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TMDLs are essentially a water quality improvement plan
To identify and restore impaired waters, the federal Clean Water Act requires states to:
1. Assess all waters of the state to determine if they meet water-quality standards.
— Learn more about assessment on MPCA’s website
2. List waters that do not meet water quality standards (also known as the 303d List) and
update every even-numbered year
— The 2008 Impaired Waters List (303(d) List) 
— Learn more about the Impaired Waters List on MPCA's website
3. Conduct TMDL studies in order to set pollutant reduction goals needed to restore waters.
— Learn about TMDL studies on MPCA's website
Simply put, the TMDL study is a study that identifies water quality problems, possible causes of those problems and proposes solutions. The broad goal of the TMDL study is to determine how water quality standards will be attained.
Once a waterbody is placed on the impaired waters list, the stream or lake needs a water quality improvement plan written. You may have heard a water quality improvement plan by its more technical name – a TMDL. This term stands for “Total Maximum Daily Load.” A TMDL is the maximum amount of a given pollutant that a water body can absorb and still maintain its designated uses (e.g., drinking, fishing, swimming, shellfish harvesting).
The goal of a TMDL is to restore the full use of a water body that has limited quality in relation to one or more of its uses. The TMDL defines an environmental target by determining the extent to which a certain pollutant must be reduced in order to attain and maintain the affected use.
The federal Clean Water Act requires that TMDLs to be developed for all waters that are not meeting their designated uses and, consequently, are defined as “impaired waters.” A TMDL study is required for each pollutant that causes a water body to fail to meet its designated use and associated state water quality standards. A TMDL study identifies both point and non-point sources of each pollutant that fails to meet water quality standards. TMDL also refers to the process of allocating pollutant loadings among point and nonpoint sources. Rivers and streams may have several TMDLs, one for each pollutant.
EPA's proposed definition is: "A written plan and analysis of an impaired waterbody established to ensure that the water quality standards will be attained and maintained throughout the waterbody in the event of reasonably foreseeable increases in pollutant loads."
Learn more about Minnesota’s Impaired Waters on the MPCA website
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