Large-scale patterns of hyporheic exchange are predictable within some river systems, but our understanding of the factors driving hyporheic processes and the magnitude of hyporheic exchange needed to influence biophysical patterns at larger scales remains limited. We investigated the patterns, magnitude, and potential effects on biota of reach-scale hyporheic exchange in an alluvial river of the Pacific Northwest. The river was topographically similar to and in the same geographic region as other systems where large-scale hyporheic exchange and associated biological responses have been observed. We hypothesized that predictable reach-scale patterns of hyporheic exchange would occur in alluvial valley segments of the river and that hyporheic upwelling would be associated with reach-scale patterns of physical and biological characteristics. We used in-channel piezometers and synoptic stream flow measurements to quantify hyporheic exchange. We measured temperature, dissolved O₂, pH, specific conductivity, chlorophyll a biomass, primary production, and benthic macroinvertebrates as indicators of physical and biological responses. Contrary to our expectations, we found no evidence, physical or biological, of reach-scale hyporheic exchange. Hyporheic connectivity in this river system probably is constrained by geologic and geomorphic characters as well as the legacy of human land use in the basin. Thus, our results illustrate the variability of hyporheic processes that can occur among alluvial river systems and may have implications for watershed management.

Low organic matter supply and reduced spatiotemporal heterogeneity severely constrain biodiversity in groundwater. At the aquifer scale, spatial variation in the groundwater recharge rate is considered a key factor in generating groundwater patches with distinctly different food supplies and spatiotemporal heterogeneity. Our study addressed the role of groundwater recharge in sustaining biodiversity in a phreatic aquifer by testing differences in the density and species richness of invertebrate assemblages among 11 reference sites and 13 sites artificially recharged with storm water. The vertical distribution pattern of invertebrate assemblages was examined using well clusters at one shallow water-table recharge site and one nearby reference site. Groundwater recharge elevated dissolved organic C (DOC) concentrations in groundwater and increased spatiotemporal physicochemical heterogeneity. The thickness of the vadose zone (VZT) controlled DOC input to groundwater at recharge sites but did not reduce spatiotemporal heterogeneity. The higher density and richness of invertebrate assemblages at the well-cluster recharge site than at the well-cluster reference site also was controlled by VZT, suggesting that organic matter supply was a primary factor determining biodiversity patterns in groundwater. Invertebrate density increased and species composition shifted with increasing depth below the groundwater table at the shallow water-table well-cluster site. Some taxa, including several epigean species, preferentially occurred near the water table, whereas others, including several hypogean species, colonized deeper groundwater layers.

We characterized channel unit types in 13 steep, headwater streams in British Columbia, Canada, based on physical variables, to determine the influence of channel unit type on benthic macroinvertebrate assemblages. Macroinvertebrate abundance was highest in riffles, followed by rapids, pools, boulder cascades, chutes, and bedrock cascades. Heptageniidae, Nemouridae, Chironomidae, Leptophlebiidae, Enchytraeidae, Chloroperlidae, Lepidostomatidae, and Tricladida were most abundant in either riffles or rapids. Baetidae and Simuliidae preferred bedrock cascades and chutes, and predominance of these families resulted in a distinct assemblage structure within these 2 channel unit types compared with other types. Benthic assemblages within riffles, rapids, pools, and boulder cascades were not distinctive from each other. Significant interstream variation was apparent in abundance of all taxonomic groups studied. In general, invertebrates were more abundant in streams with perennial flow regimes compared to those with intermittent or ephemeral flow. Assemblage structure within ephemeral streams was distinct because of the preponderance of Enchytraeidae. Defining physical characteristics of stream channel units using benthic macroinvertebrate assemblages is a useful means of discriminating habitat conditions within small, high-gradient streams.

Leaf breakdown, a key functional process, was examined in 3 glacial streams, 3 alpine springbrooks, and a rock-glacier stream in the Swiss Alps. Environmental conditions within each stream type were expected to significantly affect leaf breakdown rates among streams. Individual leaf packs (7 g fresh mass) of alder (Alnus viridis) were immersed in each stream and replicate samples were collected at periodic intervals over 49 d. Collected leaves were assessed for N and P, ergosterol (a measure of fungal biomass), % lignin (an indicator of recalcitrance), and aquatic macroinvertebrates. Leaf breakdown was faster in the springbrooks (k = −0.0085) and rock-glacier stream (k = −0.0073) than in the glacial streams (k = −0.0027) when expressed in terms of time. However, leaf breakdown rates based on degree days were similar among streams, ranging from k = −0.0011 to −0.0020. Leaf P levels increased over time in all sites, and reflected respective increases in ergosterol. Leaf ergosterol levels increased from 62 μg/g to ∼300–500 μg/g leaf dry mass during the study, and were highest in the springbrooks and rock-glacier stream. The % lignin of leaves displayed a similar pattern as breakdown rates, reaching 20 (glacial streams) to 30% (springbrooks) leaf dry mass by day 49. Chironomids dominated leaf packs in glacial streams, whereas leaf packs in springbrooks were additionally colonized by Ephemeroptera, Plecoptera, and Trichoptera. We conclude that differences in environmental conditions (interaction between abiotic and biotic properties) strongly influence ecosystem functioning, as inferred from leaf breakdown characteristics, among alpine streams.

We tested the efficacy of DNA barcodes in identifying mayfly species primarily from the northeastern United States and central Canada. We sequenced a 630-base-pair segment of the mitochondrial gene, cytochrome c oxidase 1 (COI), from 1 individual of each of 80 species to create a reference sequence profile. We used these reference sequences to identify 70 additional specimens representing 32 of the species that were in the profile. DNA barcodes correctly identified 69 of the 70 test specimens. The sole exception was an individual identified morphologically as Maccaffertium modestum that showed deep genetic divergence from other M. modestum specimens. Mean sequence divergence within species was 1%, whereas mean divergence among congeneric species was an order of magnitude greater (18%). We conclude that DNA barcoding can provide a powerful tool for mayfly species identification.

We compared the ability of simple random sampling (SRS) and a variety of systematic sampling (SYS) designs to estimate abundance, quantify spatial clustering, and predict spatial distribution of freshwater mussels. Sampling simulations were conducted using data obtained from a census of freshwater mussels in a 40 × 33 m section of the Cacapon River near Capon Bridge, West Virginia, and from a simulated spatially random population generated to have the same abundance as the real population. Sampling units that were 0.25 m² gave more accurate and precise abundance estimates and generally better spatial predictions than 1-m² sampling units. Systematic sampling with ≥2 random starts was more efficient than SRS. Estimates of abundance based on SYS were more accurate when the distance between sampling units across the stream was less than or equal to the distance between sampling units along the stream. Three measures for quantifying spatial clustering were examined: Hopkins Statistic, the Clumping Index, and Morisita's Index. Morisita's Index was the most reliable, and the Hopkins Statistic was prone to false rejection of complete spatial randomness. SYS designs with units spaced equally across and up stream provided the most accurate predictions when estimating the spatial distribution by kriging. Our research indicates that SYS designs with sampling units equally spaced both across and along the stream would be appropriate for sampling freshwater mussels even if no information about the true underlying spatial distribution of the population were available to guide the design choice.

Catchment characteristics determine the inputs of sediments and nutrients to streams. As a result, natural or anthropogenic disturbance of upland soil and vegetation can affect instream processes. The Fort Benning Military Installation (near Columbus, Georgia) exhibits a wide range of upland disturbance levels because of spatial variability in the intensity of military training. This gradient of disturbance was used to investigate the effect of upland soil and vegetation disturbance on rates of stream metabolism (ecosystem respiration rate [ER] and gross primary production rate [GPP]). Stream metabolism was measured using an open-system, single-station approach. All streams were net heterotrophic during all seasons. ER was highest in winter and spring and lowest in summer and autumn. ER was negatively correlated with catchment disturbance level in winter, spring, and summer, but not in autumn. ER was positively correlated with abundance of coarse woody debris, but not significantly related to % benthic organic matter. GPP was low in all streams and generally not significantly correlated with disturbance level. Our results suggest that the generally intact riparian zones of these streams were not sufficient to protect them from the effect of upland disturbance, and they emphasize the role of the entire catchment in determining stream structure and function.

