Linking Hydrologic Alteration to Biological Impairment in Urbanizing Streams of the Puget Lowland,Washington, USA1

Abstract: We used a retrospective approach to identify hydrologic metrics with the greatest potential for ecological relevance for use as resource management tools (i.e., hydrologic indicators) in rapidly urbanizing basins of the Puget Lowland. We proposed four criteria for identifying useful hydrologic indicators: (1) sensitive to urbanization consistent with expected hydrologic response, (2) demonstrate statistically significant trends in urbanizing basins (and not in undeveloped basins), (3) be correlated with measures of biological response to urbanization, and (4) be relatively insensitive to potentially confounding variables like basin area. Data utilized in the analysis included gauged flow and benthic macroinvertebrate data collected at 16 locations in 11 King County stream basins. Fifteen hydrologic metrics were calculated from daily average flow data and the Pacific Northwest Benthic Index of Biological Integrity (B‐IBI) was used to represent the gradient of response of stream macroinvertebrates to urbanization. Urbanization was represented by percent Total Impervious Area (%TIA) and percent urban land cover (%Urban). We found eight hydrologic metrics that were significantly correlated with B‐IBI scores (Low Pulse Count and Duration; High Pulse Count, Duration, and Range; Flow Reversals, T_Qmean, and R‐B Index). Although there appeared to be a great deal of redundancy among these metrics with respect to their response to urbanization, only two of the metrics tested – High Pulse Count and High Pulse Range – best met all four criteria we established for selecting hydrologic indicators. The increase in these high pulse metrics with respect to urbanization is the result of an increase in winter high pulses and the occurrence of high pulse events during summer (increasing the frequency and range of high pulses), when practically none would have occurred prior to development. We performed an initial evaluation of the usefulness of our hydrologic indicators by calculating and comparing hydrologic metrics derived from continuous hydrologic simulations of selected basin management alternatives for Miller Creek, one of the most highly urbanized basins used in our study. We found that the preferred basin management alternative appeared to be effective in restoring some flow metrics close to simulated fully forested conditions (e.g., T_Qmean), but less effective in restoring other metrics such as High Pulse Count and Range. If future research continues to support our hypothesis that the flow regime, particularly High Pulse Count and Range, is an important control of biotic integrity in Puget Lowland streams, it would have significant implications for stormwater management.     

Prioritizing Stream Barrier Removal to Maximize Connected Aquatic Habitat and Minimize Water Scarcity

Instream barriers, such as dams, culverts, and diversions, alter hydrologic processes and aquatic habitat. Removing uneconomical and aging instream barriers is increasingly used for river restoration. Historically, selection of barrier removal projects used score‐and‐rank techniques, ignoring cumulative change and the spatial structure of stream networks. Likewise, most water supply models prioritize either human water uses or aquatic habitat, failing to incorporate both human and environmental water use benefits. Here, a dual‐objective optimization model identifies barriers to remove that maximize connected aquatic habitat and minimize water scarcity. Aquatic habitat is measured using monthly average streamflow, temperature, channel gradient, and geomorphic condition as indicators of aquatic habitat suitability. Water scarcity costs are minimized using economic penalty functions while a budget constraint specifies the money available to remove barriers. We demonstrate the approach using a case study in Utah's Weber Basin to prioritize removal of instream barriers for Bonneville cutthroat trout, while maintaining human water uses. Removing 54 instream barriers reconnects about 160 km of quality‐weighted habitat and costs approximately US$10 M. After this point, the cost‐effectiveness of removing barriers to connect river habitat decreases. The modeling approach expands barrier removal optimization methods by explicitly including both economic and environmental water uses.     

