Lakes and streams that do not meet Missouri water quality standards are considered impaired, listed on the 303(d) list, and require the development of a Total Maximum Daily Load (TMDL). Impairment of water quality can be attributed to both point and non-point pollution sources. The TMDL is a plan that includes a mathematical calculation of the amount of a specific pollutant a waterbody can absorb and still meet water quality standards, and management practices to return the waterbody to a healthy state.

Department staff members conduct detailed field investigations to define the nature and extent of the impairment, and provide the data required for developing the TMDL. These types of field investigations are commonly referred to as wasteload allocation studies. After management controls have been implemented on an impaired waterbody, it is often necessary to perform additional studies to reassess water quality. This allows the department's Water Pollution Control Program to determine the success or failure of controls that were implemented.