Rain.
I've been saying the lake levels were on the rise for almost 5 years now. Rain expected across Michigan again today and what doesn't evaporate heads downstream to the lakes. The same happens everywhere else in te great lakes watershed.
This interconnectedness makes the lakes “like one massive river system,” said Andrew Gronewold, a hydrologist in NOAA’s Great Lakes Environmental Research Laboratory.
Lake Ontario receives water from precipitation, runoff from the surrounding basin, and water from the Niagara River. This spring, all three sources dumped more water than normal into the lake.