Which is the only Great Lake entirely within the USA?

Michigan - the other four are shared with Canada. It is also, hydrologically, not a separate lake at all: Michigan and Huron are joined at the Straits of Mackinac by a channel deep and wide enough that they behave as a single body at the same surface elevation. They are two lakes by convention and one lake by physics.

Together the five hold about a fifth of the world's surface fresh water, and they were dug by ice - the Laurentide sheet gouging out basins and leaving them to fill as it withdrew. The whole system drains through the St Lawrence to the Atlantic, which is why an organism introduced to one lake tends to reach all of them.

That connectivity has been costly. Zebra and quagga mussels arrived in ballast water in the 1980s and have restructured the food web - they filter so effectively that the water is visibly clearer, which sounds good and means the plankton that fed the fish is gone. The sea lamprey, which reached the upper lakes after the Welland Canal bypassed Niagara Falls, collapsed the lake trout fishery, and is now held down only by continuous chemical treatment of spawning streams - a permanent bill for a canal built in 1829.