Urban+streams

=Urban streams =

//by Justin Reale//
Abstract: In the coming century, climate change, shifts in the freshwater supply, consumption, and population increases will alter the water cycle considerably. Humans are becoming a more urban species. As urban populations continue to grow, the stress placed upon aquatic resources will continue to increase. Urban streams are endangered ecosystems that require change at the watershed level to alleviate the stress placed upon them by urban communities. Previous studies have shown multiple stressors affect urban aquatic ecosystems. Multiple stressors, as byproducts of urbanization, can make it difficult to infer a specific stressor as the causal factor. This paper focuses on the hydrology, geomorphology, stormwater management, stream ecosystem community responses, stream ecosystem processes of urban streams and catchments, and current practices to improve urban streams. A better understanding of these dynamic ecosystems will provide useful information for urban planners, scientists, engineers, and water managers to reduce the effects of urbanization on urban streams. By understanding the affects of urbanization on aquatic ecosystems, restoration of these endangered aquatic ecosystems can be designed and implemented.

**Introduction**
In the coming century, climate change and a growing imbalance among freshwater supply, consumption, and population will alter the water cycle considerably (Jackson et al. 2001). Humans are becoming a more urban species (Meyer et al. 2005). During the last two centuries worldwide urbanization has taken place and accelerated greatly in the 20th century (Cohen 2003). In 1800, roughly 2% of people lived in cities; in 1900, 12%; and in 2000, more than 47% (Cohen 2003). In 2000, nearly 10% of city dwellers lived in cities of 10 million people or larger (Cohen 2003). As urban populations continue to grow, the stress placed upon aquatic resources will continue to increase. Historically most ecologists have studied ecosystems, communities, species, and populations in pristine or relatively pristine systems (Cairns and Heckman 1996). The increase in urban populations has lead to increased research on ecology in urban settings in the last two decades (Walsh et al. 2005b). The analysis of urbanization is a complex examination of the numerous, layered, and interacting factors that reflect urban land use (Wenger et al. 2009). Urban stream syndrome is defined as a flashier hydrograph, elevated concentrations of nutrients and contaminants, fewer small streams in the network, altered channel morphology and stability, and reduced biotic richness with the dominance of more pollution tolerant species (Paul and Meyer 2001, Walsh et al. 2005b). Identifying the stressors that cause urban stream syndrome may help scientists, engineers, water resource managers, and planners put forth strategies to lessen the impact of urban stream syndrome. Conceptual models for urban impacts on streams have been developed by multiple researchers (Figure 1) (Walsh et al. 2005b, Wenger et al. 2009). These models use stream metabolism, biota, hydrological characteristics, water quality, morphology, flow, land use, riparian buffers, impervious land cover, and watershed characteristics to determine the impacts on urban streams. This paper focuses on the hydrology, geomorphology, stormwater management, stream ecosystem community response, stream ecosystem processes, and restoration of urban streams and catchments.



 Figure 1: Conceptual Model of Urban Impacts on Streams (Wenger et al. 2009). Arrows provide major pathways, but many were omitted for readability. Dashed box in center= water quality, Dashed box (right) =stream ecosystem variables,EI= effective impervious, Mgt= management.

**Abiotic Processess of an Urban Stream**
**Hydrology** In urban catchments impervious surfaces alter the hydrology of the catchment (Leopold 1968, Walsh et al. 2004). In a natural catchment precipitation can take several hydrological paths (Figure 2a-2b). A large portion of precipitation is lost via evaporation and transpiration. Water uptake from the soil by plants is transpired and released through their stomata. Precipitation that is not lost via evapotranspiration will drain to the receiving water by three possible pathways. The first is sub-surface flow, where water enters shallow water-bearing zones, then travels down gradient to a gaining stream. The retention time for sub-surface flows is much slower than overland flows (Walsh et al. 2004). The second is percolation, where the water enters deeper water-bearing units and recharges the local aquifer. The third possible pathway is overland flow, where precipitation stays on the surface and discharges directly into receiving waters. In urban catchments, bare soils and vegetation that has been replaced with impervious land cover, which limits subsurface interactions and increases overland flows. Overland flows have extremely short retention times compared to the other hydrological processes discussed. This short retention time increases the frequency and magnitude of high-flow events (Walsh et al. 2005b).

