Just to add a bit more info:

I attended the public hearing of the IJC study group in Collingwood a year or so ago. 2 points that were made that evening were (1)changes to the St. Clair River had a relatively minor effect. The study concluded that the difference was in the 70-140 cm range(see below). (2)The two primary variables affecting the Michigan-Huron water levels were, on the - side, evaporation from Mich-Huron and, on the + side, direct flow into the watershed i.e that flow overshadowed flow from Superior.

From their website:
http://www.iugls.org/Decision_tree_tool
Key Findings
ST. CLAIR RIVER HAS CHANGED, ACCOUNTING FOR 7-14 CM OF TOTAL HEAD DECLINE


INCREASING PRECIPITATION HAS SOMEWHAT OFFSET INCREASING EVAPORATION ON MICHIGAN-HURON AND ERIE, BUT NOT ON SUPERIOR
Description
The Study sought to improve the accuracy and consistency in net basin supply (NBS) estimates through modification of existing models, development of new models, collection of new data, and improvement of a range of methodologies that have been used for lake level estimation. It was concluded that the improved estimates of runoff, evaporation and over-lake precipitation still incorporate and introduce significant uncertainty into the overall water balance. Despite uncertainties, the Study concluded that it is clear that lake evaporation is increasing and likely will increase for the foreseeable future, likely due to the lack of ice-cover, increasing surface water temperatures and wind speeds. Analysis indicates that in the Lake Michigan-Huron basin, this increased evaporation is being largely offset by increases in local precipitation. In the Lake Superior basin, however, increasing evaporation over the past 60 years has not been compensated for by increased precipitation. As a result, NBS have been declining in general in the basin. This trend is consistent with the current understanding of climate change. Unless changes in the precipitation regime occur, which is possible, NBS in Lake Superior will continue to decline, on average, despite the possibility of higher water supplies at times. It will be important to ensure that further climate analysis be undertaken to explore these dynamics and provide more certainty of future NBS estimates.