This report describes the computer program "WASP" written to test the feasibility of simulating a river channel/lake/hydroelectric power-station system on a digital computer. WASP is a program package consisting of a main program called CONTROL, and three principal subroutines: RIVER3 (models open channels), JUNCTN (models channel bifurcations) and POND (models flows through lakes and power-stations). To simulate a river system using WASP, a user is required to write a new subroutine called SYSTEM, the statements of which consist of CALL statements. Each of these CALL statements calls one or another of RIVER3, JUNCTN or POND (the argument list referring to a particular part of the river system). The CALL statements of subroutine SYSTEM describe the architecture of the system. At its present stage of development, WASP is capable of fast and accurate simulation of open channels carrying steady or slowly fluctuating unsteady flows. Further work is required before it is able to satisfactorily simulate the water flow through a hydroelectric power-station.