Brian P. Bledsoe

Stream restoration strategies for reducing river nitrogen loads

Despite decades of work on implementing best management practices to reduce the movement of excess nitrogen (N) to aquatic ecosystems, the amount of N in streams and rivers remains high in many watersheds. Stream restoration has become increasingly popular, yet efforts to quantify N-removal benefits are only just beginning. Natural resource managers are asking scientists to provide advice for reducing the downstream flux of N. Here, we propose a framework for prioritizing restoration sites that involves identifying where potential N loads are large due to sizeable sources and efficient delivery to streams, and when the majority of N is exported. Small streams (1st–3rd order) with considerable loads delivered during low to moderate flows offer the greatest opportunities for N removal. We suggest approaches that increase in-stream carbon availability, contact between the water and benthos, and connections between streams and adjacent terrestrial environments. Because of uncertainties concerning the magnitud...

Logistic analysis of channel pattern thresholds: meandering, braiding, and incising

A large and geographically diverse data set consisting of meandering, braiding, incising, and post-incision equilibrium streams was used in conjunction with logistic regression analysis to develop a probabilistic approach to predicting thresholds of channel pattern and instability. An energy-based index was developed for estimating the risk of channel instability associated with specific stream power relative to sedimentary characteristics. The strong significance of the 74 statistical models examined suggests that logistic regression analysis is an appropriate and effective technique for associating basic hydraulic data with various channel forms. The probabilistic diagrams resulting from these analyses depict a more realistic assessment of the uncertainty associated with previously identified thresholds of channel form and instability and provide a means of gauging channel sensitivity to changes in controlling variables.

Developing linkages between species traits and multiscaled environmental variation to explore vulnerability of stream benthic communities to climate change

Abstract Forecasting responses of benthic community structure and function to anthropogenic climate change is an emerging scientific challenge. Characterizing benthic species by biological attributes (traits) that are responsive to temperature and streamflow conditions can support a mechanistic approach for assessing the potential ecological responses to climate change. However, nonclimatic environmental factors also structure benthic communities and may mitigate transient climatic conditions, and these must be considered in evaluating potential impacts of climate change. Here we used macroinvertebrate and environmental data for 279 reference-quality sites spanning 12 states in the western US. For each sampling location, we described 45 environmental variables that spanned reach to catchment scales and that represented contemporary climate drivers, hydrologic metrics, and nonclimatic habitat features, as well as purely spatial metrics. We described benthic community composition at each site in terms of 7 species traits, including those considered sensitive to temperature increases and streamflow changes. All combined environmental variables explained 67% of the total trait variation across the sites, and catchment-scale climatic and hydrologic variables independently accounted for 19%. Sites were clustered into 3 community types based on trait composition, and a classification-tree analysis confirmed that climatic and hydrologic variables were important in partitioning these groups. Sensitivity of benthic communities to projected climate change was assessed by quantifying the proportion of taxa at sites having the traits of either cold stenothermy or obligate rheophily. Regression-tree analysis showed that temperature and hydrologic variables mostly accounted for the differences in proportion of sensitivity traits across the sites. We examined the vulnerability of sites to climate change by superimposing regional-scale projections of late-21st-century temperature and runoff change on the spatial distribution of temperature- and runoff-sensitive assemblages. Sites with high proportions of cold stenotherms and obligate rheophiles occur throughout the western US, but the degree of temperature and runoff change is projected to be greatest for reference sites in the Upper Colorado River and Great Basin. Thus, our results suggest that traits-based sensitivity coupled with intraregional variation in projected changes in temperature and runoff will cause reference sites in the western US to be differentially vulnerable to future climate change.

VEGETATION ALONG HYDROLOGIC AND EDAPHIC GRADIENTS IN A NORTH CAROLINA COASTAL PLAIN CREEK BOTTOM AND IMPLICATIONS FOR RESTORATION

We described the vegetation of two alluvial swamp forest stands along Durham Creek in Beaufort County, North Carolina, USA in relation to elevation, hydrologic, and edaphic gradients. Over 3,000 surveyed elevations of individual plant microsites were used in conjunction with 26 years of stream gage data to examine individual species responses to annual and growing season flooding frequencies. Direct gradient analyses combined with plot ordinations derived from detrended correspondence analysis and canonical correspondence analysis suggested that differences in vegetation between the stands were primarily the result of variations in elevation, growing season flooding frequency, percent base saturation, exchangeable acidity, and soil physical properties. Although the stands were less than 4.5 km apart and without significant intermediate tributaries, growing season flooding frequency and duration were magnified in the lowest elevations of the downstream stand. An elevation difference of as little as 10 cm resulted in a 20% difference in the frequency of surface flooding during the growing season. Species distributions were significantly correlated with depth to mottling (r2=0.75), flooding frequency (r2=−0.57), elevation (r2=0.70), and several soil chemical properties. The two stands had very similar annual surface flooding regimes, but subtle differences in growing season flooding frequency, soil characteristics, and disturbance history have apparently resulted in dissimilar plant community composition and structure. These results suggest that the lack of quantitative data on vegetation-environment interactions occurring at the microtopographic scale (10−1 m) in alluvial swamp forests makes precise prediction, planning, or design of created or restored wetland composition and function a formidable challenge.

Channel‐reach morphology dependence on energy, scale, and hydroclimatic processes with implications for prediction using geospatial data

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.

Management of Large Wood in Streams: An Overview and Proposed Framework for Hazard Evaluation†

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.

Physical Effects of Wet Weather Flows on Aquatic Habitats: Present Knowledge and Research Needs

This study explores the current state of knowledge with respect to the effects of wet weather flows from urban areas on the physical character of aquatic habitat. It identifies knowledge gaps with respect to our ability to define the cause-effect relationships, examines the comprehensiveness of the data used in support of the published literature in the subject area, and makes a qualitative determination of the usefulness of those data for further analysis to increase our knowledge in the subject area. Finally, it recommends further research studies that will increase our knowledge in the subject area, with emphasis on pilot-scale projects that can be used to develop practical protocols for preventing or mitigating the effects. Major findings and conclusions are: 1) we lack a solid conceptual framework for predicting the impact of large-scale watershed modifications and wet weather flows on ecological processes that influence stream communities; 2) there is a need for longer-term monitoring; 3) there is no widely accepted system for quantifying geomorphic instability and degradation of physical habitat; 4) there is a need for process-based stream classification; 5) specific links between urbanization characteristics and stream degradation are lacking; 6) there is a need for urban best management practice (BMP) assessment standards; and 7) developing a multi-scale understanding of habitat potential in human-dominated watersheds is needed. The report recommends a research program that first and foremost, includes comprehensive, long-term monitoring augmented with mathematical modeling of the linkages between development style/drainage system design, flow regime, and multi-scale changes in physical habitat and biotic response. Improved diagnosis and predictive understanding of future change require multifaceted, multiscale, and multidisciplinary studies based on a firm understanding of the history and processes operating in a drainage basin. Detailed long-term analyses of the influence of hydrologic regime and channel morphology on differences between communities in recruitment, immigration/emigration, mortality, and age structure are also needed. Finally, future research should directly examine tradeoffs between: 1) flood mitigation versus channel roughness, habitat heterogeneity, debris inputs, and riparian protection; 2) chemical water quality improvement through extended detention versus geomorphically-based flow regime controls; and, 3) rehabilitation of aquatic habitat using static features versus allowing the potential for dynamic adjustments in channel form and habitat structure. It is extremely important that the research be pragmatic, and focus on developing pilot/demonstration studies that will lead to design guidance that municipalities can use to design new systems, or improve existing systems, that will protect not only the safety and welfare of the citizenry that it serves, but also the aquatic ecosystems in the streams that receive the wet weather discharges from these urbanized sites. This title belongs to WERF Research Report Series ISBN: 9781843396482 (Print) ISBN: 9781780403243 (eBook)

Relationships of Stream Responses to Hydrologic Changes

Rural to urban land use change is a ubiquitous and formidable challenge in watershed management. Decades of research have revealed that urbanization frequently results in severe stream degradation, but the complexity and variability of stream responses inhibit prediction and informed decision-making. Associations between gross measures of total imperviousness or human population and stream characteristics provide little meaningful feedback for understanding key processes and creating practical mitigation strategies. In contrast, examining the effectiveness of mitigation strategies relative to fundamental biophysical linkages provides a foundation for improved management of aquatic ecosystems in rapidly changing watersheds. The objective of this paper is to provide a process-oriented view of what is known about the physical response of streams to urbanization and stormwater controls, to identify some critical information gaps, and to suggest useful approaches and analysis tools for filling these gaps. In particular, variable responses to altered flow and sediment regimes across different stream types, riparian conditions, and spatio-temporal scales are considered. Decision-based models of channel instability that account for the relative sensitivity of stream types to changes in flow and sediment regimes can improve our ability to set priorities and tailor mitigation strategies to the response potential of receiving waterbodies.

Streams and Urbanization

“Urbanization” encompasses a diverse array of watershed alterations that influence the physical, chemical, and biological characteristics of streams. In this chapter, we summarize lessons learned from the last half century of research on urban streams and provide a critique of various mitigation strategies, including recent approaches that explicitly address geomorphic processes. We focus first on the abiotic conditions (primarily hydrologic and geomorphic) and their changes in streams that accompany urbanization, recognizing that these changes may vary with geomorphic context and climatic region. We then discuss technical approaches and limitations to (1) mitigating water-quantity and water-quality degradation through site design, riparian protection, and structural stormwater-management strategies; and (2) restoring urban streams in those watersheds where the economic, social, and political contexts can support such activities.

Comment on Lewin and Brewer (2001): ¿Predicting channel patterns¿, Geomorphology 40, 329¿339

With the aim of assessing basic alluvial channel planforms as a function of the main determining parameters, a number of stability diagrams have been published during the past several decades, starting with the well-known plots of channel slope versus bankfull discharge of Leopold and Wolman (1957) and Lane (1957). In more recent versions of these stability diagrams, a parameter representing flow energy is plotted against some geometric or grainsize parameter. In one way or another, the diagrams indicate that straight, meandering and braided patterns represent a trend of increasing flow energy (sensu Ferguson, 1987, and Knighton and Nanson, 1993). However, the discriminators could not be used in a truly predictive way, as the value of one or both of the ‘‘independent’’ variables was predicated upon a priori knowledge of one or more geometric properties of the pattern that was to be predicted, such as the bankfull width, depth or slope of the channel (Parker, 1976; Fredsøe, 1978; Struiksma and Klaassen, 1988). Therefore, a diagram was proposed by the first author using the parameters potential stream power (based on valley gradient as opposed to channel gradient) and median grainsize, variables that can be considered almost independent of channel pattern (Van den Berg, 1995). Potential specific stream power, x, was defined as: