I usually just put a cloud around a tapering end of a river, and then it's smaller at the bottom. I don't have many good examples of waterfalls in 2d and if I can, I'll try to do an isometric or cartographer-was-sitting-there-drawing-from-real-life angle for it instead. Waterfalls are easily my least favorite top down element that almost everyone wants on their map, and I admit I haven't got an awesome solution for them yet.