WATERSHED AND INSTREAM IMPACTS ON THE FISH POPULATION IN THE SOUTH FORK OF THE CLEARWATER RIVER,IDAHO1

ABSTRACT: This paper presents the results of a statistical analysis performed for the watershed and stream corridor in the South Fork of the Clearwater River (SFCR) basin, in north central Idaho. The analysis was performed for 61 six‐field hydrological unit codes (HUCs) of the SFCR basin using an extensive record (up to 100 years) for 50 watershed and in‐stream parameters, including hydrologic, flow, fish, anthropogenic, and natural activity data. The objective of this research was twofold: first, the development of quantitative relations that describe the Index of Fish Density (IFD) of particular fish species as a function of watershed and instream parameters; and second, to provide a robust confirmation for the effects of some of these parameters, previously recorded by the fisheries profession, by using well established statistical techniques. The uniqueness of this work is the compilation and statistical analysis of large data sets to quantitatively describe the impacts of watershed and instream parameters on the IFD of all salmonids and specific fish species. Factor analysis was employed to regroup parameters that are highly correlated to each other into a set of single factors and to relate the IFD to these factors. Using factor extraction, 12 factors were developed from the 50 watershed and instream parameters. Multiple regression diagnostic tests indicated that only 7 of the 12 factors are strong predictors offish indicators. The strongest predictors are longitude, latitude, elevation, watershed gradient, and water temperature. The analysis indicated that the present model has reasonable predictive power, considering the uncertainty involved in estimating the interdependence of IFD with watershed parameters.     

TECIIMQUES FOR DETECTING HYDROLOGIC CHANGE IN HIGH RESOURCE STREAMS1

ABSTRACT: A “synthetic paired basin” technique that combines hydrologic monitoring and watershed modeling proves to be a useful tool in detecting hydrologic change in creeks draining basins undergoing urbanization. In this approach, measured stream flow following subbasin treatment (a period of urbanization) is compared with flow from a control subbasin over the same time period. The control subbasin is the pretreatment subbasin itself as represented by a well‐calibrated hydrologic model that is input with post‐treatment meteorological data. The technique is illustrated for stream monitoring sites at the outlets of two high‐resource sub‐basins in the Bear Creek basin of King County, Washington. Application of this technique holds promise to provide earlier warning of cumulative, human impacts on aquatic resources and to better inform adaptive watershed management for resource protection.     

Coupling PCSWMM and WASP to Evaluate Green Stormwater Infrastructure Impacts to Storm Sediment Loads in an Urban Watershed

We coupled rainfall–runoff and instream water quality models to evaluate total suspended solids (TSS) in Wissahickon Creek, a mid‐sized urban stream near Philadelphia, Pennsylvania. Using stormwater runoff and instream field data, we calibrated the model at a subdaily scale and focused on storm responses. We demonstrate that treating event mean concentrations as a calibration parameter rather than a fixed input can substantially improve model performance. Urban stormwater TSS concentrations vary widely in time and space and are difficult to represent simply. Suspended and deposited sediment pose independent stressors to stream biota and model results suggest that both currently impair stream health in Wissahickon Creek. Retrofitting existing detention basins to prioritize infiltration reduced instream TSS loads by 20%, suggesting that infiltration mitigates sediment more effectively than detention. Infiltrating stormwater from 30% of the watershed reduced instream TSS loads by 47% and cut the frequency of TSS exceeding 100 mg/L by half. Settled loads and the frequency of high TSS values were reduced by a smaller fraction than suspended loads and duration at high TSS values. A widely distributed network of infiltration‐focused projects is an effective stormwater management strategy to mitigate sediment stress. Coupling rainfall–runoff and water quality models is an important way to integrate watershed‐wide impacts and evaluate how management directly affects urban stream health.     

Evaluating stream restoration projects

River and stream restoration projects are increasingly numerous but rarely subjected to systematic postproject evaluation. Without conducting such evaluation and widely disseminating the results, lessons will not be learned from successes and failures, and the field of river restoration cannot advance. Postproject evaluation must be incorporated into the initial design of each project, with the choice of evaluation technique based directly upon the specific project goals against which performance will be evaluated. We emphasize measurement of geomorphic characteristics, as these constitute the physical framework supporting riparian and aquatic ecosystems. Techniques for evaluating other components are briefly discussed, especially as they relate to geomorphic variables. Where possible, geomorphic, hydrologic, and ecological variables should be measured along the same transects. In general, postproject monitoring should continue for at least a decade, with surveys conducted after each flood above a predetermined threshold. Project design should be preceded by a historical study documenting former channel conditions to provide insights into the processes suggest earlier, potentially stable channel configurations as possible design models.     

