Lakes Michigan and Superior have seen a small drop over the last 20 years, which likely depends on overall precipitation. Others have not. From Wikipedia: 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. This is balanced by evaporation and drainage, making the level of the lakes historically constant.[10]
As for pollution, do you have any references? I'd argue that the lakes have gotten cleaner since the 70s, but I don't have evidence other than my own visuals.
Useful water level link below shows history back to the 1800s.
http://www.glerl.noaa.gov/data...