Defining impaired waters

MPCA staff member doing biological monitoring in the Des Moines River. A body of water is considered “impaired” if it fails to meet one or more water quality standards. Minnesota water quality standards protect lakes, rivers, streams, and wetlands by defining how much of a pollutant such as bacteria or nutrients can be in water before it is no longer drinkable, swimmable, fishable, or useable in other, designated ways (called “beneficial uses”). Waters that do not meet their designated uses because of water quality standard violations are impaired.

The federal Clean Water Act requires the MPCA to:

  1. Assess all waters of the state to determine if they meet water quality standards
  2. Create a list of impaired waters that do not meet standards, and update the list every even-numbered year.
  3. Set pollutant-reduction goals needed to restore impaired waters, called the total maximum daily load

States must develop a list of impaired waters that require total maximum daily load (TMDL) studies, and submit an updated list to the U.S. Environmental Protection Agency (EPA) every even-numbered year for approval. The TMDL is the maximum amount of a pollutant a body of water can receive without violating water quality standards, and an allocation of that amount to the pollutant’s sources

The Impaired Waters List is accompanied by a manual that describes Minnesota's monitoring and assessment strategy, tools, and process. The purpose of this guidance is to define the required data and information and lay out the criteria by which bodies of water are assessed to determine if they are impaired.