In urbanized systems there is a decrease in the perviousness of the catchment to precipitation which decreases the hydrological functions discussed above. The percent of imperviousness in urban systems varies with lot size of residential areas (Leopold 1968), where Impervious Cover (IC) increases as lot size decreases. IC is more efficient in transporting precipitation to receiving waters (Walsh et al. 2005b). The difference between the center of precipitation volume to the center of runoff volume, or lag time, is shortened in urban catchments (Espey et al. 1966). This causes urban streams and catchments to be more “flashy”, where large flow events occur more frequently with faster ascending and descending rates in the hydrograph (Walsh et al. 2005b).

 Figure 2a: The hydrological cycle in a forested catchment and in an urbanized catchment with a conventional stormwater drainage system (not considering imports of water supply or export of wastewater). The size of arrows indicates qualitative differences in the relative size of annual water volumes through each pathway. Water that falls on the catchment and is not evaporated or transpired may reach the stream by three possible paths: overland flow (O: almost all of which is transmitted to the stream by stormwater pipes in the urban catchment), subsurface flow through permeable topsoil (S), or percolation (P) into groundwater flow (G) (Walsh et al. 2004).



Figure 2b: The properties of a rainfall event in an catchment before and after urbanization (from Leopold, 1968)

The hydrology of urban catchments has been severely altered. To alleviate some of this stress, riparian buffers could be added to increase retention time and increase evapotranspiration rates within urban areas. These riparian buffers will also allow vegetation and bare soils to exist which will allow increased subsurface alluvial flow and groundwater recharge. Improved urban stormwater management techniques for new developments and redevelopment can also improve subsurface and groundwater recharge. Shifting practices away from culverts, siphons, and drains that lead to municipal stormwater networks and towards techniques that promote infiltration such as downspout infiltration trenching, dry wells, and unlined and vegetated detention ponds (Washington State Department of Ecology 2005) will reduce the effects of IC on urban aquatic ecosystems.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">**Geomorphology** <span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">Hydrologic alteration of a catchment area and change in sediment supply can alter the geomorphology of urban streams. Urban stream channels typically have increased bed and bank erosion that leads to increased widths and cross-sectional areas compared to nonurban streams unless artificially constrained (Leopold 1968). Alteration of drainage density, which is a measurement of stream length per catchment area (km/km2) is impacted by urbanization which changes basin morphometry (Paul and Meyer 2001). When small tributaries are filled in, paved or constrained the natural channel density decreases dramatically in urban catchments (Dunne and Leopold 1978). Channel density can also increase when artificial channels are added, due to the increase of tributaries that add to increased flood velocity (Meyer and Wallace 2001).

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">A long-standing paradigm is that urban stream channels first undergo a period of sedimentation from construction, subsequently experience channel enlargement from increased storm flows, and eventually stabilize (Dunne and Leopold 1978, Chin 2006). As long-term bankfull discharge and sediment supply are altered, streams adjust their channel width and depth (Dunne and Leopold 1978). Urbanization affects both bankfull discharge and sediment supply (Paul and Meyer 2001). In the initial construction phase of urbanization, vegetation is removed and earthwork allows exposed soils to become mobilized and exported in large, episodic events (Wolman 1967, Leopold 1968). This increased sediment supply leads to a channel aggradation phase as sediment loads fill urban channels (Paul and Meyer 2001). As sediment loads increase the stream depth decreases and there is a loss in channel capacity which can lead to overbanking flows and sediment deposits (Wolman 1967).High IC associated with urbanization increases the frequency of bankfull floods (Dunne and Leopold 1978). Deepening and widening of the channel also occur to accommodate the increase in flow discharge. If increases in discharge associated with increasing IC occur, channel enlargement can occur gradually in the erosional phase (Paul and Meyer 2001). The degree of geomorphic response will vary longitudinally along a stream with age of development, catchment slope, geology, sediment characteristics, type of urbanization and land use history (Gregory et al. 1992).

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">The alteration of sediment supply can also change the pattern of the channel in urban streams (Paul and Meyer 2001). During the construction phase, increased sediment supply has converted meandering streams to braided patterns or to more linear channelized patterns (Arnold et al. 1982). Channelized systems lead to increased slope and therefore higher in-stream velocities especially in systems were artificial channel alteration is conducted to increase conveyance efficiency (Pizzuto et al. 2000). Urban streams also have an altered sediment texture (Paul and Meyer 2001). A decrease in gravel classes and fine sediments, and an increase of coarse sand fractions have been observed in urban streams as a result of altered stream velocities which changes the historic sediment supply of the system (Pizzuto et al. 2000). There have also been documented urban systems where large woody debris has decreased (Finkenbine and Atwater 2001), which is important to the geomorphology and ecology of some stream ecosystems (Finkenbine and Atwater 2001, Pease et al. 2011).