A headwater stream adjacent to an abandoned As mine was investigated to determine the influence of As on stream biota and organic matter processing using an upstream (reference reach) and downstream (mine-influenced reach) comparative approach. Field assessments of stream chemistry, macroinvertebrate abundance and composition, and leaf breakdown were coupled with laboratory experiments addressing As influences on leaf biofilm respiration. Streamwater As concentrations varied from below detection limit (2.5μg/L) in the upstream reach to >12,000 μg/L. Concentrations were low in the reference reach, increased immediately adjacent to tailing piles, and climbed significantly with distance along the mine-influenced reach. Compared to the reference reach, macroinvertebrate density (7869 vs 154 individuals/m²), shredder abundance (3340 vs 22 individuals/m²), and species richness (11.9 vs 0.8 species/sample) were significantly lower in the mine-influenced reach. For both white oak and red maple leaf packs, breakdown rates in the reference reach (k = 0.0048 and 0.009/d, respectively) were significantly greater than in the mine-influenced reach immediately downstream of waste piles (k = 0.0019 and 0.003/d) and further downstream (k = 0.0014 and 0.005/d). In one experiment, laboratory assays showed that short-term exposure to elevated As concentrations did not alter leaf biofilm respiration rates. In a 2nd experiment addressing chronic exposure, respiration rates for extant leaf biofilms in the reference reach (0.37 ± 0.01 μg O₂ mg ash-free-dry-mass [AFDM]⁻¹ h⁻¹) were significantly greater than in the mine-influenced reach (0.29 ± 0.01 μg O₂ mg AFDM⁻¹ h⁻¹), but rates in both reaches were typical of forested headwater streams not exposed to elevated As concentrations. Together, these data suggest that elevated As concentrations in the stream have led to altered organic matter processing not by reducing microbial activity but primarily by decreasing invertebrate densities, limiting shredder abundance, and decreasing litter breakdown rates.

Spatially patchy and temporally varied cycles of timber harvest across a landscape may have subtle effects on stream conditions that are difficult, but important, to assess. The objective of our study was to examine the relationship between benthic diatom composition and timber harvest in coastal Oregon watersheds. Physical habitat conditions, water chemistry, and periphyton composition were characterized for 46 sites from 2 subbasins with different timber harvest intensities (0.3 km²/y vs 3 km²/y, between 1972–1998). Landscape variables including geology, vegetative cover types, and harvest intensity, were quantified for the watershed upstream of each sample point. Nonmetric Multidimensional Scaling analysis of periphyton composition showed that the 1^st axis was primarily driven by Achnanthidium minutissimum (r = −0.91) whereas the 2^nd axis was driven by Nitzschia inconspicua (r = 0.77). The 1^st axis was positively correlated with % of upstream area harvested between 1972 and 1998 (r = 0.54) and water-quality variables such as total P (TP) (e.g., r_TP = 0.74). A subset comparison (n = 12) between harvested (30% harvested 1972–1998, n = 6) and unharvested (0% harvested 1972–1998, n = 6) watersheds with similar geology (>80% basalts), broadleaf vegetative cover (8–35% broadleaf), and other reach-scale characteristics revealed higher total N, TP, turbidity, and conductivity in the harvested than the unharvested watersheds (p < 0.05). Shannon diversity and species richness also were higher in the harvested group (p < 0.05). Our data suggest that diatom assemblages may be useful in assessing the long-term impact of timber harvest within coastal Oregon watersheds.

We assessed the potential effects of urbanization on bioavailability of dissolved organic C (DOC) in stream ecosystems, by quantifying bacterial extracellular enzymatic activity (EEA). We measured activities of 6 enzymes by incubating water samples with 4-methylumbelliferyl substrates from streams across an urbanization gradient east of Melbourne, Australia. A principal components analysis (PCA) ordination separated streams according to their relative urbanization, as indicated by effective imperviousness (proportion of each catchment covered by impervious surface directly connected to streams by pipes). Activities of leucine aminopeptidase and esterase were higher in streams in more urbanized catchments than in less-urbanized streams, where moderate activities of a diverse range of enzymes were observed. High relative contributions of leucine aminopeptidase and esterase to overall enzyme activity in urban streams stressed the increased importance of peptides as a C source for heterotrophic bacteria and nonspecific extracellular hydrolytic activity. In contrast, high contributions of β-N-acetyl glucosaminidase and β-xylosidase in less-urbanized streams highlighted the significance of microbial detrital material as a C source and processing of plant-derived xylooligosaccharide substrates in these environments. Our results suggest a shift in organic C bioavailability across streams of contrasting urbanization, despite all streams having roughly similar DOC concentrations. We propose that relative EEA rates show promise as an ecological indicator of stream health across an urban gradient.

