Urban Development Creates Multiple Stressors That 

Can Degrade Stream Health by Changing Stream 

Hydrology, Habitat, and Chemistry

Analyses of how changes in stream hydrology, habitat, 

and chemistry relate to the species composition of biological 

communities indicate that no single environmental factor was 

universally important across all the study areas in explaining the 

effects of urban development on stream ecosystems. Even within 

a single study area, the three biological communities that were 

surveyed—algal, invertebrate, and fish—had different responses 

to urban development and changing environmental factors. The 

algal, invertebrate, and fish communities have different life cycles 

and requirements for food, shelter, and reproduction; consequently, 

their responses typically vary with stressors that arise from 

urban-related changes in physical and chemical factors. Different 

responses among the three communities, however, can provide 

important clues about the types of stressors that occur with urban 

development, which then can be linked to management actions 

that may be appropriate for improving stream conditions.

Dallas

Boston

Denver

Raleigh

Atlanta

Portland

Milwaukee

Birmingham

Salt Lake City

Figure 1. Streams from nine metropolitan areas across the Nation were sampled to assess the effects of urban 

development on stream ecosystems. Understanding how stream ecosystems change as levels of urban development increase 

in watersheds can assist in the development of management actions to protect and rehabilitate urban stream ecosystems. 

The photograph shows a stream in Milwaukee, Wisconsin, that was lined with concrete, but has been reconstructed to 

replicate a more natural channel.

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3071

October 2012

National Water-Quality Assessment Program

Urban Development Results in Stressors 

That Degrade Stream Ecosystems

In 2003, eighty-three percent of Americans lived in metropolitan areas, and 

considerable population increases are predicted within the next 50 years. Nowhere are 

the environmental changes associated with urban development more evident than in 

urban streams. Contaminants, habitat destruction, and increasing streamflow flashiness 

resulting from urban development have been associated with the disruption of biological 

communities, particularly the loss of sensitive aquatic biota. Every stream is connected 

downstream to other water bodies, and inputs of contaminants and (or) sediments to 

streams can cause degradation downstream with adverse effects on biological communities 

and on economically valuable resources, such as fisheries and tourism. Understanding 

how algal, invertebrate, and fish communities respond to physical and chemical stressors 

associated with urban development can provide important clues on how multiple stressors 

may be managed to protect stream health as a watershed becomes increasingly urbanized.

This fact sheet highlights selected findings of a comprehensive assessment by the 

National Water-Quality Assessment Program of the U.S. Geological Survey (USGS) of the 

effects of urban development on stream ecosystems in nine metropolitan study areas (fig. 1).
Hydrology—Urban Development Leads to Increased 

Variability in Streamflow

Urban development typically increases the amount 

of water entering a stream after a storm and decreases 

the time that it takes for the water to travel over altered 

land surfaces before entering the stream. Efforts to reduce 

flooding by draining water quickly from roads and parking 

lots can result in increased amounts of water reaching a 

stream within a short period of time, which can lead to 

stream flashiness and altered stream channels (fig. 2). 

Erosion caused by increased streamflow can also degrade 

the spawning and feeding habitat for fish and reduce the 

living space for invertebrates. Additionally, rapid runoff 

reduces the amount of water available to infiltrate the soil 

and recharge the aquifers, which often results in lower 

sustained streamflows, especially during the summer. 

Habitat—Urban Development Can Alter 

Stream Channels

Stream habitats can be severely degraded where 

urban development occurs along the streambanks, such as 

where a stream has been straightened by channelization 

or where manmade structures have replaced natural 

riparian vegetation. Additionally, urban development 

that occurs throughout a watershed (but not necessarily 

directly along the streambank) can result in degraded 

habitat within a stream channel through flow alteration 

and sediment erosion. Urban development often 

results in deeper stream channels or an increase in the 

stream-channel cross-sectional area. The magnitude of 

these effects depends on natural environmental factors, 

such as the geology and soils that can influence the 

geomorphic characteristics of a stream and its watershed.

