The Lakes have a relatively small watershed in the U.S. Much of it is the State of Michigan. Minnesota, Ohio and New York have a reasonable amount, but Wisconsin, Illinois, Indiana, and Pennsylvania have very little of it. In fact, there is a dispute in Wisconsin about whether Milwaukee suburbs situated in the Mississippi drainage basin should get a diversion of Lake Michigan water that it will not have to return to the Lake in a cleaned-up form. The watershed is that narrow around Milwaukee. The same situation exists around Chicago, but Chicago got a diversion a long, long time ago, and it probably won't be eliminated.
Recent drought around Lake Superior and the northern parts of Lakes Michigan and Huron has contributed much to the current low levels. If you check the link below,
you'll see an animation of the drought maps from the last 2-3 months. Notice how dry it has been around Lake Superior and in the northern part of the Lower Peninsula between Lakes Michigan and Huron. Hot, dry air means lots of evaporation and not much inflow.
Superior's level is somewhat held up by the locks and dams at Sault Ste. Marie at its eastern end (just east of the Whitefish Bay of "Edmund Fitzgerald" fame for you Gordon Lightfoot fans). No such constructions hold up Michigan and Huron, both of which drain into Erie through a river flowing south from Huron past Detroit. That river has been dredged deep by the Army Corps of Engineers for commercial shipping. Recently, I've seen articles suggesting that the last time the Corps dredged, it dredged too deep. Deep dredging leads to more water exiting from Huron, lowering the levels of both Huron and Michigan. Erie is held up by Niagara Falls, and Ontario by another lock and dam system.
IMHO, there should be a similar construction holding up Michigan and Huron to help keep commercial shipping viable around the Lakes. Many of the Lake ports are relatively shallow, and may become unusable in the hot and dry future without some help. Shipping by water, including barges that are Lake worthy, uses much less liquid fuel than trucks and less than rail. Ships are powered now sometimes by lower grades of petroleum fuel or coal, both of which may be available in the future (despite CO2 problems) when lighter fuels are in short supply and that may be more difficult to use in rail or truck engines. It is also possible that ships could once again be outfitted with some kind of sail, perhaps a kite-like device, that would lessen the modest carbon fuel use. Before good quality roads were constructed, there was a tremendous amount of commercial shipping on the Lakes, and it was not just bulk carriers, but small freighters. It wouldn't surprise me if those days were to return.