Faculty of Engineering School of Civil and Enviromental Engineering 9308302w fredb@ananzi.co.za / The Renoster River is a minor left-bank tributary of the Vaal River located some 120 km south-west of Johannesburg in the Free State Province of South Africa. The river is dammed by the Koppies Dam, which divides the river into upper and lower basins. No water is released directly from the Koppies Dam into the Renoster River channel except as a protective measure during natural flooding events. Irrigation releases are made through a separate canal system. The lower Renoster River was modeled as a series of channels, crossing four quaternary catchments. Hydraulic routing was used to estimate potential flow losses for discrete flow events. Out-of-bank flow was found to occur when flow exceeded 300 m3/s. A Pitman monthly rainfall-runoff hydrologic model was used to model longer term historical streamflow data and associated losses. Flow losses for individual flood events along the lower Renoster River were found to vary between 3% and 17%. For a longer term constant flow regime, losses can be expected to range between 10% and 40%. Based on both the hydraulic and hydrologic models, the primary cause of flow loss was found to be evaporation, with insignificant transpiration and transmission losses. For eighty years flow in the lower Renoster River has been artificially controlled. Within that time period the river has adjusted itself to the imposed anthropogenic flow regime. Water losses associated with planned releases into the lower Renoster River can be minimized by using a high discharge rate, to a maximum of 300 m3/s, combined with larger volumetric flows. Evaporation losses can be significant, and release strategies will need to take this into consideration.