Instream Restoration to Improve the Ecohydrologic Function of a Subalpine Meadow: Pre‐implementation Modeling with HEC‐RAS

Vegetation in subalpine meadows in the Sierra Nevada Mountains is particularly vulnerable to lowering of groundwater levels because wet meadow vegetation is reliant upon shallow groundwater during the dry summer growing season. These ecosystems are especially vulnerable to channel incision as meadow aquifers are hydrologically connected to tributaries, and many have not yet recovered from previous anthropogenic influences. While instream restoration projects have become a common approach, lack of postrestoration monitoring and communication often result in a trial‐and‐error approach. In this study we demonstrate that preimplementation modeling of possible instream restoration solutions, chosen to raise stream stage and subsequently groundwater levels, is a useful tool for evaluating and comparing potential channel modifications. Modeling allows us to identify strategic locations and specific methods. Results show additional sediment depth and roughness on tributaries along with introduced woody debris (simulated by high roughness) on the Tuolumne River are the most effective means of raising stream stage. Results demonstrate that restoration efforts are most efficient in tributary streams. Managers and planners can more efficiently direct resources while minimizing the potential for negative impacts or failed restoration projects by modeling the possible effects of multiple restoration scenarios before implementation.     

Creating False Images: Stream Restoration in an Urban Setting

Stream restoration has become a multibillion dollar business with mixed results as to its efficacy. This case study utilizes pre‐ and post‐monitoring data from restoration projects on an urban stream to assess how well stream conditions, publicly stated project goals, and project implementation align. Our research confirms previous studies showing little communication among academic researchers and restoration practitioners as well as provides further evidence that restoration efforts tend to focus on small‐scale, specific sites without considering broader land use patterns. This study advances our understanding of restoration by documenting that although improving ecological conditions is a stated goal for restoration projects, the implemented measures are not always focused on those issues that are the most ecologically salient. What these projects have accomplished is to protect the built environment and promote positive public perception. We argue that these disconnects among publicized goals for restoration, the implemented features, and actual stream conditions may create a false image of what an ecologically stable stream looks like and therefore perpetuate a false sense of optimism about the feasibility of restoring urban streams.     

USING MARYLAND'S STREAM CORRIDOR ASSESSMENT SURVEY TO PRIORITIZE WATERSHED RESTORATION EFFORTS1

ABSTRACT: The Stream Corridor Assessment survey has been developed by the Maryland Department of Natural Resources as a watershed management tool to identify environmental problems and to help prioritize restoration opportunities on a watershed basis. Potential environmental problems commonly identified during the survey include: stream channel alterations, excessive bank erosion, exposed pipes, inadequate stream buffers, fish migration blockages, trash dumping sites, near stream construction, pipe outfalls, and unusual conditions. In addition, the survey records information on the location of potential wetlands creation sites and collects data on the general condition of instream and riparian habitats. Over the past several years, in collaboration with the Maryland Conservation Corps, watershed associations, and local governments, more than 3,293 km (2,046 miles) of Maryland streams have been surveyed. Overall, the survey has proven to be a cost effective starting point for many watershed restoration efforts.     

NATURAL RESTABILIZATION OF STREAM CHANNELS IN URBAN WATERSHEDS1

ABSTRACT: Stream channels are known to change their form as a result of watershed urbanization, but do they restabilize under subsequent conditions of constant urban land use? Streams in seven developed and developing watersheds (drainage areas 5–35 km²) in the Puget Sound lowlands were evaluated for their channel stability and degree of urbanization, using field and historical data. Protocols for determining channel stability by visual assessment, calculated bed mobility at bankfull flows, and resurveyed cross‐sections were compared and yielded nearly identical results. We found that channel restabilization generally does occur within one or two decades of constant watershed land use, but it is not universal. When (or if) an individual stream will restabilize depends on specific hydrologic and geomorphic characteristics of the channel and its watershed; observed stability is not well predicted by simply the magnitude of urban development or the rate of ongoing land‐use change. The tendency for channel restabilization suggests that management efforts focused primarily on maintaining stability, particularly in a still‐urbanizing watershed, may not always be necessary. Yet physical stability alone is not a sufficient condition for a biologically healthy stream, and additional rehabilitation measures will almost certainly be required to restore biological conditions in urban systems.     

