You've tried to do that, but the system won't let you.
When you can put down a river corner, it's outline is green; when it won't -- then it's outline is red.
So what is the secret?
Paths!!!
This page shows the sequence to place a river corner onto another river corner to make a waterfall.
Turn on "Remove Space Between Fields".
This must be checked for stacking to work.
Stacked items on your farm: