<P> The Great Lakes are also connected by canal to the Gulf of Mexico by way of the Illinois River (from the Chicago River) and the Mississippi River . An alternate track is via the Illinois River (from Chicago), to the Mississippi, up the Ohio, and then through the Tennessee--Tombigbee Waterway (a combination of a series of rivers and lakes and canals), to Mobile Bay and the Gulf . Commercial tug - and - barge traffic on these waterways is heavy . </P> <P> Pleasure boats can also enter or exit the Great Lakes by way of the Erie Canal and Hudson River in New York . The Erie Canal connects to the Great Lakes at the east end of Lake Erie (at Buffalo, New York) and at the south side of Lake Ontario (at Oswego, New York). </P> <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> <P> The water level of Lake Michigan--Huron had remained fairly constant over the 20th century, but has nevertheless dropped more than 6 feet from the record high in 1986 to the low of 2013 . One newspaper reported that the long - term average level has gone down about 20 inches because of dredging and subsequent erosion in the St. Clair River . Lake Michigan--Huron hit all - time record low levels in 2013; according to the US Army Corps of Engineers, the previous record low had been set in 1964 . By April 2015 the water level had recovered to 7 inches (17.5 cm) more than the "long term monthly average". </P>

What's at the bottom of the great lakes