Riparian vegetation instream flow requirements: A case study from a diverted stream in the Eastern Sierra Nevada,California, USA

A methodology is described that allows determination of instream flow requirements for maintenance of riparian trees. Tree-ring data revealed strong relationships between tree growth and stream flow volume for riparian species at Rush Creek, an alluvial stream within an arid setting; these relationships allowed development of models that predict growth rates from hydrologic variables. The models can be used to assess instream flow requirements under the assumption that certain levels of growth are necessary to maintain the population. There is a critical need for development and use of instream flow methodologies for riparian vegetation, since present methodologies focus on needs of aquatic animals (e.g., fish) and may underestimate needs of the entire riparian ecosystem.     

Alluvial Sedimentation and Erosion in an Urbanizing Watershed,Gwynns Falls,Maryland1

Abstract: Earlier measurements of stream channel geometry on 19 reaches were repeated to provide a longitudinal study of stream channel adjustment over 13 years (1987‐2000) in the urbanizing Gwynns Falls, Maryland watershed. We observed both enlargement and reduction in channel size, depending on the extent of upstream development, the timing and location of urbanization and upstream channel adjustment, and the presence of hydrologic constrictions and grade controls. Based on a relatively simple visual assessment of the composition, size, and extent of instream sediment storage, we categorized stream reaches into three phases: aggraded (7 sites), early erosion (7 sites), and late erosion (5 sites). Aggraded sites had point and lateral bars mantled with fine‐grained sediment and experienced some reduction in cross‐sectional area, primarily through the deposition of fine‐grained material on bars in the channel margins. Early erosion sites had smaller bars and increases in channel cross‐sectional area as a consequence of the evacuation of in‐channel fine‐grained sediment. Fine‐grained sediments were either entirely absent or found only at a few high bar elevations at late erosion sites. Sediment evacuation from late erosion sites has both enlarged and simplified channels, as demonstrated by an increase in cross‐sectional area and a strong decrease in channel width variation. Channel cross‐sectional area enlargement, reduced channel width variation, and channel incision were ubiquitous at erosion sites. As a result, overbank flows were less common in the erosion sites as determined by high water marks left by a 2‐year flood that occurred during the study period. Principal causes for channel changes appear to be increased high flow durations and reduced sediment supply. Spatial variation in channel conditions could not be tied simply to sub‐basin impervious cover or watershed area. In‐channel sediment storage is a useful indicator of channel form and adjustment. When combined with information on development and sedimentation conditions in the contributing drainage, instream sediment storage can be used to effectively assess future channel adjustments.     

REVIVING URBAN STREAMS: LAND USE,HYDROLOGY, BIOLOGY,AND HUMAN BEHAVIOR1

ABSTRACT: Successful stream rehabilitation requires a shift from narrow analysis and management to integrated understanding of the links between human actions and changing river health. At study sites in the Puget Sound lowlands of western Washington State, landscape, hydrological, and biological conditions were evaluated for streams flowing through watersheds with varying levels of urban development. At all spatial scales, stream biological condition measured by the benthic index of biological integrity (B‐IBI) declined as impervious area increased. Impervious area alone, however, is a flawed surrogate of river health. Hydrologic metrics that reflect chronic altered streamflows, for example, provide a direct mechanistic link between the changes associated with urban development and declines in stream biological condition. These measures provide a more sensitive understanding of stream basin response to urban development than do treatment of each increment of impervious area equally. Land use in residential backyards adjacent to streams also heavily influences stream condition. Successful stream rehabilitation thus requires coordinated diagnosis of the causes of degradation and integrative management to treat the range of ecological stressors within each urban area, and it depends on remedies appropriate at scales from backyards to regional storm water systems.     

