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Chapter 1: Impacts of Urban Stormwater Runoff

In urban areas, the impervious surfaces created by buildings and pavement cause rainwater and snowmelt to flow quickly over the landscape, rather than soaking naturally into the soil or being absorbed by plants. This can change stream flows, increase flooding, endanger private and public infrastructure, erode stream banks and channels, and destroy fish habitat. Runoff also carries pollutants such as oil, heavy metals, bacteria, sediment, pesticides and fertilizers into streams or groundwater. The combined impacts of hydrologic changes and water pollution can be disastrous for streams and rivers in urban areas.

Altered Hydrologic Flow 

Urbanization significantly alters the way water flows in a watershed. In natural areas, most rainfall and snowmelt soaks into the ground to replenish groundwater or is absorbed or transpired by plants, and a significantly smaller amount runs directly into rivers. In urbanized areas, water flows rapidly off of the hard, “impervious surfaces” of buildings, streets and sidewalks, and it is piped into streams and rivers or discharged underground. Even lawns can contribute to urban runoff because their soils have been compacted. A one-acre paved parking lot generates 16 times more runoff than a meadow of the same size.12 The unnaturally high volume and rate of urban stormwater runoff erodes streambanks and streambeds, changes the shape and dimension of river channels, and alters aquatic habitat and channel stability. Increases in stormwater runoff can damage or degrade private and public infrastructure, such as property that is lost or damaged due to widening stream channels and unnatural flooding, and washed out roads, bridges, culverts and sewer lines.



Image source: Maryland Department of the Environment


The total impervious surface area of a watershed can be estimated by associating a percentage of imperviousness with different land uses and totaling them up. Typical total imperviousness in medium-density, single-family home residential areas ranges from 25% to nearly 60%. Total imperviousness at strip malls or other commercial and industrial sites can approach 100%. Clean Water Services has estimated that 54.5% of the impervious surfaces in a suburban watershed are for “car habitat” (roads, driveways and parking lots) and 44.6% are for “human habitat” (buildings and sidewalks).13


Numerous studies show that aquatic life is impacted by very small amounts of impervious surface in a watershed.14,15,16,17 Once about 10% of a watershed (or less, depending on the watershed’s physical and biological characteristics) has been converted to impervious surfaces, significant ecological damage has already been done. Therefore, ensuring that new development manages stormwater runoff in a way that protects the natural hydrology of the system is much less costly and more beneficial to the environment than allowing urban runoff to degrade groundwater and streams and then attempting to restore them later. Impervious surfaces have less impact on watersheds when green infrastructure techniques are used to allow the stormwater that collects on those surfaces to infiltrate into and be filtered by the soil, thereby disconnecting the impervious surface from the storm drainage system. 

Increased Pollution

Stormwater accumulates a variety of pollutants as it runs over roofs, lawns, sidewalks, streets, compacted soils and parking lots before entering streams or groundwater. This type of pollution is often called non-point source pollution because it comes from multiple sources, making it difficult to control. There are also some point-source contributors to stormwater pollution, such as industrial facilities and construction sites. Pollutants commonly found in urban stormwater include heavy metals, pesticides and fertilizers, oil and grease, bacteria, and sediment. Stormwater runoff contributes to water quality problems that endanger human heath and wildlife.


Rapidly flushing stormwater can increase erosion from the land, including streambanks and streambeds. Soil exposed by construction activities is especially vulnerable to erosion during storm events. Poorly managed construction sites can contribute significant amounts of sediment to urban runoff. Stormwater then transports the eroded soil downstream into nearby storm drains and waterways. Eventually, when sediment-laden water slows down, that sediment settles to the bottom of the stream, river, lake, or estuary. When sediment settles out, it may cover fish eggs or destroy important habitat such as spawning beds and submerged aquatic vegetation. Sediment can also plug underground injection systems when stormwater is discharged underground instead of to surface waters.

Sediment is commonly listed by the DEQ as a pollutant causing water quality problems in Oregon’s waters . Impacts of excessive sediment include: damages to fish gills, increasing risk of infection and disease; reduced feeding efficiency for fish caused by reduced visibility; reduced light penetration causing limited aquatic plant growth; adverse impacts on aquatic insects, which are the base of the food chain; increased nutrients and metals carried by suspended sediments; reduced survival rates for fish eggs; destruction of fish spawning areas; and loss of storage behind reservoirs. Excessive sediment deposition over time can fill in navigation channels, increasing the maintenance and safety costs of shipping.


The effects of metals on human and aquatic health can be far reaching. Lead, which is often used as an indicator for other toxic pollutants in stormwater, can be harmful or deadly for human and aquatic life. Zinc, although not harmful to humans at concentrations normally found in stormwater, can be deadly for aquatic life. Cadmium can bioaccumulate in an ecosystem, soil microorganisms are especially sensitive to it, and it is harmful to human health. Chromium damages fish gills, causes birth defects in animals, and is also dangerous to human health. Mercury is a neurotoxin that bioaccumulates and has led to fish consumption advisories in Oregon rivers. Recent research demonstrates that low levels of copper inhibit the olfactory systems of salmonid fish, decreasing their ability to hide in response to warning signals.24 Some metals bind to soils and organic matter and are transported in sediment, while other metals dissolve in water. Rainwater is naturally slightly acidic, which increases its ability to dissolve heavy metals and compounds the health and environmental effects of stormwater runoff from urban areas.

