This paper discusses the possible impacts on the Orange River Estuary, of the proposed Noordoewer / Vioolsdrift Dam (NVD) project on the lower Orange River located 300 km upstream of the estuary. The flow patterns and sediment dynamics at the estuary were evaluated as supporting information to the feasibility study of the proposed NVD site. A statistical analysis of the entire flow regimes and monthly sediment concentrations for the Orange River estuary was performed. Six different dam scenarios ranging in height between 35 m and 80 m are compared to the present and the natural states for the 52-year period from October 1931 to September 1983. Another two scenarios were considered for 35 m and 70 m high dams with freshet environmental releases in the range of 20 to 40 m3/s (monthly average data). Under “natural” conditions (1920), the sediment load at the Orange River mouth was 51.4 million t/a, but was decreased due to dam development (mainly until 1975) to 19.6 million t/a for the present state, but the proposed NVD scenarios will further decrease the sediment loads at the mouth, to as low as 0.5 million t/a for the 80 m high dam. All the proposed dam scenarios cause a 39% to 92% reduction in the 1-year to 5-year annual recurrence interval flood peaks compared to the present state. 1D hydrodynamic modelling showed significantly lower water levels and flow velocities in the estuary for the post-dam scenarios than under the natural and present scenarios for these floods. If it is assumed that that the estuary mouth closes when flow rates < 5 m3/s occur for 3 months or longer, the mouth closure events would become longer and more frequent with the construction of the NVD project and the estuary channels would become shallower and sediment-laden. Larger post-dam 1 to 5-year ARI floods are required to improve the sediment transport capacity at the estuary and to increase floodplain flooding. If the proposed dam is flushed during the flood season to manage the sedimentation in the reservoir, the number of floods and flood peaks of the small floods will increase at the estuary.