Stormwater 101

Overland Flow

What is Stormwater?

When precipitation accumulates more quickly than it can be soaked up by the ground, some of the excess water flows over the ground surface. This is stormwater runoff. In urban areas, people have built many impervious surfaces like roads, parking lots, and buildings which prevent stormwater from soaking into the ground. This increases the amount of stormwater runoff during rainfall events in populated areas.

How Can Stormwater Be a Problem?

Most stormwater runs directly into our local streams, lakes, and rivers. Any chemicals or other pollutants it flows over are picked up and carried into every water body the stormwater comes into contact with. This untreated water can harm natural ecosystems and contaminate the water we use for drinking, farming, or recreation.

What Can Polluted Stormwater Do?

Polluted stormwater adversely affects every body of water it comes into contact with. Pathogens, like bacteria, can be picked up from unhealthy areas and spread to bodies of water where people get their drinking water or like to swim.

  • Large debris, like nails, cigarettes, plastic bags, and glass, can be washed into bodies of water. These types of debris are not only harmful to people but can injure and kill local wildlife, also.
  • Harmful chemicals, like paint, gasoline, oil, or herbicides, from yards and businesses, can contaminate stormwater. People and animals can both become sick from contact with polluted water.
  • Loose sediment, especially from places like open construction sites, can be picked up in large quantities. This sediment can clog streams and cloud lakes and ponds making it more difficult for plants to grow.
  • Fertilizers picked up from yards, gardens, and farms can feed large algae blooms. Large algae blooms remove most of the oxygen from water bodies, killing fish and other aquatic creatures

Cleaning polluted water which people use as a resource for drinking is also very expensive. Preventing contamination is much cheaper than trying to clean it up, so it's important that we all do our part.

Stormwater Runoff IllustrationWhere Does Stormwater Pollution Come from?


  • Improperly disposed of household chemicals are a major cause of stormwater pollution. Paints, solvents, herbicides, pesticides, oil, and household cleaning products should never be poured onto the ground, the street, or into storm drains.
  • Lawn care products used in excess can be washed off yards into streams. These products should be used sparingly and always in accordance with the product’ recommendations. Whenever possible, natural mulches and pesticide alternatives should be used in place of strong artificial chemicals.
  • Grass clippings can clog stormwater drains or be carried into streams, altering the streams natural state.
  • Over watered lawns can contribute act like small localized stormwater runoff events. Care should be taken to water sparingly, as with a soaker hose instead of a sprinkler.
  • Yard waste and leaves should be composted or mulched to prevent them from clogging stormwater drains. Piles of landscaping materials like mulch and compost should be covered to prevent them from washing away.
  • Septic systems which are improperly maintained can leak into the soil, releasing pathogens and nutrients. Both of these can adversely affect aquatic life, and pathogens can make people very sick. Septic systems should be regularly inspected and pumped when necessary.


  • Pet waste can be filled with bacteria and other pathogens. If left on the ground or in a yard, the pet waste can contaminate local water bodies and drinking water, causing people or wildlife to become sick. Pet waste is also a source of excess nutrients like nitrogen which can alter the natural ecosystem of water bodies.
  • Care should always be taken to pick up after a pet and dispose of their waste properly.

Cars, Gas Stations, and Automotive Repair

  • Cars can leak oil or other fluids onto roads or driveways where they can be washed into water bodies by stormwater. Care should be taken to repair all leaks.
  • Degreasing car parts, oil changes, and other automotive care procedures should be done with care taken to dispose of all fluids carefully and at specially designated locations. Any fluids which are left on driveways or poured into storm drains will flush directly into local streams or lakes.
  • Soaps and waxes used to wash and buff cars will wash directly into the stormwater system unless special care is taken. Commercial car washes which recycle or treat their wastewater should be used. Or, if a car is washed at home, it should be washed in the yard, where water will soak into the ground, instead of on a driveway, where water will wash into the street.
  • Gasoline spilled at pumping stations can be washed into local water bodies by stormwater. Pumping stations should be covered and protected from stormwater runoff.
  • Oil, gas, and other automotive fluids are very hazardous to humans and wildlife. Care should be taken to clean up spills and leaks. No chemical products should ever be poured onto the street or into a storm drain. All products should be recycled or disposed of in specially designated areas.

Farm Runoff


  • Vegetation protects soils from excessive rainfall and erosion and increases a soil’s ability to soak up water. Overgrazing can strip pastures and stream banks of vegetation, increasing erosion and the amount of sediment being washed into local streams. All stream banks should be protected and vegetated.
  • Pesticides, herbicides, and excess fertilizer can poison aquatic plants and animals and lead to algae blooms, stripping the oxygen from a body of water. Fertilizers should be applied carefully in a limited fashion and away from bodies of water.
  • Livestock and their waste can contaminate water with bacteria and other pathogens, contaminating drinking water and causing aquatic animals to become sick or unhealthy. Livestock should, therefore, be kept away from streams and be provided with an alternative source for dring water.
  • A thick forest canopy protects a landscape from excessive precipitation and roots and vegetation protect soils from erosion. Poorly managed logging practices can expose a landscape to much-increased rates of erosion and damage streams by increasing stormwater runoff and the amount of sediment entering them.
  • When logging, care should be taken to minimize the environmental impact by limiting stream and soil disturbance. Also, cleared areas should be revegetated as quickly as possible.

Storm Drain


Construction removes vegetation and disturbs soil, increasing the amount of sediment in stormwater runoff. contamination. Stormwater should be diverted away from all areas which have been disturbed or had their vegetation removed.

  • Oil and other automotive fluids collect in parking lots and on streets only to be washed into streams by stormwater. Chemical spills should be reported as quickly as possible to the municipality to ensure safe cleanup and to prevent the spread of contamination.
  • Trash, litter, and other debris can collect on streets and washed onto storm drains, clogging them. All debris should be regularly swept and collected and all storm drains should be maintained in clean working order.

Rain Garden

How Can We Help?

  • Rain barrels can collect rainfall from rooftops, decreasing the amount of water that will flow into storm drains. The same water can be later be used to water your lawn. Rain barrels are cheap, easy to install, and are often given away for free by some cities or municipalities.
  • Rain gardens are low lying depressions planted with hydrophilic plants which can collect water from excess rainfall events. Instead of flowing into a storm drain, excess rainfall can feed a garden. Rain gardens can reduce stormwater flow by as much as 75% during regular rainfall events, are cheap, and can be beautiful, as well.
  • Vegetated filter strips are lines and patches of grasses or other plants along streams or roadways which can slow stormwater flow and trap debris and pollutant. They are cheap, easy to maintain, and can protect streams during major storm events.

Where Can I Learn More?

There are many great resources for learning more about stormwater and getting to stormwater experts and agencies around the country, state, and locally.

There are also many great resources provided by local members of the RSWP: