Urban Streams: an
endangered aquatic
ecosystem
Justin Reale
March XX, 2012
CE 598 Watershed and River Restoration
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, and restoration of
urban streams and catchments. 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 and restore these endangered aquatic
ecosystems.
2
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
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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.
4
Discussion
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 2). 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
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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 2: 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).
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
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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.
Geomorphology
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/km 2) 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).
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).
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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).
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
8
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).
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.
Impervious Land Cover and Stormwater Management
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
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flow during high-flow events represent another characteristic of the hydrograph
consistently altered by urbanization (Walsh et al. 2005b).
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.
Stream Ecosystem Community Response
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 peerreviewed journals between 1988 and 2008 has increased dramatically. Specifically,
there has been a dramatic increase in studies looking at the urban effects on fishes,
10
macroinvertebrates, and algae. Less work has been published on birds, herpetofauna
and other vertebrates. Little is known about the microbial communities in urban streams.
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).
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
11
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 minnowbased 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.
Biogeochemisty of Urban Catchments
Urban Stream Ecosystem Processes
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.
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.
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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.
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 welloxygenated 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.
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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).
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
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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.
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.
Restoration of Urban Systems
Humans have become dependent on the ecosystem services that healthy and selfsustaining catchments provide (Postel and Richter 2003). As discussed in this paper,
impaired urban streams and catchments have altered ecological, hydrologic and
15
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).
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).
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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 to
reduce sewage and nutrients from streams
Conclusion
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.
17
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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