. Furthermore, this analysis provides practical charts for estimating and correcting these types of underestimation errors commonly incurred in sediment yield calculations.

Instream and floodplain wood can provide many benefits to river ecosystems, but can also create hazards for inhabitants, infrastructure, property, and recreational users in the river corridor. We propose a decision process for managing large wood, and particularly for assessing the relative benefits and hazards associated with individual wood pieces and with accumulations of wood. This process can be applied at varying levels of effort, from a relatively cursory visual assessment to more detailed numerical modeling. Decisions to retain, remove, or modify wood in a channel or on a floodplain are highly dependent on the specific context: the same piece of wood that might require removal in a highly urbanized setting may provide sufficient benefits to justify retention in a natural area or lower-risk urban setting. The proposed decision process outlined here can be used by individuals with diverse technical backgrounds and in a range of urban to natural river reaches so that opportunities for wood retention or enhancement are increased.

Low Impact Development (LID) is an alternative to conventional urban stormwater management practices, which aims at mitigating the impacts of urbanization on water quantity and quality. Plot and local scale studies provide evidence of LID effectiveness, however, little is known about the overall watershed scale influence of LID practices. This is particularly true in watersheds with a land cover that is more diverse than that of urban or suburban classifications alone. We address this watershed-scale gap by assessing the effects of three common LID practices (rain gardens, permeable pavement, and riparian buffers) on the hydrology of a 0.94 km2 mixed land cover watershed. We used a spatially-explicit ecohydrological model, called Visualizing Ecosystems for Land Management Assessments (VELMA), to compare changes in watershed hydrologic responses before and after the implementation of LID practices. For the LID scenarios, we examined different spatial configurations, using 25%, 50%, 75% and 100% implementation extents, to convert sidewalks into rain gardens, and parking lots and driveways into permeable pavement. We further applied 20 m and 40 m riparian buffers along streams that were adjacent to agricultural land cover. The results showed overall increases in shallow subsurface runoff and infiltration, as well as evapotranspiration, and decreases in peak flows and surface runoff across all types and configurations of LID. Among individual LID practices, rain gardens had the greatest influence on each component of the overall watershed water balance. As anticipated, the combination of LID practices at the highest implementation level resulted in the most substantial changes to the overall watershed hydrology. It is notable that all hydrological changes from the LID implementation, ranging from 0.01 to 0.06 km2 across the study watershed, were modest, which suggests a potentially limited efficacy of LID practices in mixed land cover watersheds.

Whitewater parks (WWPs) are increasingly popular recreational amenities, but the effects of WWPs on fish habitat and passage are poorly understood. This study investigated the use of a two-dimensional (2-D) model as compared with a three-dimensional (3-D) hydrodynamic model (FLOW-3D ® ) for assessing effects of WWPs on fish habitat. The primary aims of this study were to (1) examine the utility of 3-D modelling versus 2-D modelling in a hydraulically complex WWP and (2) compare modelled habitat quality for resident fishes with actual fish abundance and biomass generated from field sampling surveys. Two reaches of a wadeable river in Colorado were modelled: a natural reach and a reach containing a WWP. A 2-D habitat suitability analysis for juvenile and adult brown trout, juvenile and adult rainbow trout, longnose dace and longnose sucker predicted the same or higher habitat quality in the WWPs than the natural pools for all four species and for all modelled flow rates; however, results from fish sampling found significantly higher fish biomass for all four species in natural pools compared with WWP pools. All hydraulic metrics (depth, depth-averaged velocity, turbulent kinetic energy, 2-D and 3-D vorticity) had higher magnitudes in WWP pools than in natural pools. In the WWP pools, 3-D model results described the spatial distribution of flow characteristics or the magnitude of variables better than 2-D results. This supports the use of 3-D modelling for complex flows found in WWPs, but improved understanding of linkages between fish habitat quality and 3-D hydraulic descriptors is needed. Copyright © 2015 John Wiley & Sons, Ltd.

