system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do fish survive tall waterfalls?

Not only survive the impact, but wouldn't they still hit the rocks at bottom? Why aren't there fish corpses all along the river banks at the bottoms?
gpt
Couple things contribute.

Fish are fairly small so they don't gather much kinetic energy. They're also shaped much more streamlined than most animals so they enter water much more smoothly and gracefully. 

The surface tension of the water, which is usually what would kill, say, a human, is broken by the water tumbling down onto the pool underneath. That "impact" is more or less gone as long as the fish can reach the water "inside" the stream of the waterfall. 

Usually there's also enough depth underneath for them to decelerate without dangerously impacting rocks.

That being said, it's not always a given that a fish will survive a fall. Whenever one of these conditions no longer exist, such as if the fish leaves the stream of falling water into falling through the air, there's a good chance of the fish being killed or at least stunned on impact. In fact, they can be stunned even when all of those conditions are favorable, simply if the fall is big enough.