Michigan's upper peninsula (UP) has something like 50 waterfalls IIRC. Tahquamenon Falls is great. Also Bond Falls near Paulding. The west unit of Porcupine Mountains SP has a river with 3 falls, and you can walk along the river on both sides to see them. Canyon Falls (a roadside park) south of the flyspeck village of Alberta on US41 is another nice place to walk alongside the river and see the rushing water cascading.
The highway north of Duluth along Lake Superior has been mentioned. There is a 50' escarpment running parallel to the shore, so one can see a series of waterfalls at the several state parks along there. The view from Palisade Head is not to be missed; look for climbers rappelling down the cliff toward the lake below. Also at Grand Marais, walk out past the coast guard station to Artist's Point. The tough part in this area is finding a campsite along the shore; it's very popular, for good reason.
Iowa has some decent state parks and a few waterfalls, or so I've read.
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