In 2009, the lakes contained 84 % of the surface freshwater of North America; if the water were evenly distributed over the entire continent 's land area, it would reach a depth of 1.5 meters (5 feet). The source of water levels in the lakes is tied to what was left by melting glaciers when the lakes took their present form. Annually, only about 1 % is "new '' water originating from rivers, precipitation, and groundwater springs that drain into the lakes. Historically, evaporation has been balanced by drainage, making the level of the lakes constant. While the lake levels have been preserved, intensive human population growth only began in the region in the 20th century and continues today. At least two human water use activities have been identified as having the potential to affect the lakes ' levels: diversion (the transfer of water to other watersheds) and consumption (substantially done today by the use of lake water to power and cool electric generation plants, resulting in evaporation).
where does the water for the great lakes come from
what was left by melting glaciers when the lakes took their present form