General Information Product 143

October 2012

U.S. Department of the Interior

U.S. Geological Survey

Dragonfly 

Greenside darter larvae

Smallmouth bass

Epithemia 

Cymbella

Isopod

Green

algae

Common carp Leech Blue-green algae

Fathead minnow

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/.

Drawing by Frank Ippolito, Production Post Studios, 110 North Fulton St., Bloomfield, N.J. Drawing by Frank Ippolito, Production Post Studios, 110 North Fulton St., Bloomfield, N.J.

Photographs by Alan M. Cressler

Rainfall gradually reaches 

a stream in a natural or 

undeveloped setting by 

flowing over the land 

surface into the stream 

and by seeping into the soil 

and flowing underground 

(as groundwater) toward 

the stream. These natural 

seasonal patterns of 

hydrology, together with 

seasonal changes in light 

and temperature, serve 

as life cycle cues to the 

biological communities.

Stream habitat is the 

physical living space of 

aquatic biota and includes 

the channel size and shape, 

water depth and velocity, 

and structures within the 

stream, such as woody 

debris and boulders. Slow 

moving, deeper areas of a 

stream are called pools, and 

faster flowing shallow areas 

are referred to as riffles. A 

natural stream with multiple 

habitats generally will have a 

diverse biological community.

Some chemicals and 

nutrients, such as nitrogen and 

phosphorous, are required for 

all stream life. Nutrients are 

incorporated into algae, which 

are then consumed by other 

biota, such as invertebrates 

and fish, thus introducing 

the nutrients into the aquatic 

food web. Oxygen dissolved 

in water is essential for all 

biological communities, and 

adequate amounts of oxygen 

are necessary to support a 

diverse biological community.

Fish have life cycles 

that can span years and 

are affected by stream 

hydrology, habitat, 

chemistry, and other 

biological communities. 

Fish are relatively mobile 

along the stream as they 

search for food. Smallmouth bass can hide under 

logs or undercut banks, 

along stream edges or 

in pools, and emerge to 

feed on crayfish and small 

fish. Greenside darters 

live in riffle habitats of 

streams, where they feed 

on aquatic invertebrates, 

such as dragonfly larvae.

Invertebrates have complex 

life cycles that occur over time 

spans of weeks to years. Most 

aquatic insects spend nearly all 

their life in the water as eggs 

and larvae, and then leave the 

water and develop wings as 

adults. This dragonfly larva lives 

in areas of slower streamflow, 

where it preys on other invertebrates and even some small fish.

Many species of dragonflies 

are sensitive to pollution, as 

are mayflies and stoneflies. 

These invertebrates crawl on 

the surfaces of rocks and feed 

by gathering and shredding leaf 

debris, scraping off algae, or 

preying on other insects.

Native fishes that are 

sensitive to changes in the 

stream ecosystem generally 

become less abundant 

with increased urban 

development, while tolerant 

fishes can thrive. The 

fathead minnow, although 

native to streams in the 

United States, tolerates 

muddy, low-oxygen water 

that is typical of many 

urban streams. Fish that 

are more tolerant to urban 

stressors are often nonnative species, such as the 

common carp, that prefer 

slow or still water and silty 

stream sediments.

Urban development leads 

to a loss of invertebrate

species that are sensitive to 

pollution, such as mayflies 

and stoneflies, and an 

increase in more tolerant 

species, such as leeches 

and isopods. The loss of 

species that are sensitive 

to pollution can begin at 

very low levels of urban 

development. Tolerant 

species, such as leeches 

are most common in warm, 

protected shallow areas 

of streams. Isopods prefer 

slower moving streams with 

relatively low dissolved 

oxygen levels.

An increase in urban 

development often results 

in a high abundance of 

algae that are tolerant of 

pollution. Diatom algae tend 

to decrease and non-diatom 

algae tend to increase with 

urban development. Some 

non-diatom algae, such as 

green or blue-green algae 

that appear as a green 

coating on the surface of 

the water and rocks, are in 

low abundance in natural 

streams but might increase 

in abundance to nuisance 

levels from open sunlightand nutrient-rich conditions 

in many urban streams.

Natural Stream Ecosystem Hydrology, Habitat, Chemistry Conditions Natural Stream Ecosystem Biological Communities Urban Stream Ecosystem Hydrology, Habitat, Chemistry Conditions Urban Stream Ecosystem Biological Communities

Natural

Stream 

Ecosystem

Urban

Stream 

Ecosystem

Natural Stream Ecosystem Urban Stream Ecosystem

Urban development in 

a watershed alters the 

hydrology or movement of 

water through a watershed. 