Chemistry—Concentrations of Contaminants 

in Water Increase With Urban Development

Concentrations of contaminants, including 

nitrogen, chloride, insecticides, and polycyclic aromatic 

hydrocarbons (PAHs), increased with urban development, 

although few measurements exceeded any human or 

aquatic-life benchmarks. The total concentration of 

insecticides increased with urban development in seven 

of the nine study areas. The numbers of individual 

insecticides detected and the relation between total 

concentrations of these compounds and the amount 

of urban development varied across the study areas 

and appeared to coincide with regional pesticide-use 

patterns. For instance, higher concentrations of the 

insecticides chlorpyrifos and chlordane were detected in 

urban streams in the Atlanta, Dallas, and Raleigh study 

areas because of the historical use of these compounds for 

termite control in these areas. These results underscore 

the importance that regional differences need to be taken 

into account when comparing the influence of urban 

development on aquatic biota in different areas.

Aquatic Biota—Loss of Sensitive Species Was the Most 

Consistent Biological Response to Urban Development 

Urban development generally results in a shift in the 

species composition of the algal, invertebrate, and fish 

communities. The most consistent change in any of the 

biological communities, however, was the loss of sensitive 

invertebrate species and a shift to a community with a 

higher percentage of species more tolerant to physical 

and chemical stressors. A loss in the numbers (richness) 

of sensitive species, which is related to a decrease in 

Ephemeroptera (mayflies), Plecoptera (stoneflies), and 

Trichoptera (caddisflies) invertebrate species (EPT), 

was a common response in all study areas where urban 

development occurred in forested watersheds. The number 

of EPT species is a biological condition metric that is 

used in many biomonitoring programs across the country 

because it is sensitive to environmental degradation. A 

reduction of more than 50 percent of EPT species was 

observed in some study areas as the percentage of urban 

development increased in the watersheds from low 

to high levels.

Figure 2. Increases in streamflow volume and frequency 

can lead to flooding and damage to roads and bridges. 

These flooding events also erode the streambank, sending 

sediment downstream. This road crossing was washed 

out during a flood near Atlanta, Georgia, in 2009.

2
The Invertebrate Community Begins to Degrade at the 

Earliest Stages of Urban Development 

The invertebrate communities begin to degrade at the 

onset of urban development, which indicates that some 

invertebrate species are highly sensitive to physical and 

chemical changes associated with urban development (fig. 3). 

There was no evidence that the biological communities were 

resistant to even low levels of urban development, based on 

the observation that sensitive species were being lost over 

the initial stages of development in relatively undisturbed 

watersheds. Likewise, over moderate to high levels of 

urban development, no exhaustion threshold was observed 

(typically a “flat line” response) that would have indicated the 

biological communities were degraded so severely that only 

the most tolerant species remained. The absence of such an 

exhaustion threshold indicates that stream-restoration efforts 

could have a positive effect on the biological condition 

of a stream regardless of the level of urban development 

in the watershed.

 Management Implications 

• Stormwater-management strategies that result in 

disconnecting impervious cover from the storm-drainage 

systems may reduce streamflow flashiness, thereby 

reducing channel erosion and protecting stream habitat. 

• Strategies that result in slowing the runoff of precipitation 

from developed areas to nearby streams can increase 

infiltration to groundwater, which can help sustain 

streamflows during the summer and fall and reduce the 

amounts of contaminants that are transported to streams 

in stormwater runoff. 

• Because the response of algae, invertebrates, and fish to 

physical and chemical stressors varies, monitoring all three 

communities provides multiple lines of evidence when 

assessing the effects of disturbances to aquatic systems. 

• Strategies that carefully manage development in 

undisturbed watersheds are important for minimizing 

detrimental effects on aquatic biological communities 

because the degradation of these communities begins 

at the onset of urban development. 

• The lack of an exhaustion threshold in the biotic response 

indicates that stream-restoration efforts are likely to 

improve the biological condition of many streams in 

urban areas. 

• The continuous decline in the biological condition of 

a stream as the amount of urban development increases 

in a watershed indicates that biological diversity and 

food-web complexity may be reduced. These types of 

changes may make the stream, and the ecosystem as a 

whole, more vulnerable to other changes, such as the 

introduction of non-native species. 

Figure 3. By the 

time that urban 

development in the 

Boston study area 

was approaching 

20 percent, the 

invertebrate 

community had 

declined by 

approximately 

25 percent, based 

on changes in 

community condition.

Percent urban development

0 20 40 60 80 100

Approximately 

25-percent change

0

100

50

Invertebrate community condition

3

In urban stream ecosystems, 

environmental stressors can have 

a negative effect on the overall 

health of the streams. These 

stressors may include 

eroding banks, chemical 

contaminants, 

leaking pipes, 

garbage, 

impervious 

surface runoff, 

invasive and 

exotic plants 

and animals, and 

removal 

of riparian 

trees and plants.

Drawing by Frank Ippolito, Production Post Studios, 110 North Fulton St., Bloomfield, N.J.
Urban Development Affects Stream Ecosystems 

Differently Across the Country 

Regional environmental factors, such as climate, geology, 

topography, and land cover, strongly influence the species 

composition of the biological communities in streams. 

Regardless of the level of urban development in a watershed, 

the biological communities in sampled streams still had similar 

community composition characteristics distinct for the region. 

These regional differences are important to consider when 

assessing stream health and when designing management 

strategies for stream remediation. 

Regional Differences in the Types of Land Undergoing Urban 

Development Influenced the Response of Stream Chemistry 

and Aquatic Biota 

In-stream concentrations of total nitrogen increased 

with urban development in the Portland, Atlanta, Raleigh, 

and Boston study areas, which were regions where land cover 

prior to urban development was primarily forested with total 

nitrogen concentrations generally at or near background 

concentrations. In the Denver, Dallas, and Milwaukee study 

areas, the predominant land cover prior to urban development 

was agriculture and grassland, which can contribute to elevated 

total nitrogen. Nitrogen concentrations were relatively high 

in these streams, even under low levels of urban development. 

Consequently, relations between urban development and 

nitrogen concentrations were not as strong in the Denver, 

Dallas, and Milwaukee study areas as in study areas where 

urban development occurred on forested land. 

The pre-urban land cover was also an important factor 

in how the biological communities responded to urban 

development in the Denver, Dallas, and Milwaukee study 

areas. In these three study areas, the decline in biological 

communities was consistently less than in the other study 

areas. The reason for this difference was not because 

biological communities in these regions are more resilient to 

stressors from urban development but because the biological 

communities had already lost sensitive species to stressors 

from pre-urban agricultural land-use activities. Thus, the 

biological communities in these three study areas had already 

been degraded before the onset of urban development (fig. 4).

0 4 8 12 16

Boston

Portland

Salt Lake City

Birmingham

Atlanta

Raleigh

Denver

Dallas

Milwaukee

Land cover before urban development

Smallest loss of sensitive species

because of prior degradation

Greatest loss of sensitive species

Forested land

Difference in number of EPT invertebrate species

between the least and most developed sites in 

each study area

Agricultural land

Figure 4. The loss of Ephemeroptera, Plecoptera, and Trichoptera (EPT) invertebrate species was greatest when urban 

development occurred on forested land (green bars). Fewer species were lost in streams of agricultural watersheds 

(yellow bars) because the biological communities already had endured some degree of degradation associated with 

agricultural land-use activities, and relatively sensitive EPT species were already absent in streams in these areas.

4
Figure 5. The Biological Condition Gradient can be used to measure and communicate characteristics of stream health. 

The transition from tiers 1 through 6 (y-axis) indicates a declining biological condition and stream health as the stressor 

levels increase (x-axis). Modified from Davies and Jackson (2006).

How Can This Information Be Used to Manage Urban 

Stream Ecosystems?

When evaluating alternative strategies designed to reduce 

the effects of urban development, managers need methods 

that account for the dynamic nature of stream ecosystems so 

that findings can be presented in a way that is scientifically 

valid and readily understood. Models can play important 

roles in describing how the physical, chemical, and biological 

components of these systems interact and for predicting what 

will happen if changes are made to one or more components.

Using the Biological Condition Gradient to Describe 

Stream Health

A model that predicts how urban development affects 

stream ecosystems is most effective as a management tool 

if its endpoints and outcomes are stated in terms that are 

meaningful to scientists, managers, and the public. The 

Biological Condition Gradient is one modeling approach 

that can be used to characterize endpoints and outcomes for 

meeting this goal.

Essentially, this approach provides a standardized 

framework for evaluating stream health as measured by 

the biological community (fig. 5). The Biological Condition 

Gradient is defined by six levels or tiers of stream health, 

ranging from excellent (natural habitat) to poor (altered 

habitat), as the biological community degrades from 

stressors, such as those associated with urban development. 

When linked to biological endpoints that are characteristic 

of streams for a particular region, the tiers are helpful in 

evaluating management strategies designed to improve stream 

health. This approach can be used to set realistically attainable 

goals for stream rehabilitation and to diagnose and correct 

problems before they become larger.

