There is an excellent resource at the Great Lakes Environmental Research Laboratory
where you look at actual data over time. A cursory examination of the trends suggests (to me) that the upper lakes (Superior, Huron & Michigan) are into a significant declining period which may be unprecedented. Since the late 1990s these lakes have been well below the long-term average. There was a similar low-level period in the late 1950s to early 1960s, but the current levels appear to be deeper, and more stable at their low level rates.
For Erie and Ontario, what strikes me is how stable the depths have been since the late 1990s. There's more variability with Erie vs Ontario, but both appear to be hovering right around the long-term average depth. This suggests that the lower lakes' water levels are being more actively managed.
So the reality indicates that the lower lakes are pretty stable around the long-term average depths, while the upper lakes are into a decline which may become unprecedented. The major reasons for the lower water levels in the upper lakes appears to be decreased inflow from precipitation in the basin, but more importantly, the increased
evaporation rates due to diminishing ice coverage in the winter.
According to a recent NOAA paper
, "Total annual average ice cover on the Great Lakes has shown an overall decline of 71%, while the annual maximum ice cover shows an overall decline of 52% over the period 1973-2010 (38 years)." According to another recent paper published in the Journal of Great Lakes Research
looking at Lake Superior's ice rates, "Analysis of the data indicates that the duration of ice cover on Lake Superior at Bayfield, Wisconsin has decreased over the past 150 years at the rate of approximately 3 days/decade or 45 days over the course of the study."
With regard to the St. Claire River dredging, the research I've read suggests that while there is some contribution to lowering Huron, Michigan and Superior, it remains a small effect compared to evaporation and diminished water inputs (rain and snow).