As the amount of impervious 

surface and artificial drainage 

systems (for example, storm 

drains) increases with urban 

development, stormwater 

runoff from developed sites 

occurs more quickly. The 

higher streamflows that 

often result can alter stream 

channels through streambank 

erosion and can increase the 

magnitude of seasonal floods 

to a level that damages 

homes and property near the 

stream and in the flood plain.

Urban development 

can alter habitats that 

provide living spaces for 

the biota in and around 

the stream. Plants and 

trees near a stream can 

be removed to increase 

the amount of light 

reaching streams, and 

cement or rock can be 

added to the channel 

to protect it from high 

streamflow. Sediment 

from erosion can fill 

spaces between rocks 

on the stream bottom, 

thus reducing living 

space or habitat for the 

biological communities.

Urban development might 

increase the inputs of 

chemicals to levels that 

greatly exceed those that 

occur naturally in streams 

and can be toxic to the 

biological communities. For 

example, excess amounts 

of nutrients from fertilizers 

can lead to an abundance 

of algae and might result 

in extreme high and low 

levels of dissolved oxygen 

in a stream. Pesticides from 

lawn care or insect control 

and heavy metals from 

industry and vehicles can be 

ingested or absorbed by the 

biological communities.

Algae, such as these 

diatoms, are microscopic 

plants and are the foundation of aquatic foodwebs. 

Algae have short life 

cycles of days to weeks, 

and they can respond 

rapidly to changes in 

sunlight, water chemistry, 

and streamflow. The most 

common algae reported in 

natural streams of small-tomoderate size are diatoms, 

which attach to underwater 

surfaces, such as rocks and 

aquatic plants. Cymbella

is found in riffles, while

Epithemia is found in both 

pools and riffles.

Stream Ecosystems Change With Urban Development

The healthy condition 

of the physical living 

space in a natural 

stream—defined by 

unaltered hydrology 

(streamflow), high 

diversity of habitat 

features, and natural 

water chemistry— 

supports diverse 

biological communities 

with aquatic species 

that are sensitive 

to disturbances.

In a highly degraded 

urban stream, the 

poor condition of the 

physical living space—

streambank and tree 

root damage from 

altered hydrology, low 

diversity of habitat, 

and inputs of chemical 

contaminants—

contributes to biological 

communities with 

low diversity and high 

tolerance to disturbance.

By Amanda H. Bell, James F. Coles, and Gerard McMahon

National Water-Quality 

Assessment Program

What is a stream ecosystem? A stream ecosystem is 

defined by the hydrology, habitat, and chemistry conditions 

and the biological communities within the stream, all of 

which are influenced by activities in the surrounding watershed. A complex and well-balanced ecosystem provides 

recreation, aesthetics, food, water, nutrients, and many 

other valuable assets to humans, animals, and plants that 

live in the area. Natural stream ecosystems are well adapted 

Urban development is associated with changes in the 

natural environment such as alterations to the hydrology, 

habitat, and chemistry of a stream, which result in stressors 

to biota in stream ecosystems. Impervious surfaces, such as 

parking lots, roads, and rooftops, limit the amount of rainwater 

seeping into the ground, which increases stormwater runoff. 

Urban areas often experience a rapid rise in streamflow after 

a rainfall, which can erode streambanks and bottoms and 

to seasonal environmental changes, such as annual flooding 

and drought cycles.

Every stream is connected downstream to other water 

bodies including rivers, reservoirs, and ultimately coastal 

waters. Inputs of chemical contaminants or sediments at any 

point along the stream can cause degradation downstream with 

adverse effects on biological communities and on economically 

valuable resources, such as fisheries and tourism.

degrade fish spawning and feeding habitats. Stream channels 

are often reinforced with concrete or large rock to minimize 

erosion and control flooding. Water temperature increases when 

tree cover is removed along the banks, thus exposing the stream 

to more sunlight. Chemicals, wastes, and sediment—from 

industry, animal production, water treatment, and runoff from 

impervious surfaces—increase in the stream and can be toxic to 

biological communities. Biological communities have different 

life cycles and requirements for food, shelter, and reproduction; 

consequently, their responses also vary with changes in 

physical and chemical conditions related to urban development.

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 can be managed to protect stream 

health as a watershed becomes increasingly urbanized.