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.
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.