Ecologists have described an urban stream syndrome with attributes such as elevated nutrients and contaminants, increased hydrologic flashiness, and altered biotic assemblages. Ecosystem function probably also varies with extent of urbanization, although there are few stream networks in which this prediction has been studied. We examined functional characteristics of 6 tributaries of the Chattahoochee River near Atlanta, Georgia, USA, whose catchments differed in degree of urbanization. We conducted short-term NH₄- and PO₄-addition experiments to measure nutrient uptake velocity, which is the rate at which a nutrient moves through the water column toward the benthos. Both NH₄ and soluble reactive P uptake velocities decreased as indicators of urbanization (i.e., % of catchment covered by high-intensity urban development) increased. The amount of fine benthic organic matter (FBOM) also decreased with increasing urbanization, and uptake velocities were directly related to FBOM. Uptake velocities were not related to ecosystem metabolism (gross primary production [GPP], community respiration [CR], or net ecosystem production) as measured with diel oxygen curves. However, NH₄ uptake velocity increased as total stream metabolism (GPP CR) increased in these streams as well as in other North American streams, suggesting that biotic demand drives NH₄ uptake velocities across a wide range of stream ecosystems. Measures of ecosystem function responded differently to urbanization: ecosystem metabolism was not correlated with indicators of urbanization, although breakdown rate of Acer barbatum leaves was positively correlated and nutrient uptake velocities were negatively correlated with indicators of urbanization. Elevated nutrient concentrations associated with urbanization are usually attributed to increased inputs from point and non-point sources; our results indicate that concentrations also may be elevated because of reduced rates of nutrient removal. Altered ecosystem function is another symptom of an urban stream syndrome.

Stream water often diverges from the main channel into sediments below the stream surface, gravel bars next to the stream, or organic debris dams in the middle of the stream. These geomorphic structures have the potential to support processes that produce or consume inorganic N (NH₄ , NO₃⁻) and thus affect streamwater quality. We measured production (potential net mineralization and nitrification) and consumption (denitrification potential, net immobilization) of inorganic N, respiration, and organic-matter content in sediments from geomorphic structures in 4 streams in and around Baltimore, Maryland, USA. We sampled sediments from stream pools, riffles, gravel bars (vegetated and nonvegetated), and organic debris dams in forested reference and suburban catchments, and also sampled degraded (incised channel) and restored reaches of one stream. Denitrification potential was highest in organic debris dams and organic-rich gravel bars—structures with high organic matter content. Organic debris dams in suburban streams had higher denitrification than debris dams in the forested reference stream, likely because of higher NO₃⁻ concentrations in suburban streams. These results suggest that denitrification in debris dams increases in response to high NO₃⁻ levels and that denitrification may be an important sink for NO₃⁻ in urban or suburban streams. However, such denitrifying structures as organic debris dams may be difficult to maintain in urban streams because of high storm flows and downstream displacement. Geomorphic structures in N-rich streams also supported higher rates of nitrification than structures in a forested reference stream, suggesting that these structures can become sources of NO₃⁻. The ultimate effect of different structures on NO₃⁻ concentrations in urban streams will depend on the balance of these production and consumption processes, which is a complex function of a stream's ability to retain organic matter and resist hydrologic changes associated with urbanization and elevated NO₃⁻ levels.

Nutrient spiraling in theory and application provides a framework for comparing nutrient retention efficiency of urban streams to relatively unaltered streams. Previous research indicated that streams of the southwestern USA deserts are highly retentive of N because of N limitation, high productivity, and high channel complexity (in particular, extensive transient storage associated with the hyporheic zone). Most southwestern urban streams have extensively modified channels and experience N loading from urban runoff and inputs of NO₃⁻-contaminated groundwater. Therefore, we predicted southwestern urban streams are neither N-limited nor retentive. For some urban streams, however, restoration efforts reestablish flow in long-dry channels, create nonstructural flood-management solutions, and design riparian areas as a public recreation amenity. These human modifications may, in part, restore N retention functions if channel complexity and heterogeneity are as important to N retention efficiency as believed. We conducted experimental tracer studies using ¹⁵N-NO₃⁻, as part of the Lotic Intersite Nitrogen eXperiment (LINX) project, and several separate nutrient-addition experiments (using slight increases in NO₃⁻ concentration), to evaluate N retention in southwestern urban streams. We present preliminary results of those experiments, comparing results to similar experiments in unaltered streams to test our predictions. Our results allow an evaluation of the use of nutrient spiraling metrics as a tool for assessing the status of stream ecosystem services in urban restoration projects.

