The many wetlands along the Mekong River, including Cambodia's Tonle Sap (Grand Lac), are major sources of fish for the riparian peoples and an important part of the regional economy. This resource may be affected by the development of dams and diversions in the basin. The Semi-Distributed Land-Use Runoff Process (SLURP) hydrological model has been applied to the basin. First, only data from the Internet and from public-domain databases were used to provide a comprehensive simulation of the daily hydrological cycle for the main river and tributaries. Simulating the complex hydraulics of the river connecting the Tonle Sap and the Mekong required local data to develop routing relationships. The model takes into account river diversions and dams where data are available. Model outputs can be used to investigate water allocations and the effects of land-use change or climate change on water resources, to evaluate the effects of proposed structures on fisheries and environmental interests and on the performance of irrigation schemes.