Analytical channel design tools have not advanced appreciably in the last decades, and continue to produce designs based upon a single representative discharge that may not lead to overall sediment continuity. It is beneficial for designers to know when a simplified design may be problematic and to efficiently produce alternative designs that approximate sediment balance over the entire flow regime. The Capacity/Supply Ratio (CSR) approach, an extension of the Copeland method of analytical channel design for sand channels, balances the sediment transport capacity of a design reach with the sediment supply of a stable upstream reach over the entire flow duration curve (FDC) rather than just a single discharge. Although CSR has a stronger physical basis than previous analytical channel design approaches, it has not been adopted in practice because it can be a cumbersome and time-consuming iterative analysis without the use of software. We investigate eighteen sand-bed rivers in a comparison of designs based on the CSR approach and five single-discharge metrics: the effective discharge (Qeff), the 1.5-year recurrence interval discharge (Q1.5), the bankfull discharge (Qbf), and the discharges associated with 50th (Qs50) and 75th (Qs75) percentiles of the cumulative sediment yield curve. To facilitate this analysis we developed a novel design tool using the Visual Basic for Applications (VBA) programming language in Excel® to produce stable channel slope-width combinations based on the CSR methodology for both sand- and gravel-bed streams. The CSR Stable Channel Design Tool’s (CSR Tool) code structure was based on Copeland’s method in SAM and HEC-RAS (Hydrologic Engineering Center’s River Analysis System) and was tested with a single discharge to verify outputs. The Qs50 and Qs75 single-discharge designs match the CSR output most closely, followed by the Qbf, Qeff, and Q1.5. The Qeff proved to be the most inconsistent design metric because it can be highly dependent on the binning procedure used in the effectiveness analysis. Furthermore, we found that the more rigorous physical basis of the CSR analysis is potentially most important in designing ‘labile’ channels with highly erodible substrate, high perennial flow ‘flashiness,’ low width-to-depth ratio, and high incoming sediment load. The CSR Tool provides a resource for river restoration practitioners to efficiently utilize in-depth design techniques that can promote sediment balance in dynamic fluvial systems.

Nutrient pollution is a pervasive water quality problem. Stream restoration has been proposed as a novel approach to reduce loading and increase nutrient processing within streams. We summarize evidence from the literature on the efficacy of stream restoration for reducing nutrient loading and increasing nutrient removal in stream ecosystems. We also analyze published data on streambank phosphorus concentrations and riparian and stream denitrification rates to improve understanding of the potential benefits of stream restoration for phosphorus retention and nitrogen removal. Finally, we discuss the role of stream restoration in nutrient management and provide recommendations for practice and future research.

Theoretical approaches to magnitude-frequency analysis (MFA) of sediment transport in channels couple continuous flow probability density functions (PDFs) with power law flow-sediment transport relations (rating curves) to produce closed-form equations relating MFA metrics such as the effective discharge, Qeff, and fraction of sediment transported by discharges greater than Qeff, f+, to statistical moments of the flow PDF and rating curve parameters. These approaches have proven useful in understanding the theoretical drivers behind the magnitude and frequency of sediment transport. However, some of their basic assumptions and findings may not apply to natural rivers and streams with more complex flow-sediment transport relationships or management and design scenarios, which have finite time horizons. We use simple numerical experiments to test the validity of theoretical MFA approaches in predicting the magnitude and frequency of sediment transport. Median values of Qeff and f+ generated from repeated, synthetic, finite flow series diverge from those produced with theoretical approaches using the same underlying flow PDF. The closed-form relation for f+ is a monotonically increasing function of flow variance. However, using finite flow series, we find that f+ increases with flow variance to a threshold that increases with flow record length. By introducing a sediment entrainment threshold, we present a physical mechanism for the observed diverging relationship between Qeff and flow variance in fine and coarse-bed channels. Our work shows that through complex and threshold-driven relationships sediment transport mode, channel morphology, flow variance, and flow record length all interact to influence estimates of what flow frequencies are most responsible for transporting sediment in alluvial channels.

Changes in the physical structure of urban streams can occur abruptly due to flashy high-flow events and subsequently alter stream processes, including transient storage and nitrate uptake. We examined temporal variability in transient storage and nitrate uptake by exploring the effects of altered physical characteristics resulting from a single high-flow event in three reaches of Spring Creek, an urban stream in Fort Collins, Colorado, USA. Study reaches of varying geomorphic and hydraulic characteristics were chosen to represent distinct geomorphic settings in terms of substrate size, sinuosity, bed slope, and degree of rehabilitation and structural controls. We performed detailed physical characterizations and multiple nutrient injections of Br− and NO3− to estimate transient storage and nitrate uptake in each reach. A comparison of pre-flood and post-flood data indicates that transient storage and nitrate uptake are highly context specific and mediated by interactions between geomorphic setting and flood discharge. In the two reaches that showed significant post-flood increases in transient storage (250% to 350% increases in Fmed200), the pool-riffle reach exhibited a significant increase in uptake velocity, while the channelized reach did not. In contrast, transient storage decreased post-flood in the third reach containing hydraulic structures. These complex responses likely reflect reach-specific differences in hyporheic versus in-channel storage. This study shows that repeat injections are necessary to describe nutrient dynamics because transient storage and nitrate uptake can be highly variable over time (showing changes on the order of 100%) due to variation in discharge and geomorphically influential flow events. Copyright © 2014 John Wiley & Sons, Ltd.