The transportation system is a primary source of metals in stormwater runoff to urban streams and groundwater.25 Cadmium, copper, cobalt, iron, nickel, lead and zinc are deposited into the environment by vehicle exhaust, brake linings, and tire and engine wear. They accumulate on roads, waiting to be washed into storm drains with the next rainfall. Pollutant concentrations in roadway runoff are positively correlated with traffic volume.26 All cars, even the cleanest vehicles, shed small amounts of metals, fluids, and other pollutants.

Galvanized metal rooftops, gutters and downspouts, and moss killer are also a source of zinc in stormwater. Some copper comes from architectural uses and treated wood, and a primary source is brake pads. Outdoor storage of scrap metal can also contribute to metal pollution. Soil erosion is a significant source of mercury.27


Excessive nutrient levels in waterways stimulate the growth of plants and algae, which can reduce dissolved oxygen levels and harm the entire aquatic ecosystem. The primary nutrients are phosphorous and nitrogen. Phosphates and nitrates enter stormwater from fertilizers applied to lawns and golf courses, decomposition of natural rock and soils, air deposition from vehicle exhaust, detergents used to wash cars on the street, and pet waste.


The Willamette and numerous other Oregon rivers are contaminated with DDT and dieldrin, pesticides that were banned in the seventies. These chemicals are highly persistent in the environment, and they continue to enter our waterways via the erosion of soils. In addition to the impacts of these legacy chemicals, a significant amount of other pesticides are currently applied in urban areas.

While the total amount of pesticides applied on agricultural lands is higher than the amount applied in urban areas, homeowners use up to ten times more pesticides per acre on their lawns than farmers apply to their crops per acre.28 Also, residential users do not have training in safe pesticide use. There has been little monitoring of the amounts of pesticides currently applied in urban areas, or their levels in Oregon’s rivers and stormwater.


The presence of E. coli and fecal coliform bacteria in our rivers, lakes and oceans makes them unsafe for swimming, as the pathogens can cause skin ailments and illness in humans and they indicate the presence of feces.

Stormwater often picks up bacteria from the fecal matter of domesticated pets and wild birds. If one considers the small number of wolves that would naturally live in a given area and compares that with the number of dogs in our cities, one can see that our pets do have a significant impact. Multnomah County Animal Services has about 40,000 registered dogs, and only about one-third of dogs are typically registered. With 120,000 dogs creating about five pounds of waste a week, that’s 600,000 pounds of waste entering the watershed each week.29 Under pre-development conditions, much of the bacteria contained in runoff would be filtered out by soil and plants, but in urban areas that bacteria is quickly swept into surface waters.

A new research method analyzing the DNA of fecal coliform bacteria can determine which type of animal the bacteria came from. A DNA study of bacteria in Tualatin Basin streams and stormwater sites found that birds are the most significant source of bacteria in that area.30 Human behavior can contribute to the congregation of birds near waterways. Some cities are putting up signs asking residents not to feed ducks and geese.

Another major source of bacteria in some rivers is Combined Sewer Overflows. During storm events, these older systems found in some cities expel raw sewage directly into rivers because their pipes do not have enough room to accommodate both sewage and stormwater. The City of Portland is working aggressively to reduce Combined Sewer Overflows by building a “big pipe” and reducing the amount of stormwater that enters the stormdrain system by promoting downspout disconnects and green infrastructure.

Hydrocarbons and vehicle byproducts

Vehicles contribute a number of pollutants to urban stormwater in addition to metals and nitrogen. Engine coolants and antifreeze containing ethylene glycol and propylene glycol can be toxic and contribute to water quality impairments. Oil, grease, and other hydrocarbons related to vehicle use and maintenance also pollute urban runoff. They come from disposal of used oil and other fluids on the ground or into storm drains, spills of gasoline or oil, and leaks of oil and other fluids from vehicles. In addition, hydraulic oil is ubiquitous at industrial sites and is difficult for facilities to control at the source, contributing these hydrocarbons to stormwater. Runoff from residential car washing also contributes oil and grease to the stormwater system. The vehicle exhaust that is deposited on roads also contributes dioxins and polycyclic aromatic hydrocarbons (PAHs), highly toxic chemicals that persist in the environment. PAHs also leach from coal tar-based sealants used on paved roads and parking lots.

High temperatures

In addition to carrying pollution and altering stream flows, stormwater may contribute to the unnaturally warm water temperatures found in many of Oregon’s rivers. Because most rainfall occurs in the winter when river temperatures are cool, stormwater’s direct impact on river temperatures is minor. Of greater concern is the fact that the impervious surfaces of urbanized areas reduce the amount of rainwater that recharges groundwater. This means there is less cool groundwater to supply streams and rivers in the warm summer months, which can have a significant impact on water temperatures and base flow levels.


National Low Impact Development (LID) Atlas

LID map

This Low Impact Development (LID) Atlas was created for the National Nonpoint Education for Municipal Officials (NEMO) Network by the Connecticut NEMO Program and the California Center for Water and Land Use to highlight innovative LID practices around the country. Its goal is to encourage and educate local officials and others about low impact development practices by providing specific, local examples of their use.

by the National NEMO Network
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