Under the Clean Water Act, states are required to compute Total Maximum Daily Loads (TMDLs) for their priority water bodies. A TMDL determines the maximum pollutant loading from both point and nonpoint sources that a receiving water can accept without exceeding an allowable frequency of water quality excursions. Computing a TMDL is difficult because point source loadings are continuous in time while nonpoint source loadings occur only intermittently. A framework for determining a TMDL and its allocation among sources is developed, based on a modified form of continuous simulation. The approach is applied to an example problem of lead toxicity control within an urban catchment. Results show that it is possible to define the TMDL in an operationally useful way for simple receiving water systems, that the computed TMDL value depends on the level of nonpoint source source control selected, and that multiple combinations of equally effective point and nonpoint source control levels are possible.