This method compares an unimpaired watershed of similar size, geology and land use distribution to the impaired watershed. Computer simulation models are then used to estimate the loading rates in each watershed and to determine the load reductions of pollutants needed to correct the impairment. A load allocation is assigned to each contributing source, and those sources identified as the causes of impairment are given prescribed reductions. The TMDL sets the stage for citizens to define a plan to correct the impairments. For nonpoint source TMDLs, the input of local citizens replaces the regulated implementation procedures for point source TMDLs.
Lakes have characteristics that differentiate TMDLs from other waters.
Lakes are not free-flowing like streams, but are contained waters that trap pollutants for long periods. Most lake impairments are from high nutrient or sediment loads. Wherever possible, lake TMDLs are developed with the information in the lake study reports that were sponsored by local watershed groups or other local interests. Target acceptable pollutant loads are set at the level of a watershed largely unaffected by human-induced impacts. Load allocations are given to the pollutant sources using the same methods as nonpoint source TMDLs.