Traditional approaches to water resource engineering have sought to maintain a static, optimized state of system performance in providing reliable water supplies, energy, and flood protection. However, delivery of these services has been associated with the disruption of freshwater ecosystem functioning, driving global-scale declines of biodiversity and the loss of ecosystem services. Climate change is presenting new challenges for water and ecosystem managers alike. Yet, climate change is also creating new opportunities to consider ecological resilience in the design and management of water systems. Here, we describe a set of climate-informed ecological resilience principles and associated indicators, which can support integration of ecosystem needs within water resource engineering decision-making. These have the potential to guide climate-adaptive water resource management while also provisioning broad benefits to both people and ecosystems in a shifting operating environment.

Summary Semiarid channels exhibit an extreme sensitivity to upstream urban development, particularly in unconfined valleys with unprotected grades. For example, one of our study streams in southern California has increased its cross-sectional area by nearly 14-fold relative to its pre-developed channel form in a watershed that has been only lightly developed (10.4% imperviousness). Multivariate regression models of cross-sectional channel enlargement at 61 sites were highly dependent on the ratio of post- to pre-urban sediment-transport capacity over cumulative duration simulations of 25 yrs (Lr), which explained nearly 60% of the variance. The proximity of a channel hard point such as bedrock or artificial grade control was also significant, indicating that channel enlargement increased moving upstream from grade control. The enlargement models point to the importance of balancing the post-developed sediment transport to the pre-developed setting over an entire range of flows rather than a single flow in order to reduce the risk of adverse channel responses to hydromodification. The need for controlling a wide range of flows was underscored by logistic-regression analyses that indicated a high risk of instability in systems with Lr > 1, especially for fine-grained systems (i.e., d50

Dams and water diversions can dramatically alter the hydraulic habitats of stream ecosystems. Predicting how water depth and velocity respond to flow alteration is possible using hydraulic models, such as Physical Habitat Simulation (PHABSIM); however, such models are expensive to implement and typically describe only a short length of stream (102 m). If science is to keep pace with development, then more rapid and cost-effective models are needed. We developed a generalized habitat model (GHM) for brown and rainbow trout that makes similar predictions to PHABSIM models but offers a demonstrated reduction in survey effort for Colorado Rocky Mountain streams. This model combines the best features of GHMs developed elsewhere, including the options of desktop (no-survey) or rapid-survey models. Habitat–flow curves produced by PHABSIM were simplified to just two site-specific components: (i) Q95h (flow at 95% of maximum habitat) and (ii) Shape. The Shape component describes the habitat–flow curves made dimensionless by dividing flow increments by Q95h and dividing habitat (weighted usable area) increments by maximum habitat. Both components were predicted from desktop variables, including mean annual flow, using linear regression. The rapid-survey GHM produced better predictions of observed habitat than the desktop GHM (rapid-survey model explained 82–89% variance for independent validation sites; desktop 68–85%). The predictive success of these GHMs was similar to other published models, but survey effort to achieve that success was substantially reduced. Habitat predicted by the desktop GHM (using geographic information system data) was significantly correlated with the abundance of large brown trout (p

The river management and restoration community has devoted much effort to predicting the bankfull discharge, Qbf, and associated channel geometry at Qbf for the purposes of channel study, classification, and design. Four types Qbf prediction methods predominate: (1) direct estimation based on field indicators of bankfull stage, (2) downstream hydraulic geometry equations, (3) a flood peak discharge with a specified return interval based on the annual maximum flood series (e.g., the 1.5- to 2-year flood) or regional flood peak statistical relations, and (4) process-based approaches that incorporate the magnitude and frequency of sediment transport such as the most effective discharge Qeff. Process-based Qbf predictors are calculated using sediment transport data from 95 gauged sites across the United States including coarse, bed load–dominated channels and fine, suspended load–dominated channels with drainage areas ranging from 6 to 1.4×105 km2. These values are compared with observations of Qbf m...

Changes in streamflow and sediment loading associated with urban development have the potential to exacerbate channel erosion, and result in impacts to wetland, riparian, and stream habitats, as well as infra- structure and property losses. The typical ''one-size-fits-all'' management prescription of flow control with reten- tion or detention basins has not been wholly effective, pointing to a need for improved management strategies and tools for mitigating the impacts of ''hydromodification.'' We present an approach for developing screening- level tools for assessing channel susceptibility to hydromodification, and describe a novel tool for rapid, field- based assessments of the relative susceptibility of stream segments. The tool is based on the results of extensive field surveys, which indicate that susceptibility is the driver of channel response, not the magnitude of urbaniza- tion. A combination of relatively simple, but quantitative, field indicators are used as input parameters for a set of decision trees that follow a logical progression in assigning categorical susceptibility ratings to the channel segment being assessed. The susceptibility rating informs the level of data collection, modeling, and ultimate mitigation efforts that can be expected for a particular stream segment type. The screening approach represents a critical first step toward tailoring hydromodification management strategies and mitigation measures to differ- ent stream types and geomorphic settings. (KEY TERMS: fluvial geomorphology; urbanization; stormwater; channel evolution; channel instability; risk; adaptive management.)