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">To improve the geomorphology of urban streams, sediment criteria may be implemented similar to minimum flow criteria. To return to their historic sediment regimes, sediment mobilization projects can provide degrading reaches with additional loads to restore historic channel morphology. In-channel modifications such as channel realignment, high-flow channels and bank destabilization can also provide measures that will alleviate the geomorphic effects of urbanization on aquatic ecosystems.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">**Impervious Land Cover and Stormwater Management**

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">The conveyance of stormwater in urban systems depends heavily on the municipality’s stormwater management system. This can include collection and discharge into tributaries, which then carry stormwater effluents to larger rivers, retention ponds that collect stormwater which then evaporates (or infiltrate if unlined), or the use of manmade and naturally occurring conveyance channels (arroyos, diversions channels, agricultural return flow channels etc.). Depending on the substrate of these stormwater management tools, there are wide ranges of infiltration rates observed. Amplified peak flow during high-flow events represent another characteristic of the hydrograph consistently altered by urbanization (Walsh et al. 2005b).

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">Total impervious cover (TI) is used to measure urban density. Unlike TI, a synergist effect is not seen for subsurface contaminants and regulated point source discharges (Wenger et al. 2009). In 2003, the Center for Watershed Protection developed an Impervious Cover Model (ICM). This model determined streams with low TI can vary greatly in degradation, from pristine to highly impacted. The ICM model also found as TI increases, the impact on streams also increases. TI is a functional indicator for stream impairment because it affects streams via multiple mechanistic pathways. Figure1 in Paul and Meyer (2001) provides a conceptual site model for changes in hydrologic flows with increasing TI which is a causal mechanism for multiple degraded stream ecosystem functions. TI decreases storm water infiltration, and alters stream hydrology which then alters water chemistry.

<span style="display: block; font-family: arial,helvetica,sans-serif; font-size: 16pt; text-align: left;"> **Biotic Responses to Urbanization** **<span style="font-family: "Arial","sans-serif";">Ecological Stoichiometry of Urban Catchments ** <span style="display: block; font-family: "arial","sans-serif"; font-size: 14px; text-align: left;"> Nitrogen and phosphorus are considered limiting nutrients in freshwater ecosystems. The Redfield ratio (106 Carbon: 16 Nitrogen: 1 Phosphorus) is a widely accepted optimum nutrient ratio for aquatic systems (Wetzel 2001). Ecological stoichiometry deals with the mass balance of multiple key elements including carbon, nitrogen and phosphorus (Cross et al. 2005). When the ecological stoichiometry of a freshwater system is altered it can have ecological effects and impact the biological communities. Increased urbanization has lead to higher concentrations of limiting nutrients which alters the ecological stoichiometry of rivers (Wenger et al. 2009). Phosphorus loading can be from wastewater effluent and fertilizers. In the urban setting, individual homeowners applying fertilizers are the primary source of phosphorus in urban rivers (in Paul and Meyer 2001). In former agricultural areas, excess nutrients can be stored in the soil. When agricultural lands are