This is the first modular waterfall I created in unreal engine around 3 years ago.
I learned a lot of tips while working on this project which eventually led to releasing easy waterfall pack last year.
Easy waterfall pack does not use Niagara and allow you to create performant waterfalls simply by using modular assets.
It include a post process volume that create the under waterfall effect too and many more assets which you can check and buy on:
https://www.fab.com/listings/2cddfe96-bbf3-459a-9d9b-f8bbf0e46761
