Three potential damsites on the lower Murray River near Mannum were investigated by drilling and geological mapping. Two holes at Site 4 and one at Site 5 showed Tertiary sediments overlying granite bedrock at depth exceeding 24m. Exploration was concentrated at the more favourable Site 1, where metamorphic bedrock outcrops or is present at shallow depth. Drilling consisted of four series of holes: ...46 cable tool holes testing the geological sequence along the centreline of the dam; ...188 seismic shot holes by auger drilling; ...27 auger holes to test depth to bedrock; ...16 rotary holes and 8 cable tool holes at 3 sites to test the permeability of the limestone. This work showed greywackes of the Kanmantoo Group outcropping in the right abutment and forming part of the river bed, and occurring slightly weathered at shallow depth under the river flood plain and left abutment. Overlying sediments are mainly river alluvials. Leakage under and around a structure at this site would be controllable, and leakage into the limestones forming the reservoir rim would be a mior concern, comparable to the potential loss by evaporation.