Hawley, Robert J., Brian P. Bledsoe, Eric D. Stein, and Brian E. Haines, 2012. Channel Evolution Model of Semiarid Stream Response to Urban-Induced Hydromodification. Journal of the American Water Resources Association (JAWRA) 48(4): 722-744. DOI: 10.1111/j.1752-1688.2012.00645.x Abstract: We present a novel channel evolution model (CEM) that qualitatively describes morphologic responses of semiarid channels to altered hydrologic and sediment regimes associated with urbanization (hydromodification). The CEM is based on southern California data from 83 detailed channel surveys, hundreds of synoptic surveys, and historical analyses of aerial photographs along 14 reaches. Channel evolution sometimes follows the well-known sequence described by Schumm et al. (Incised Channels: Morphology, Dynamics, and Control, Water Resources Publications, Littleton, Colorado, 1984) for incising, single-thread channels; however, departures from this sequence are common and include transitions of single thread to braided evolutionary endpoints, as opposed to a return to quasi-equilibrium single-thread planform. Thresholds and risk factors associated with observed channel response are also presented. In particular, distance to grade control and network position emerged as key controls on channel response trajectory. The CEM and quantitative extensions provide managers with a framework for understanding channel responses and rehabilitation alternatives, and may be transferable to other semiarid settings. It also offers insights regarding channel susceptibility to hydromodification, highlights key boundary conditions for high-risk channels, and underscores critical knowledge gaps in predicting the complex, discontinuous response trajectories that are highly prevalent in urbanized watersheds.

summary In 15 mountain stream reaches containing instream wood, we characterized velocity and flow resistance at bankfull through low flows. These data were: (1) used to assess the accuracy of previously published velocity prediction techniques for high-gradient channels; and (2) were combined with field data from other studies to develop general methodologies for predicting velocity and flow resistance in alluvial and mixed alluvial-bedrock channels both with and without step-forming instream wood. Velocity and flow resistance were poorly predicted by variables characterizing grain size and relative grain submergence. Conversely, methods based on detrended standard deviation of bed elevations (rz) and relative bedform submergence (h/rz) explained up to 84% of the variance in the measured flow resistance coefficients and 97% of the variance in dimensionless velocity. With an average velocity of 0.44 m/s for the collected measurements, velocity was predicted with RMS (root mean square) error as low as 0.071 m/s (16% of average) when discharge and bedform geometry is known and 0.10 m/s (23%) when only bedform geometry is known. Additionally, an empirical relationship indicates V=u � ¼ h=r z, supporting previouslypublished laboratory findings using a field-based dataset in complex high-gradient channels. Interactions between instream wood and clasts result in substantially enhanced step heights and flow resistance. This compound effect defies description by grain size and relative grain submergence. However, rz and h/rz quantify variability due to both clasts in combination with wood and clasts alone, providing relatively accurate predictions for the tested dataset and indicating substantial predictive capabilities in channels where bedforms are the primary source of flow resistance. Published by Elsevier B.V.

The understanding of nutrient uptake in streams is impeded by a limited understanding of how geomorphic setting and flow regime interact with biogeochemical processing. This study investigated these interactions as they relate to transient storage and nitrate uptake in small agricultural and urban streams. Sites were selected across a gradient of channel conditions and management modifications and included three 180-m long geomorphically distinct reaches on each of two streams in north-central Colorado. The agricultural stream has been subject to historically variable cattle-grazing practices, and the urban stream exhibits various levels of stabilisation and planform alteration. Reach-scale geomorphic complexity was characterised using highly detailed surveys of channel morphology, substrate, hydraulics and habitat units. Breakthrough-curve modelling of conservative bromide (Br−) and nonconservative nitrate (NO3−) tracer injections characterised transient storage and nitrate uptake along each reach. Longitudinal roughness and flow depth were positively associated with transient storage, which was related to nitrate uptake, thus underscoring the importance of geomorphic influences on stream biogeochemical processes. In addition, changes in geomorphic characteristics due to temporal discharge variation led to complex responses in nitrate uptake. Copyright © 2011 John Wiley & Sons, Ltd.

s u m m a r y Forty-three US Geological Survey gauges with records greater than �15 yrs located in watersheds less than �250 km 2 were used to model the effects of suburbanization on streams in semi-arid southern California. The watersheds spanned a gradient of urban development, ranging 0–23% total impervious area in 2001. With little flow control at the subdivision scale, most impervious area in the region is relatively well-connected to surface-drainage networks and hydrologically effective. Consequently, total impervious area was an effective hydrologic surrogate for urbanization, emerging from an expansive array of geospatially-derived hydrologic variables as a statistically-significant ( p < 0.05) predictor of instantaneous peak-flow rates at the 1.5- and 2-yr recurrence intervals and the durations of all geomorphically-important flows. To represent the effects of urbanization on flow durations, we developed duration density functions by using power functions (typical R 2

Growing water demand across the world is increasing the stress on river ecosystems, causing concern for both biodiversity and people. River-specific environmental flow assessments cannot keep pace with the rate and geographic extent of water development. Society needs methods to assess ecological impacts of flow management at broad scales so that appropriate regional management can be implemented. To meet this need in Colorado, USA, we developed a Watershed Flow Evaluation Tool (WFET) to estimate flow-related ecological risk at a regional scale. The WFET entails four steps: (i) modelling natural and developed daily streamflows; (ii) analysing the resulting flow time series; (iii) describing relationships between river attributes and flow metrics (flow–ecology relationships); and (iv) mapping of flow-related risk for trout, native warm-water species and riparian plant communities. We developed this tool in two watersheds with differing geomorphic settings and data availability. In one of the two watersheds, the WFET was successfully implemented to assess ecological risk across the 3400-km2 watershed, providing consistent watershed-wide information on flow-related risk. In the other watershed, active channel change and limited data precluded a successful application. In Colorado, the WFET will be used to evaluate the risk of impacts on river ecosystems under future climate change and water development scenarios (e.g. for energy development or municipal water supply). As water continues to be developed for people, the WFET and similar methods will provide a cost-effective means to evaluate and balance ecosystem needs at large scales. Copyright © 2011 John Wiley & Sons, Ltd.

Reduced streamflow via flow diversion has the potential to limit the sediment-transport capacity of downstream channels and lead to accumulation of fine sediments and habitat degradation. To investigate, we examined the effects of variable levels of flow diversion on fine-sediment deposition, hydraulic conditions and geomorphic alteration. Our study consisted of a detailed field analysis pairing reaches above and below diversion dams on 13 mountain streams in north-central Colorado and southern Wyoming USA. Diversions are ubiquitous across the American West, yet previous comparative studies on the effects of flow diversion have yielded mixed results. Through application of strict site-selection criteria, multiple fine-sediment measures, and an intensive sampling scheme, this study found that channels downstream of diversions contained significantly more fine sediment and slow-flowing habitat as compared to upstream control reaches. Susceptibility to fine-sediment accumulation was associated with decreasing basin size, decreasing bankfull depth and smaller d84, and it appears to be magnified in streams of less than 3% slope. Copyright © 2010 John Wiley & Sons, Ltd.

Mountain snowpack is the main source of water in the semi-arid Colorado River Basin (CRB), and while the demands for water are increasing, competing and often conflicting, the supply is limited and has become increasingly variable over the 20th Century. Greater variability is believed to contribute to lower accuracy in water supply forecasts, plus greater variability violates the assumption of stationarity, a fundamental assumption of many methods used in water resources engineering planning, design and management. Thus, it is essential to understand the underpinnings of hydroclimatic variability in order to accurately predict effects of climate changes and effectively meet future water supply challenges. A new methodology was applied to characterized time series of temperature, precipitation, and streamflow (i.e., historic and reconstructed undepleted flows) according to the three climate regimes that occurred in CRB during the 20th Century. Results for two tributaries in the Upper CRB show that hydroclimatic variability is more deterministic than previously thought because it entails complementary temperature and precipitation patterns associated with wetter or drier conditions on climate regime and annual scales. Complementary temperature and precipitation patterns characterize climate regime type (e.g., cool/wet and warm/dry), and the patterns entail increasing or decreasing temperatures and changes in magnitude and timing of precipitation according to the climate regime type. Accompanying each climate regime on annual scales are complementary temperature (T) and precipitation (P) patterns that are associated with upcoming precipitation and annual basin yield (i.e., total annual flow volume at a streamflow gauge). Annual complementary T and P patterns establish by fall, are detectable as early as September, persist to early spring, are related to the relative magnitude of upcoming precipitation and annual basin yield, are unique to climate regime type, and are specific to each river basin. Thus, while most of the water supply in the Upper CRB originates from winter snowpack, statistically significant indictors of relative magnitude of upcoming precipitation and runoff are evident in the fall, well before appreciable snow accumulation. Results of this study suggest strategies that may integrated into existing forecast methods to potentially improve forecast accuracy and advance lead time by as much as six months (i.e., from April 1 to October 1 of the previous year). These techniques also have applications in downscaling climate models and in river restoration and management.

