Go Waterfalling’s page on the Slate River Falls shares that:
Slate River Falls is, unsurprisingly, on the Slate River. This is the largest of many drops over a three mile stretch of the river. This is a wild waterfall, with no fences, and the only trail is one left behind by the curious feet of others.
The falls are located along Skanee Road between L’Anse and Skanee, about 10 miles east of L’Anse. There is a sign marking the Slate River, so the falls are easier to find than some.
If you continue upstream past the falls a few hundred yards you can find two smaller falls, Slide Falls and Ecstasy Falls (so named by kayakers). About 3 miles upstream you can find Quartzite Falls, Black Slate Falls and more unnamed drops.