So I've tried doing some research but have not found anything that I could understand or was looking for. Perhaps the community can help!

I understand the basic principles of the waterfall. Logically, there are two ways a waterfall (I also understand there is some contention around what a waterfall is...) can exist.

1) it's from a basin on a high surface and falls off an elevated surface to water level ground.
2) it's from water level to a land mass under water level.

Now my question is about the 2nd form.

If land is below water level, where does the water run? I'd assume it would go to some underground cave system, or if it goes no where it would fill until it became ground level water?

I ask for the purpose of a D&D map I'm making. Trying to think logically about a fantasy world! heh.

Thanks for any help the community can throw my way!