Protecting Public Health and the Environment.

Stormwater in Idaho

Stormwater is water from rain or melting snow that does not immediately soak into the ground. Stormwater runs off of land and hard surfaces such as streets, parking lots, and rooftops, and picks up pollutants, such as fertilizers, dirt, pesticides, and oil and grease. Eventually, stormwater soaks into the ground or discharges to surface water (usually through storm drains), bringing the pollutants with it.

Construction activities that disturb one acre or more of land, including clearing, grading, and excavation activities; industrial activities specifically listed by EPA; and municipal separate storm sewer systems (MS4), which are a city's or town's storm drains are considered "point" sources of pollution (A point source is a source of pollution that comes from a discrete pipe or other "point.") and require coverage by a National Pollutant Discharge Elimination System (NPDES) stormwater permit.

Environmental Impacts of Stormwater

In Idaho, stormwater is channeled to rivers, streams, or lakes, or underground to ground water (it is also channeled to the ocean in coastal areas of the U.S.).  It is not cleaned at a wastewater treatment plant. This means all of the pollutants carried by stormwater are also channeled to these water bodies. If you pour something down a storm drain or in a gutter, it is just as though you poured it directly into your favorite swimming hole or fishing spot, or even into the source of your drinking water.

Stormwater Management in Idaho

Federal, state, and local government agencies; business and industry; and individual land owners all share responsibility for stormwater management in Idaho.

Federal Government

The U.S. Environmental Protection Agency (EPA), Region 10, is the NPDES permitting authority for Idaho and as such is responsible for issuing NPDES stormwater permits

State Government

  • Idaho Department of Environmental Quality: DEQ provides technical assistance and support for controlling stormwater in Idaho. DEQ's Catalog of Stormwater Best Management Practices for Cities and Counties includes site design techniques for controlling stormwater runoff associated with land development activities. DEQ also provides plan and specification review for facilities that control, treat, or dispose of stormwater if requested by the developer or design engineer.
  • Idaho Transportation Department: The Idaho Transportation Department maintains the storm drain system that lies within the state highway right-of-way and incorporates erosion and sediment controls into its construction projects to keep sediment out of stormwater. The Idaho Transportation Department also periodically conducts erosion and sedimentation control workshops.
  • Idaho Department of Water Resources: The Idaho Department of Water Resources administers the Idaho waste disposal and injection well program and the stream channel alteration program. Injection wells can be used for stormwater runoff disposal; stream channel alteration permits are required when construction activities impact a stream below the mean high water mark. This includes constructing a stormwater outfall along a river, stream, or lake.

Local Government

Many communities have stormwater NPDES permits and related stormwater ordinances that impact everyone, including individuals. Stormwater ordinances are designed to minimize the environmental threat to Idaho's rivers, lakes, and streams by prohibiting certain activities that would directly discharge into stormwater sewer systems. For more information about local ordinances in your community, contact your local public works department, highway district, or county.

Business, Industry, and Land Owners

Businesses, industry, and land owners are responsible for stormwater runoff from their property and may need to obtain a stormwater NPDES permit from EPA and/or comply with their city's municipal stormwater NPDES permit. Compliance with a stormwater permit may require the use of stormwater best management practices; their use is recommended although not required.

Compliance with TMDLs in Idaho

A total maximum daily load (TMDL) is a water quality improvement plan that provides a calculation of the maximum amount of a pollutant that a water body can receive and still meet water quality standards. TMDLs are calculated for surface water bodies that do not meet water quality standards; their purpose is to improve poor water quality. Since stormwater can discharge pollutants to streams, lakes, and rivers, stormwater discharges must be consistent with the requirements of a TMDL that has been developed for a particular water body.

If a TMDL has been established for the stream, lake, or river where stormwater will discharge, the stormwater discharger should contact DEQ to determine if there are specific TMDL stormwater requirements. Following BMPs from the Catalog of Stormwater Best Management Practices for Idaho Cities and Counties described below is generally sufficient to meet TMDL requirements, but it is best to discuss BMPs with DEQ before implementing if a TMDL is in place.

Stormwater Best Management Practices

A best management practice, or "BMP," is a technique of preventing or reducing pollution that has been determined to be an effective, practical method of doing so in a specific situation. Stormwater BMPs are used in Idaho to help prevent stormwater runoff from polluting Idaho's streams and rivers. DEQ has developed a Catalog of Stormwater Best Management Practices for Idaho Cities and Counties to provide technical guidance for the selection and site design of stormwater BMPs.

In general, there are two types of BMPs for stormwater pollution control.

  • Source control measures focus on minimizing or eliminating the source of the pollution so that pollutants are prevented from contacting runoff or entering the drainage system. An example of a source control is maintaining existing vegetation in specific areas of a construction site to help control erosion.
  • Treatment control measures are designed to remove pollutants after they have entered runoff. They tend to be more expensive than source control measures. An example of a treatment control is a oil/water separator.

What You Can Do to Prevent Stormwater Pollution

Each of us can help prevent stormwater pollution through simple actions we take every day:

  • Never pour anything other than pure water into a gutter or down a storm drain.
  • Only use as much pesticide and fertilizer as necessary (read package instructions). Extra product (and your money!) goes down storm drains.
  • Position sprinklers so you only water your lawn (not the driveway, sidewalk, etc.), and only use as much water as necessary. Extra goes down storm drains and takes pollutants with it.
  • Use a commercial car wash. Commercial car washes recycle their wastewater and some also treat it before it is sent into the sewer system. If you do wash your car at home, use a bucket or a nozzle that you can turn off so the hose isn't running the entire time and wash your car on the lawn so the extra water soaks into the ground.  Use cleaners sparingly.
  • Pick up after your pet. Pet waste on the ground adds bacteria and nutrients to the stormwater, and eventually to local water bodies.
  • Recycle, trade, or properly dispose of household products that contain chemicals. Do not pour them onto the ground or into gutters or storm drains.
  • Participate in a program to educate your neighbors about stormwater pollution by marking storm drains with special "Dump No Waste" markers.  Contact your local city public works department or DEQ Regional Office for more information on marking programs.