Detailed mechanistic modeling of hydrogeomorphic processes in fluvial systems is extremely chal- lenging, expensive, and of limited usefulness without explicit knowledge of prediction uncertainty. Accordingly, there is a need for parsimonious tools that support probabilistic scientific assessments of physical-biological link- ages in streams and rivers. This paper introduces GeoTools, a suite of analysis tools for fluvial systems written in Visual Basic for Applications ⁄Excel. Based on flow time series and basic geomorphic data, GeoTools auto- mates computation of numerous hydrologic, hydraulic, and geomorphic descriptors including effective discharge, sediment transport and yield, temporal distributions of hydraulic parameters (e.g., shear stress and specific stream power), cumulative erosion potential, channel stability indices, and over 100 flow regime metrics. GeoTools accepts input flow records in standard USGS format and a variety of other formats and temporal den- sities. The package also serves as a post-processor for SWMM, and HSPF ⁄BASINS model output. Three case studies illustrate specific applications of GeoTools: a channel restoration project, a stormwater manage- ment ⁄hydromodification study, and an analysis of the effects of flow regulation below an impoundment dam. (KEY TERMS: decision support systems; planning; environmental indicators; fluvial processes; urbanization; sediment transport; geomorphology; restoration.)

Streamflow prediction in ungauged basins provides essential information for water resources planning and management and ecohydrological studies yet remains a fundamental challenge to the hydrological sciences. A methodology is presented for stratifying streamflow regimes of gauged locations, classifying the regimes of ungauged streams, and developing models for predicting a suite of ecologically pertinent streamflow metrics for these streams. Eighty-four streamflow metrics characterizing various flow regime attributes were computed along with physical and climatic drainage basin characteristics for 150 streams with little or no streamflow modification in Colorado, Washington, and Oregon. The diverse hydroclimatology of the study area necessitates flow regime stratification and geographically independent clusters were identified and used to develop separate predictive models for each flow regime type. Multiple regression models for flow magnitude, timing, and rate of change metrics were quite accurate with many adjusted R2 values exceeding 0.80, while models describing streamflow variability did not perform as well. Separate stratification schemes for high, low, and average flows did not considerably improve models for metrics describing those particular aspects of the regime over a scheme based on the entire flow regime. Models for streams identified as ‘snowmelt’ type were improved if sites in Colorado and the Pacific Northwest were separated to better stratify the processes driving streamflow in these regions thus revealing limitations of geographically independent streamflow clusters. This study demonstrates that a broad suite of ecologically relevant streamflow characteristics can be accurately modeled across large heterogeneous regions using this framework. Applications of the resulting models include stratifying biomonitoring sites and quantifying linkages between specific aspects of flow regimes and aquatic community structure. In particular, the results bode well for modeling ecological processes related to high-flow magnitude, timing, and rate of change such as the recruitment of fish and riparian vegetation across large regions.

The importance of hydrologic variability in sustaining natural riverine ecosystems is now well accepted. Over the last 15 years or so, many typologies and assessment tools have been developed to assist ecologists and managers in describing natural flow regimes in quantitative terms. In the course of this recent progress, however, some critical questions have arisen concerning the degree to which generalizations about flow regime characteristics are geographically dependent both within and among regions, and the degree to which flow variability alone captures critical environmental variability. In this paper we address these issues in a hierarchical framework that allows comparative statements about hydrologic variability to be made a multiple spatial scales, from local to global. First, we examined hydrologic variability among 463 readily available daily streamflow gauges from five continents/ countries around the world: Australia, New Zealand, South Africa, Europe, and the United States. Using ordination and clustering techniques, we identified similarities and differences among these gauges. We found that the US gauges exhibited the greatest overall flow variability among a suite of 66 hydrologic indicators, whereas Australian streams showed the greatest influence by interannual variability in flow. Similarities in overall flow regime were greatest between Australia and the US, whereas New Zealand streams were most regionally distinctive. These results support the idea of intercontinental distinction in streamflow variability at a global scale; however, they also point to important similarities in flow characteristics among continents/countries. Second, within the continental United States, we examined how hydrologic variability changes along river profiles as catchment area increases for five river basins arrayed across a gradient of hydroclimatic variation. Using historical streamflow records that precede river impoundment, we found that small 'headwater' streams exhibit the greatest similarity in flow characteristics across the basins, as compared to mid-sized and larger river reaches, which often diverged among the rivers. These results reveal the importance of more carefully defining the spatial domain of allowable hydrologic extrapolation from individual stream gauges and emphasize the need to stratify within basins when considering hydrologic variability at regional scales. Third, we used a modeling approach to illustrate how geomorphic setting provides a context for assessing the ecological consequences of flow variation at the local scale of stream reaches. For modeled channels having the same sediment size distribution but with either entrenched or floodplain morphology, we found that the effective regime of bed movement for three hydrologically distinct streams depended as much on geomorphic setting as on flow regime per se. These results emphasize the need to integrate hydrology with geomorphology to characterize 'disturbance regimes' at the channel reach scale to allow generation of spatially explicit mapping of flow-mediated habitat dynamics for entire drainage networks within specific regions. In sum, if riverine scientists wish to develop a general framework for comparing hydrologic variability across basins, regions, and continents, a hierarchical approach is advised. At very broad scales, intercontinental differences in flow regimes could allow a stratification of basins to identify similar hydroecological settings. Within continents or hydroclimatically similar regions, finer-scale spatial analysis of flow regime types would further assist in hydrologic stratification, based only on the regionally-relevant components of flow variability. Finally, within hydrologically homogeneous sub-regions, geomorphic stratification could be applied to identify stream reaches or segments having similar hydrogeomorphic properties.

An extensive group of datasets was analyzed to examine factors affecting widths of streams and rivers. Results indicate that vegetative controls on channel size are scale dependent. In channels with watersheds greater than 10 to 100 km 2 , widths are narrower in channels with thick woody bank vegetation than in grass lined or nonforested banks. The converse is true in smaller streams apparently due to interactions between woody debris, shading, understory vegetation, rooting characteristics, and channel size. A tree based statistical method (regression tree) is introduced and tested as a tool for identifying thresholds of response and interpreting interactions between variables. The implications of scale dependent controls on channel width are discussed in the context of stable channel design methods and development of regional hydraulic geometry curves. (KEY TERMS: river restoration; river engineering; stable channel design; fluvial geomorphology; hydraulics; watershed management).

Hydrologic and sediment transport modeling were used to examine the effectiveness of typical stormwater management policies in reducing the potential for stream-channel erosion. Two bedload functions and three total-load transport relationships were applied to 8-mm gravel and 0.5-mm sand bed materials to compare the performance of the relationships in estimating detention requirements across modes of sediment transport. The various sediment-transport relationships yielded widely diverging estimates of sediment-transport capacity and yet suggested detention volume requirements that agreed within 20%. Detention design for control of cumulative sediment load required a detention storage volume 61% greater than a peak control detention facility and resulted in an altered temporal distribution of sub-bank-full shear stress. Design of stormwater facilities based on time-integrated sediment-transport capacity may inadvertently result in channel instability and substrate changes unless the approach accounts for the frequency distribution of sub-bank-full flows, the capacity to transport heterogeneous bed and bank materials, and potential shifts in inflowing sediment loads.

Incised channels are caused by an imbalance between sediment transport capacity and sediment supply that alters channel morphology through bed and bank erosion. Consistent sequential changes in incised channel morphology may be quantified and used to develop relationships describing quasi-equilibrium conditions in these channels. We analyzed the hydraulic characteristics of streams in the Yazoo River Basin, Mississippi in various stages of incised channel evolution. The hydraulic characteristics of incising channels were observed to follow the sequence predicted by previous conceptual models of incised channel response. Multiple regression models of stable slopes in quasi-equilibrium channels that have completed a full evolutionary sequence were developed. These models compare favorably with analytical solutions based on the extremal hypothesis of minimum stream power and empirical relationships from other regions. Appropriate application of these empirical relationships may be useful in preliminary design of stream rehabilitation strategies.

Incised channels are caused by an imbalance between sediment transport capacity and sediment supply to the stream. The resulting bed and bank erosion alter channel morphology and stability. Geomorphological models of incised channel evolution can provide guidance in the selection of engineering design alternatives for incised channel rehabilitation. This paper describes how incised channel evolution models may be coupled with a dimensionless stability diagram to facilitate evaluation of rehabilitation alternatives. In combination, the models provide complementary views of channel processes from geomorphic and engineering perspectives.

In the 1990's, a number of best management practices (BMPs) manuals have been developed that address the control of urban runoff to protect receiving water quality. More recently, several papers have investigated the effectiveness of these BMPs in protecting small urban watercourses, and have concluded that they do not. Investigations of both design practices and effectiveness reveals that there is a lot of ignorance in the scientific and engineering community about what constitutes a properly designed BMP and what it really achieves, with respect to environmental protection. This paper discusses the state-of-practice in BMP design in the United States and points out its strengths and weaknesses with respect to real protection of the downstream receiving water environment. The paper recommends an approach to design criteria development that can be applied over a wide variety of climatologic, topologic, and geologic conditions to protect receiving waters systems.

