Book excerpt: Accurate values of monthly hydrologic data are required for simulation, forecasting, and water resource studies of the Great Lakes and their basins. This report summarizes the monthly hydrologic data currently used by the Great Lakes Environmental Research Laboratory in their hydrologic and water resources studies of the Great Lakes. The data consist of precipitation, runoff, evaporation, connecting channel flows, diversions, beginning-of-month lake levels, and rates of change in storage.