It works in the endorheic basins in eastern Oregon pretty well. Although of course many of the "waterways" there are dry. It gets the paths correct, though.

funny, that's the first thing I wanted to look at too. I tested it on the Salton Sea and Mono Lake and it was accurate.

I didn't realize that Lake Tahoe drained to Pyramid Lake - pretty fun tool.

Yep. Just tried near Frenchglen and it did not go to the sea.

Two Ocean Creek/Pass, Wyoming is another oddity it doesn't get right.