Few ideas, first depending on how you're doing your mountains, Josh Stolarz has a great tutorial.

Historically waterfalls were represented in varying ways; for example, here's how Henry Popple did it in his 1733 Map of the British Empire in America.

Screen Shot 2019-10-27 at 1.37.57 PM.png

In this 1929 map from Touring Club Italiano, they didn't do much except a little X and labeling the waterfall. (Gullfoss is an amazing fall in Iceland and worth visiting, FWIW.)

Screen Shot 2019-10-27 at 1.52.13 PM.png

1851 R.M. Martin just labeled it.

Screen Shot 2019-10-27 at 1.56.37 PM.png

H.G. Johnson sorta 3D-ed in 1849, but he's a wild man. Look how he's displaying the buildings. Heh.

Screen Shot 2019-10-27 at 2.01.20 PM.png

John Melish did an interesting texture shift in 1891.

Screen Shot 2019-10-27 at 2.03.46 PM.png

You could also do something like this—this is a satellite view of an underwater waterfall off the island of Mauritius in the Indian Ocean.

Screen Shot 2019-10-27 at 2.06.57 PM.png