Innovative Model Predicts How Hydrology, Habitat, 

and Chemistry Can Affect Stream Health

In a pilot investigation using data from the Boston 

study area, an innovative regional model was developed 

for New England to predict how different combinations of 

urban-related stressors associated with stream hydrology, 

habitat, and chemistry affect stream health, as measured by 

changes in the invertebrate community. The tool can be used 

to evaluate how changes to one or multiple urban stressors 

can affect biological endpoints and the likelihood of attaining 

the desired stream-health goal (fig. 6). The model was 

structured to predict the probability of attaining six tiers of 

stream health (fig. 5), which makes it possible for different 

management scenarios to be evaluated for protecting stream 

health in urbanizing areas. For example, the model predicted 

that the likelihood of attaining a healthy stream would be 

only about 25 percent when the levels of urban development 

in a watershed exceeded 31 percent. Management actions 

to improve water quality and reduce stream flashiness are 

predicted to increase the likelihood of attaining a healthy 

stream to about 70 percent. Biological Condition Gradient

Level of exposure to stressor

Tiers of biological condition

1

2

3

4

5

6

Sensitive taxa markedly diminished; unbalanced distribution of major 

taxonomic groups; reduced complexity in ecosystem function. 

Natural habitat, flow regime, 

and stream chemistry.

Altered habitat, flow regime, 

and stream chemistry.

Moderate changes in structure due to replacement of some 

sensitive ubiquitous taxa by more tolerant taxa; ecosystem 

functions are largely maintained.

Evident changes in structure due to loss of some rare native taxa; 

decreased relative abundance; ecosystem level functions are 

fully maintained.

Structure and function similar to natural community with some 

additional taxa and biomass; ecosystem level functions are 

fully maintained.

Natural structural, functional, and taxonomic integrity is preserved. 

(Excellent stream health)

Extreme changes in structure and ecosystem function; wholesale 

changes in taxonomic composition; extreme alterations from 

normal densities. (Poor stream health)

5
Additional information on the effects of urban development 

on stream ecosystems is available in the following publication: 

Coles, J.F., and others, 2012, Effects of urban development on 

stream ecosystems in nine metropolitan study areas across 

the United States: U.S. Geological Survey Circular 1373, 

138 p. (Also available at http://pubs.usgs.gov/circ/1373/.)

Videos, podcasts, articles, and fact sheets describing the 

USGS assessment of the effects of urban development on 

stream ecosystems in nine metropolitan areas of the United 

States are available at http://water.usgs.gov/nawqa/urban/.

By: Amanda H. Bell, James F. Coles, 

Gerard McMahon, and 

Michael D. Woodside

For additional information contact:

Chief, National Water-Quality

Assessment Program

U.S. Geological Survey 

413 National Center 

Reston, VA 20192

Reference Cited

Davies, S.P., and Jackson, S.K., 2006, The biological condition gradient: A descriptive model for 

interpreting change in aquatic ecosystems: Ecological Applications, v. 16, no. 4, p. 1251–1266.

Fact sheet photography by Alan M. Cressler, 

USGS, unless otherwise indicated. 

Hydrology

(flashiness)

Chemistry

(specific

conductance)

Generic

richness

Habitat

(substrate)

Urban development

Biological Condition Gradient (BCG)

Likelihood

Tier of attainment

Percent

1

2

3

4

5

6 2

0 50 100

8

20

35

27

8

Reduce

flashiness

Improve

water quality

70% likelihood

of attaining BCG 

tier 3 or better

Filter feeder

relative abundance

Plecoptera +

Ephemeroptera

relative abundance

Regenerative stormwater 

conveyance systems are being 

used in Anne Arundel County, 

Maryland, to treat and detain 

stormwater, thereby reducing 

stream flashiness and improving 

water quality. (Photograph by 

Barbara Eikenberry, USGS.)

Greenstreet and parking lot 

retrofits can improve water quality 

by directing stormwater into a 

bioretention area. (Photograph by 

Center for Watershed Protection, 

Ellicott City, Maryland.)

6

Figure 6. Stream health in urban areas is often affected by multiple 

stressors associated with stream hydrology, habitat, and chemistry. 

New multi-stressor models can provide insights on how management 

actions to improve one or more of these stressors may increase the 

likelihood of obtaining a desired biological condition. For instance, 

management actions to reduce stream flashiness and improve 

water quality in the Boston study area are predicted to increase the 

likelihood of attaining a healthy stream to about 70 percent.