Show source of great lakes water the supply of water in the great lakes is maintained by drainage from the area comprising the lake basin about square miles of which 60 per cent is in this country and 40 per cent in canada the amount of water fed into the lakes by the hundreds of small rivers streams and springs varies with the season in april and may it has been estimated to average more than cubic feet per second and in november less than cubic feet per second the level of the lakes is determined by the balance between inflow and outflow artificial outflow and ad evaporation from the surface of nearly square miles in the natural state each lake was an individual reservoir maintaining its own level by natural dams at the outlets but with blasting and digging to assist transportation these haag been modified and the artificial outlets such as the ci cago drainage canal welland canal new york state barge canal etc also affect lake levels |