Worldwide, dams will continue to be built and enlarged to supply water for the growing human population. The inundation of large areas of land and vegetation in the process of filling new reservoirs provides a large input of nutrients into the aquatic system, resulting in trophic upsurge. A trophic upsurge is likely to result in a change of fish food resources and cause other water quality changes, such as a decrease in dissolved oxygen. While many reservoirs designed for domestic water supply purposes limit the influx of nutrients through the removal of vegetation and topsoil prior to filling, the vegetation and topsoil in the inundation zone of the Enlarged Cotter Dam (ECD) has not been removed. Cubic metre samples of topsoil and dominant vegetation types in the inundation zone of the ECD were collected, including blackberry, eucalypt, pine and silver wattle. The samples were submerged in 1m³ Intermediate Bulk Containers with 1m³ water extracted from Cotter River. Water samples were analysed for TN/TP, dissolved oxygen, temperature, turbidity, electrical conductivity and pH on day 1, 2, 3, 4, 6, 8, 10, 14, 17, 21, 25, 29, 37, 44, 53, 65, 74, 84, 106. Results indicate that inundation negatively impacted on water quality across all vegetation types. DO fell below 1mgL^-1 after 10-20 days in the eucalypt, pine and wattle containers. By the conclusion of the experiment DO fell to such severe levels that there would have been extreme stress and mortality of aquatic biota under all sampled conditions.