We examined patterns in Maryland fish assemblages in 1^st- through 3^rd-order nontidal streams along an urbanization gradient in the Eastern Piedmont (EP) and Coastal Plain (CP) physiographic ecoregions of Maryland, USA, using 1995 to 1997 and 2000 to 2002 data from the Maryland Biological Site Survey (MBSS). Major urbanization and other historical stressors occur in both ecoregions, and there is potential for further stress over the next 25 y as urbanization increases. We assigned each MBSS site (n = 544 streams) to a class of urbanization based on land cover within its upsite catchment. We compared observed fish abundance and species richness to the probable (expected) assemblages within each ecoregion, and also assessed the accuracy of the Maryland fish index of biotic integrity (FIBI) to indicate catchment urbanization. Relationships between urbanization and fish assemblages and FIBI varied between the 2 ecoregions. Assemblages in EP streams exhibited stronger relationships with urbanization than those in CP streams, particularly when urban land cover was >25% of the catchment. Across all EP stream orders (1^st, 2^nd, and 3^rd), high urbanization was associated with low fish abundance and richness, low FIBI, and few intolerant fish species, resulting in assemblages dominated by tolerant species. Conservation practices minimizing urbanization effects on fish assemblages may be inadequate to protect sensitive fish species because of the invasiveness of urban development and stressors related to the urban stream syndrome.

Stream biota in urban and suburban settings are thought to be impaired by altered hydrology; however, it is unknown what aspects of the hydrograph alter fish assemblage structure and which fishes are most vulnerable to hydrologic alterations in small streams. We quantified hydrologic variables and fish assemblages in 30 small streams and their subcatchments (area 8–20 km²) in the Etowah River Catchment (Georgia, USA). We stratified streams and their subcatchments into 3 landcover categories based on imperviousness (<10%, 10–20%, >20% of subcatchment), and then estimated the degree of hydrologic alteration based on synoptic measurements of baseflow yield. We derived hydrologic variables from stage gauges at each study site for 1 y (January 2003–2004). Increased imperviousness was positively correlated with the frequency of storm events and rates of the rising and falling limb of the hydrograph (i.e., storm “flashiness”) during most seasons. Increased duration of low flows associated with imperviousness only occurred during the autumn low-flow period, and this measure corresponded with increased richness of lentic tolerant species. Altered storm flows in summer and autumn were related to decreased richness of endemic, cosmopolitan, and sensitive fish species, and decreased abundance of lentic tolerant species. Species predicted to be sensitive to urbanization, based on specific life-history or habitat requirements, also were related to stormflow variables and % fine bed sediment in riffles. Overall, hydrologic variables explained 22 to 66% of the variation in fish assemblage richness and abundance. Linkages between hydrologic alteration and fish assemblages were potentially complicated by contrasting effects of elevated flows on sediment delivery and scour, and mediating effects of high stream gradient on sediment delivery from elevated flows. However, stormwater management practices promoting natural hydrologic regimes are likely to reduce the impacts of catchment imperviousness on stream fish assemblages.

Live-trapping surveys recorded populations of the platypus, Ornithorhynchus anatinus, in 73% of 45 reaches in the Dandenong Creek and Werribee, Yarra, Maribyrnong, Bunyip, and Lang Lang River catchments near Melbourne, Victoria; however, many populations occurred at low density. Our study investigated the relationship between population status and water and sediment quality along 28 stream reaches, including 17 reaches supporting a population of O. anatinus and 11 reaches lacking a resident population. Stream attributes included surface water-quality variables (summer concentrations of dissolved O₂, total P [TP], NO_X, total Kjeldahl N [TKN], dissolved organic N, NH₄-N, and 50^th, 75^th, and 90^th percentiles of suspended solids [SS]), concentrations of sediment toxicants (Zn, Pb, Cd, As, Cr, Cu, Hg, Ni), extent of catchment urbanization (as indicated by % imperviousness), and daily discharge. Reaches supporting a medium-density population (mean number of ≥0.5 adults or subadults captured per pair of nets set overnight) were characterized by significantly lower concentrations of streamwater TP, TKN, and SS (90^th percentile), significantly lower Cd, Pb, and Zn in sediments, and significantly lower catchment imperviousness than reaches lacking resident animals. The maximum imperviousness associated with a population of O. anatinus was 11%, suggesting that this species is sensitive to urban-related change. Capture rate was not significantly correlated with median summer discharge, but was inversely correlated with streamwater TP and TKN. Further studies are needed to determine if pollutants may limit urban O. anatinus populations through direct toxicity or indirectly by pollutants reducing their benthic macroinvertebrate food resource.