urbanized, phosphorus and nitrogen can be mobilized from the soil, which increases the load of these limiting nutrients into the receiving water (in Paul and Meyer 2001). Nitrogen in urban streams largely depends on the level of waste water treatment, leaking septic systems, illicit discharge and fertilization (Paul and Meyer 2001). Grimm et al. (2005) compared the nutrient retention efficiency of urban streams and unaltered streams in the southwestern U.S. They found that nitrogen retention was lower in urban streams, which had high nitrogen loads, when compared to unaltered, low nitrogen streams. 15N injections by Grimm et al (2005) found that unaltered streams have a higher nitrogen retention capacity than highly urbanized streams. Increased nutrient fluxes increase the number of algal blooms (Smith 2003). Some algal blooms can be toxic to aquatic and terrestrial organisms. Because algal toxins can be bioaccumulated, biotransferred and biomagnified through food chains and food webs, aquatic and terrestrial animals may also be indirectly exposed to toxins when they consume other animals containing toxins(in Camargo and Alonso 2006). Among the different taxonomic groups contributing to the occurrence of toxic algae, cyanobacteria, dinoflagellates and diatoms appear to predominate and may be stimulated by inorganic nitrogen pollution (in Camargo and Alonso 2006). Algal blooms can also cause anoxic conditions that are harmful to aquatic organisms. <span style="display: block; font-family: arial,helvetica,sans-serif; font-size: 14px; text-align: left;"> **Stream Ecosystem Community Response** <span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">Urban stream ecology research has repeatedly established declines in assemblage richness, diversity, and biotic integrity of algae, invertebrates, and fishes with increasing urbanization (Paul and Meyer 2001, Walsh et al. 2005b). Loss of sensitive flora and fauna is sometimes associated with the increase of tolerant species, many of which might be nonnative (Wenger et al. 2009). Wenger et al. (2009) have shown the number of papers related to effects of urbanization on biological assemblages published in peer-reviewed journals between 1988 and 2008 has increased dramatically. Specifically, there has been a dramatic increase in studies looking at the urban effects on fishes, macroinvertebrates, and algae. Less work has been published on birds, herpetofauna and other vertebrates. Little is known about the microbial communities in urban streams.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">As a part of the USGS National Water-Quality Assessment Program, Brown et al. (2009) determined that prior land use, (i.e. agriculture) has lasting effects on responses of water quality (Nitrogen and herbicides), algae, and fishes. The study found no detectible response to urbanization in areas with previous agriculture. Presumably, this is due to the impact from previously agricultural lands where sensitive species have already been eliminated by the prior urban land use (Brown et al. 2009, Wenger et al. 2009). Macroinvertebrates have been shown to be the assemblage group with the highest sensitivity to urbanization (Brown et al. 2009). This could be a result of higher macroinvertebrate diversity than fishes, and less knowledge of algal taxa compared to macroinvertebrates (Wenger et al. 2009). There is a substantial amount of variation in the urban effects on stream ecosystem processes (Walsh et al. 2005b). It has been argued that more urban stream research is needed before generalizations can be made about different taxa and different geographic locations (Walsh et al. 2005b, Wenger et al. 2009).