Assessment Tools for Urban Catchments: Developing Stressor Gradients1

Abstract: This is the first in a series of three articles designed to establish empirically defined biological indicators and thresholds for impairment for urbanized catchments, and to describe a process by which the biological condition of waterbodies in urbanized catchments can be applied. This article describes alternative gradients of urbanization for assessing and selecting a nationally applicable biological index (article 2 – Purcell et al., this issue ) and defining the potential of biological communities within a gradient of cumulative stressors (article 3 – Paul et al. this issue ). Gradients were designed to represent the most prominent mosaic of stressors found in urban settings. A primary urban gradient was assembled based on readily obtained information of urbanization to include three broad‐scale parameters: percent urban land use/land cover, population density, and road density. This gradient was used as the standard by which alternative urban gradients, which included fine‐scale instream chemical and hydrologic parameters, were assessed. Five alternative gradients were developed to provide numerous environmental management options based on availability of data from water program resources. The urban gradients were developed with the intent that they be applied throughout the country; therefore, data from three different regions of the United States (Mid‐Atlantic, Midwest, and Pacific Coast) were used to validate the urban gradient model. Our study showed that a relatively straightforward stressor gradient consisting of human population density, road density, and urban land use is useful in providing a framework for developing relevant biological indicators and evaluating the potential of biological communities as a basis for assessing attainment of designated aquatic life use.     

Influence of Restoration Age and Riparian Vegetation on Reach‐Scale Nutrient Retention in Restored Urban Streams

In urban watersheds, stormwater inputs largely bypass the buffering capacity of riparian zones through direct inputs of drainage pipes and lowered groundwater tables. However, vegetation near the stream can still influence instream nutrient transformations via maintenance of streambank stability, input of woody debris, modulation of organic matter sources, and temperature regulation. Stream restoration seeks to mimic many of these functions by engineering channel complexity, grading stream banks to reconnect incised channels, and replanting lost riparian vegetation. The goal of this study was to quantify these effects by measuring nitrate and phosphate uptake in five restored streams in Charlotte and Raleigh, North Carolina, with a range of restoration ages. Using nutrient spiraling methods, uptake velocity of nitrate (0.02‐3.56 mm/min) and phosphate (0.14‐19.1 mm/min) was similar to other urban restored streams and higher than unimpacted forested streams with variability influenced by restoration age and geomorphology. Using a multiple linear regression approach, reach‐scale phosphate uptake was greater in newly restored sites, which was attributed to assimilation by algal biofilms, whereas nitrate uptake was highest in older sites potentially due to greater channel stability and establishment of microbial communities. The patterns we observed highlight the influence of riparian vegetation on energy inputs (e.g., heat, organic matter) and thereby on nutrient retention.     

Assessing Linkages in Stream Habitat,Geomorphic Condition,and Biological Integrity Using a Generalized Regression Neural Network

Watershed managers often use physical geomorphic and habitat assessments in making decisions about the biological integrity of a stream, and to reduce the cost and time for identifying stream stressors and developing mitigation strategies. Such analysis is difficult since the complex linkages between reach‐scale geomorphic and habitat conditions, and biological integrity are not fully understood. We evaluate the effectiveness of a generalized regression neural network (GRNN) to predict biological integrity using physical (i.e., geomorphic and habitat) stream‐reach assessment data. The method is first tested using geomorphic assessments to predict habitat condition for 1,292 stream reaches from the Vermont Agency of Natural Resources. The GRNN methodology outperforms linear regression (69% vs. 40% classified correctly) and improves slightly (70% correct) with additional data on channel evolution. Analysis of a subset of the reaches where physical assessments are used to predict biological integrity shows no significant linear correlation, however the GRNN predicted 48% of the fish health data and 23% of macroinvertebrate health. Although the GRNN is superior to linear regression, these results show linking physical and biological health remains challenging. Reasons for lack of agreement, including spatial and temporal scale differences, are discussed. We show the GRNN to be a data‐driven tool that can assist watershed managers with large quantities of complex, nonlinear data.