Two rivers, the Columbia and the Incomappleux, empty into the north end of Upper Arrow Lake. The form and the general pattern of sedimentation of the two river deltas were determined by an echo sounder survey.The three classical delta sedimentation zones were recognized and observations made on sedimentation in each. Sedimentation in the topset zone is largely channel and overbank deposition similar to that taking place on river floodplains. The bottomset zone receives fine silt and clay carried out to where it can settle under lacustrine conditions. The bulk of the sediment is deposited at the river mouths near the top of the foreset zone. Following initial deposition, the sand and silt dumped at the river mouths is carried into the lake basin by a slow sliding failure of the entire foreset zone.