Restoration of streams degraded by urbanization has usually been attempted by enhancement of instream habitat or riparian zones. Such restoration approaches are unlikely to substantially improve instream ecological condition because they do not match the scale of the degrading process. Recent studies of urban impacts on streams in Melbourne, Australia, on water chemistry, algal biomass and assemblage composition of diatoms and invertebrates, suggested that the primary degrading process to streams in many urban areas is effective imperviousness (EI), the proportion of a catchment covered by impervious surfaces directly connected to the stream by stormwater drainage pipes. The direct connection of impervious surfaces to streams means that even small rainfall events can produce sufficient surface runoff to cause frequent disturbance through regular delivery of water and pollutants; where impervious surfaces are not directly connected to streams, small rainfall events are intercepted and infiltrated. We, therefore, identified use of alternative drainage methods, which maintain a near-natural frequency of surface runoff from the catchment, as the best approach to stream restoration in urban catchments and then used models of relationships between 14 ecological indicators and EI to determine restoration objectives. Ecological condition, as indicated by concentrations of water-quality variables, algal biomass, and several measures of diatom and macroinvertebrate assemblage composition, declined with increasing EI until a threshold was reached (EI = 0.01–0.14), beyond which no further degradation was observed. We showed, in a sample catchment, that it is possible to redesign the drainage system to reduce EI to a level at which the models predict detectable improvement in most ecological indicators. Distributed, low-impact design measures are required that intercept rainfall from small events and then facilitate its infiltration, evaporation, transpiration, or storage for later in-house use.

The term “urban stream syndrome” describes the consistently observed ecological degradation of streams draining urban land. This paper reviews recent literature to describe symptoms of the syndrome, explores mechanisms driving the syndrome, and identifies appropriate goals and methods for ecological restoration of urban streams. Symptoms of the urban stream syndrome include a flashier hydrograph, elevated concentrations of nutrients and contaminants, altered channel morphology, and reduced biotic richness, with increased dominance of tolerant species. More research is needed before generalizations can be made about urban effects on stream ecosystem processes, but reduced nutrient uptake has been consistently reported. The mechanisms driving the syndrome are complex and interactive, but most impacts can be ascribed to a few major large-scale sources, primarily urban stormwater runoff delivered to streams by hydraulically efficient drainage systems. Other stressors, such as combined or sanitary sewer overflows, wastewater treatment plant effluents, and legacy pollutants (long-lived pollutants from earlier land uses) can obscure the effects of stormwater runoff. Most research on urban impacts to streams has concentrated on correlations between instream ecological metrics and total catchment imperviousness. Recent research shows that some of the variance in such relationships can be explained by the distance between the stream reach and urban land, or by the hydraulic efficiency of stormwater drainage. The mechanisms behind such patterns require experimentation at the catchment scale to identify the best management approaches to conservation and restoration of streams in urban catchments. Remediation of stormwater impacts is most likely to be achieved through widespread application of innovative approaches to drainage design. Because humans dominate urban ecosystems, research on urban stream ecology will require a broadening of stream ecological research to integrate with social, behavioral, and economic research.

Undoing harm caused by catchment urbanization on stream channels and their resident biota is challenging because of the range of stressors in this environment. One primary way in which urbanization degrades biological conditions is by changing flow patterns; thus, reestablishing natural flow regimes in urban streams demands particular attention if restoration is to have a chance for success. Enhancement efforts in urban streams typically are limited to rehabilitating channel morphology and riparian habitat, but such physical improvements alone do not address all factors affecting biotic health. Some habitat-forming processes such as the delivery of woody debris or sediment may be amenable to partial restoration, even in highly disturbed streams, and they constitute obvious high-priority actions. There is no evidence to suggest, however, that improving nonhydrologic factors can fully mitigate hydrologic consequences of urban development. In the absence of effective hydrologic mitigation, appropriate short-term rehabilitation objectives for urban channels should be to 1) eliminate point sources of pollution, 2) reconstruct physical channel elements to resemble equivalent undisturbed channels, and 3) provide habitat for self-sustaining biotic communities, even if those communities depart significantly from predisturbance conditions. Long-term improvement of stream conditions is not feasible under typical urban constraints, so large sums of money should not be spent on unrealistic or unreachable targets for stream rehabilitation. However, such a strategy should not be an excuse to preclude potential future gains by taking irreversible present-day development or rehabilitative actions.