Moses Lake is part of the Columbia River basin, as Crab Creek is a tributary of the Columbia River.

It then became part of the Columbia Basin Project, built and managed by the United States Bureau of Reclamation.

Moses Lake receives water from Crab Creek, as well as from irrigated agricultural runoff.

Its water used to flow out into Crab Creek, but now flows into Potholes Reservoir, a storage reservoir created by impounding Crab Creek at O'Sullivan Dam.

Moses Lake is fairly complex in shape, with several arms, called "horns".