The Enlarged Cotter Dam project was undertaken to increase the security of potable water supply to the ACT. Hydronumerics was contracted by ACTEW Corporation to undertake numerical modelling to deliver a quantitative prediction of water quality conditions during and after filling of the enlarged dam. As part of the project ELCOM- CAEDYM was modified to allow for spatially varying sediment oxygen depletion rates. The validated ELCOM-CAEDYM model was used to assess potential operational scenarios and to investigate strategies for maintaining acceptable oxygen concentrations. Strategies modelled included staged filling rates, the use of artificial mixing devices and the removal of vegetation prior to inundation.