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">A hypothetical model for dissolved oxygen (DO) as a result of random urban pulses during fish development has been developed (Porcella and Sorensen 1980), but no empirical studies have been conducted. Helms et al. (2009) evaluated the impact of land cover on fish assemblages by examining relationships between stream hydrology, physicochemistry, in-stream habitat and their association with fish responses. This study looked at streams draining 18 watersheds of the Lower Piedmont of western Georgia. Overall, this study found that fish assemblage structure was best described by total dissolved solids (TDS) and DO. Streams with high TDS and low DO contained sunfish-based assemblages and low TDS and high DO streams contained minnow-based assemblages. Helms’ et al. (2009) results imply that altered hydrological and physicochemical conditions, induced largely by TI, may be a strong determinant of fish assemblage structure in lowland streams. This relationship allows for a more mechanistic understanding of how land use ultimately affects inland fish assemblages. Based on this understanding, modifications to flow regimes and geomorphology can help improve these aquatic ecosystems and in turn lessen the stress on urban biota.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">**Urban Stream Ecosystem Processes**

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">Primary productivity, leaf composition, and nutrient cycling are ecosystem processes that have been overlooked in urban streams, although these processes have been studied in natural systems in great detail (Paul and Meyer 2001). These ecosystem processes influence the trophic dynamics of urban streams which can alter the biological assemblages of these systems.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">Many factors control the breakdown of allochthonous material in streams (Young et al. 2008). Natural factors include climate, longitudinal position, altitude, naturally occurring nutrient concentration, hydrological regimes, river geomorphology and bed substrate, aquatic invertebrates, and riparian vegetation (Young et al. 2008). Leaf decomposition is also strongly influenced by anthropogenic disturbance to the stream ecosystem. Urban stressors include riparian vegetation alteration which increases stream temperature, alteration of nutrient concentration, alteration of sediment inputs and increased contaminant loads (Wenger et al. 2009). Walsh et al. (2005) found an inconsistent response in leaf breakdown, but Wenger et al. (2009) found that leaf breakdown rates are higher in some urban streams than in nonurban streams. Wenger et al. (2009) also note that the putative mechanisms differ.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">DO levels have long been taken as indicators of the health of a water body (Keefer et al. 1979). DO in lotic systems is usually high, uptake from the atmosphere is high and the diurnal variation is high (coupled to photosynthesis, respiration, and decomposition) (Wetzel 2001). Urban streams that receive insufficiently treated wastewater, biological and chemical oxygen demand, can lead to DO sags (reviewed in Paul and Meyer 2001). Keefer et al. (1979) analyzed 104 urban streams in the United States, of which more than 40% showed a higher probability of greater than average oxygen deficits below 2 mg/L. Most lotic organisms (fish and macroinvertebrates) are adapted to well-oxygenated systems. Severe oxygen sags can cause a decrease in biological integrity (Wenger et al. 2009). Although, in the Pacific Northwest, Finkenbine and Atwater (2001) found that urban streams had higher values of intergravel DO than in rural streams due to the decrease of fine grain sediments, which is beneficial to eggs deposited by spawning salmonids. Random flood pulses during sensitive life stages (egg growth and fry development) could cause year classes to be eliminated (Porcella and Sorensen 1980). Continued annual cycles with DO sags could eliminate the hypothetical fish species.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">River ecosystem metabolism is the combination of gross primary production (GPP; photosynthesis [P]) and ecosystem respiration (ER) which is a measurement of how much organic carbon is produced and consumed in rivers (Young et al. 2008). The ratio of P/ER or GPP/ER is informative and provides information on the relative importance of the allochthonous and autochthonous organic matter. Ecosystem metabolism provides a direct measurement of the primary production and decomposition of river ecosystems and helps to determine their life supporting capacity (Young et al. 2008). Metabolic rates at urban impacted sites can be compared with rates measured at less altered or “pristine” sites that are characterized by similar stream order and size. Low GPP does not necessarily indicate a poor aquatic ecosystem, e.g., first order, forested streams (Young et al. 2008). Extremely low rates of ER are more likely to represent a stressed aquatic ecosystem (Young et al. 2008).

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">Ecosystem processes such as primary productivity have been overlooked in urban streams, although they have been extensively studied in other types of stream ecosystems (Allan et al. 1997). A review by Paul and Meyer (2001) identified studies where ecosystem metabolism has been measured in several urban streams. Studies in Michigan (Ball et al. 1973) and Georgia (Paul 1999) found that urban rivers had higher GPP and R than in rural rivers; the GPP to respiration (P/R) ratio in urban rivers without municipal effluent was greater than in rural streams and greater than 1.0, indicating an autotrophic system; urban rivers that receive effluent from waste water treatment plants (WWTP) are found to have increased respiration rates and the P/R is ratio less than rural rivers and far less than 1.0, indicating a predominantly heterotrophic system. Uehlinger and Brock (2005) studied periphyton metabolism at five sites along 70 km of the Truckee River below the city of Reno, Nevada. The Truckee is an open channel desert river, which is influenced by WWTP effluents. In the study periphyton responded to nutrient enrichment from a point source with a significant increase in biomass and metabolism, but only under favorable light and temperature conditions prevailing during summer in this desert river. The conditions observed of negative net daily oxygen metabolism during summer have implications to the oxygen balance in this and other desert rivers, where low dissolved oxygen conditions may prevail seasonally and serve as a stressor for aquatic biota (Uehlinger and Brock 2005). A meta-analysis to asses ecosystem health using ecosystem metabolism (distribution of rates of GPP and ER) at “pristine” and impacted sites (N=213) was conducted by Young et al. (2008). These criteria could be used to determine the health of streams that are subject to urban stressors.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">Additional studies focused on primary productivity, leaf composition, and nutrient cycling may provide further insight into the effects on ecosystem metabolic processes in urban streams. Ecosystem processes influence the trophic dynamics of urban streams which alters the biological assemblage of these systems. If urban streams are managed to promote better ecosystem function, it may provide additional benefit to urban aquatic ecosystems and the biota within these systems.

