A new technique for sewer network flow routing has been developed using the state variable approach to systems analysis and synthesis. This approach is based upon a lumped mathematical model to describe unsteady, nonuniform sewer flow in terms of a set of first-order ordinary differential equations. This technique can be applied to multilevel branching sewer networks. The model is compared with other routing techniques through the use of an example sewer network. State variable modeling is not limited to sewer flow routing, but has been applied to overland flow, linear reservoir modeling, and reservoir operation.