 After a cool start to the summer season, the water temperature in the Great Lakes has risen dramatically. Temps in each of the lakes vary, but Superior is the coolest due to being the furthest north, and Lake Erie is the warmest. And just as temperatures fluctuate with the seasons, water levels in each of the Great Lakes do as well, impacting fisheries, commercial shipping, hydropower generation, boating, shoreline flooding, and more. The lakes won't reach their peak temperature until late August before they start to cool again ahead of autumn.