<span style="display: block; font-family: arial,helvetica,sans-serif; font-size: 16px; text-align: left;"> **Restoration of Urban Systems** <span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">Humans have become dependent on the ecosystem services that healthy and self-sustaining catchments provide (Postel and Richter 2003). As discussed in this paper, impaired urban streams and catchments have altered ecological, hydrologic and geomorphic characteristics. As a result, the ecosystem services that urban catchments provide, are diminished or on the brink of collapse. Restoration of urban systems can include in-stream restoration, riparian/stream buffer restoration, and watershed restoration and management depending on the goal and associated stressors. Within the USA, billions of dollars are spent on stream and river restoration (Palmer et al. 2003) and have increased exponentially in the last decade. Stream restoration activities are diverse, ranging from channel engineering, hydrologic modification, restoring riparian vegetation, bank stabilization, and habitat improvement. To maximize the benefit of stream restoration, an interdisciplinary team of geomorphologists, engineers, and ecologists should be included on a stream restoration project. Collaboration between the restoration team and community groups, watershed managers and stakeholders will also maximize the benefit of a stream restoration project (Palmer et al. 2003).

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">Many rivers and watershed have endured irreversible changes in the catchment hydrology, geomorphology, permanent infrastructure in the floodplain and catchment, or the introduction of non-native flora and fauna. Rather than attempting to recreate unobtainable historical conditions, a pragmatic approach where the goal is to achieve the least degraded and ecologically dynamic state (Palmer et al. 2005). If funding allows, the restoration should not focus on the local scale stream restoration, but instead focus on the catchment-scale impacts associated with urbanization (Walsh et al. 2005a).

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">Sewage and stormwater runoff are two major stressors that degrade receiving waters (Walsh 2000). To alleviate the stress of stormwater drainage, restoration of the catchment rather than the stream itself is needed (Walsh et al. 2005a). Impervious cover and drainage infrastructure are causal attributes determining the quality and quantity of urban stormwater (Walsh et al. 2005a). Techniques that reduce TI and promote infiltration will reduce max discharge and also stabilize base flow by allowing longer resonance time and promote discharge via subsurface flows.Techniques such as treatment wetlands, vegetated swales, and riparian buffers provide a substrate where vegetation can uptake nutrients and other urban contaminants of concern. Upgrading wastewater treatment plants to tertiary processes will further stabilize oxygen demanding substances <span style="display: block; font-family: "arial","sans-serif"; font-size: 14px; text-align: left;"> Low impact development (LID) is a design strategy with a goal of maintaining or replicating the historic hydrologic regime through the use of design techniques to create a functionally equivalent hydrologic landscape (USEPA 2000). The principles of LID are based on controlling stormwater at the source by the use of microscale controls that are distributed throughout the site (USEPA 2000). LID provides additional benefits to the infrastructure and are more cost effective and aesthetically pleasing than traditional, structural stormwater conveyance systems (USEPA 2000). LID also provides ecosystem services such as wildlife habitat, protection and preservation of sensitive features (wetlands, steep slopes, valuable trees, flood plains, woodlands and permeable soils), groundwater recharge and biofiltration of stormwater. LID can also lessen the hydrologic and geomorphic stressors previously identified by reducing the impervious land cover and promote groundwater infiltration. LID can include bioretention areas, grass swales, vegetated roof covers, permeable pavements, and other features (Figure 3). Many states are adopting and inforcing LID practices for new residential and commercial development to help alleviate the stressors of urbanization on the watershed. Existing infrastructure can also adopt many of the LID practices identified to improve urban watersheds.

Figure 3: Low Impact Hydrologic Design and Analysis Components (Coffman, 2000)

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">**Conclusion**

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 14px;">The current understanding of urban ecosystems has come a long way in the last twenty plus years. As shown, urban streams are dynamic ecosystems that have multiple stressors associated with them. As urban populations continue to grow, these stressors will also increase. Increased urban ecological research, to fill in identified data gaps, will further our understanding of the impacts of urbanization on aquatic ecosystems. The research discussed in this paper provides planners, scientists, engineers and water managers with key pathways that affect urban aquatic ecosystems. It also provides some insight on how to alleviate these adverse effects on urban streams. Urban streams provide ecosystem services that humans have become accustomed to. Urban streams and rivers also offer local communities with an easily accessible retreat from urban sprawl. Engaging the general public in the form of ecological outreach and education will give the community a sense of self-worth in regards to urban streams. Additional research and engagement with the general public will hopefully spark the restoration and better environmental management of these endangered ecosystems.

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Great paper Justin. Your paper is nicely organized and well written. It's also quite comprehensive. At this point, my only suggestion for expanding would be to include a paragraph on Low Impact Development/Green Infrastructure in your final section on 'Restoration of Urban Streams'. (100%) -MS