<P> 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 ) . </P>
where does the water for the great lakes come from
what was left by melting glaciers when the lakes took their present form