Channel instability and aquatic ecosystem degradation have been linked to watershed imperviousness in humid regions of the U.S. In an effort to provide a more process-based linkage between observed thresholds of aquatic ecosystem degradation and urbanization, standard single event approaches (U.S. Geological Survey Flood Regression Equations and rational) and continuous hydrologic models (HSPF and CASC2D) were used to examine potential changes in flow regime associated with varying levels of watershed imperviousness. The predicted changes in flow parameters were then interpreted in concert with risk-based models of channel form and instability. Although low levels of imperviousness (10 to 20 percent) clearly have the potential to destabilize streams, changes in discharge, and thus stream power, associated with increased impervious area are highly variable and dependent upon watershed-specific conditions. In addition to the storage characteristics of the pre-development watershed, the magnitude of change is sensitive to the connectivity and conveyance of impervious areas as well as the specific characteristics of the receiving channels. Different stream types are likely to exhibit varying degrees and types of instability, depending on entrenchment, relative erodibility of bed and banks, riparian condition, mode of sediment transport (bedload versus suspended load), and proximity to geomorphic thresholds. Nonetheless, simple risk-based analyses of the potential impacts of land use change on aquatic ecosystems have the potential to redirect and improve the effectiveness of watershed management strategies by facilitating the identification of channels that may be most sensitive to changes in stream power.

[1] Total flow resistance can be partitioned into its components of grain (ffgrain), form (ffstep), wood (ffwood), and spill (ffspill) resistance. Methods for partitioning flow resistance developed for low-gradient streams are commonly applied to high-gradient systems. We examined the most widely used methods for calculating each component of resistance, along with the limitations of these methods, using data gathered from 15 high-gradient (0.02 < S0 < 0.195) step-pool, cascade, and plane-bed reaches in Fraser Experimental Forest. We calculated grain resistance using three equations that relate relative submergence (R/Dm) to ffgrain as well as using an additive drag approach. The drag approach was also used for calculating ffwood and ffstep. The ffgrain contributed the smallest amount toward all reaches at all flows, although the value varied with the method used. The Parker and Peterson (1980) equation using D90 best represented ffgrain at high flows, whereas the Keulegan (1938) equation using D50 best characterized ffgrain at base flows, giving a lower bound for grain resistance. This suggests that ffgrain may be better represented if two grain sizes are used to calculate this component of resistance. The drag approach, which is used to calculate wood resistance, overestimated the significance of individual logs in the channel. The contribution of ffspill was reduced at higher flows when form drag around the step is accounted for at higher flows. We propose a method for evaluating the contribution of ffstep that accounts for form drag around the steps once they are submerged at higher flows. We evaluated the potential sources of error for the estimation of each component of resistance. Determination of the drag coefficient was one of the major sources of error when calculating drag around wood, steps, or boulders.

[1] Detailed channel and water surface surveys were conducted on 15 mountain stream reaches (9 step-pool, 5 cascade, and 1 plane-bed) using a tripod-mounted Light Detection and Ranging scanner and laser theodolite. Reach-average velocities were measured at varying discharges with dye tracers and fluorometers. Multiple regressions and analysis of variance tests were used to test hypothesized correlations between Darcy-Weisbach friction coefficient, f, and potential control variables. Gradient (S0) and relative grain submergence (Rh/D84) individually explained a low proportion of the variability in f (R2 = 0.18), where Rh is hydraulic radius, D84 is the 84th percentile of the cumulative grain size distribution, and R2 is equal to the coefficient of determination. Because channel type, grain size, and S0 are interrelated, we tested the hypothesis that f is highly correlated with all three of these variables or a combination of the above variables with flow period (a categorical variable) or dimensionless unit discharge (q*). Total resistance correlated strongly (adj-R2 = 0.74, 0.69, and 0.64) with S0, flow period, wood load (volume of wood/m2 of channel), q*, and channel type (step-pool, cascade, plane-bed). Total resistance differed between step-pool and plane-bed and between cascade and plane-bed reaches. Significant differences in f in step-pool and cascade reaches were found at the same values of flow and S0. The regression analyses indicate that discharge explains the most variability in f, followed by S0 when discharge is similar among channel reaches, but that Rh/D84 is not an appropriate variable in these steep mountain streams to represent variations in both resistance and discharge. Results also indicate that the forms of resistance among channel types are sufficiently different to change the relationship of the control variables with f in each channel type. These results can be used to further the development of predictive equations for high-gradient mountain streams.

Received 28 April 2005; revised 6 January 2006; accepted 10 March 2006; published 20 June 2006. [1] Channel types found in mountain drainages occupy characteristic but intergrading ranges of bed slope that reflect a dynamic balance between erosive energy and channel boundary resistance. Using a classification and regression tree (CART) modeling approach, we demonstrate that drainage area scaling of channel slopes provides better discrimination of these forms than slope alone among supply- and capacity-limited sites. Analysis of 270 stream reaches in the western United States exhibiting four common mountain channel types reveals that these types exist within relatively discrete ranges of an index of specific stream power. We also demonstrate associations among regional interannual precipitation variability, discharge distribution skewness, and means of the specific stream power index of step-pool channels. Finally, we discuss a conceptual methodology for predicting ecologically relevant morphologic units from digital elevation models at the network scale based on the finding that channel types do not exhibit equal energy dissipation.