Is it possible to have a waterfall big/slow enough for a human to swim up it?

It’s 2012 and Michael Phelps, uh, marries Donald Trump in a surprise wedding. Yeah. With his new-found wealth, Phelps decides to build himself a lair that only he can access… by swimming up the waterfall of doooooooom.

Is that even physically possible? Does the waterfall need a certain volume of water or a certain maximum flow rate? Does gravity’s effect on water on this planet make it totally impossible?

Or, let’s say I have a huge blimp filled with water. Could I shoot it with a slingshot, start a small leak and then shoot a torpedo up the stream?

Swimming involves pulling yourself through water that’s either inert or going the same way you are. You can try swimming upstream, but even something as slight as a riptide can kill you. So, no. A waterfall is out of the question for any human.

If the water is actually falling, as in vertical, then it will fall far too fast to swim up. If it is not falling but flowing down a steep slope then it just depends on your definition of “waterfall”. How shallow can the angle be before it is not a waterfall? Any river is a “waterfall” flowing downhill at a very, very shallow angle and course you can swim up a river.

So I suppose the answer is no, unless you are using a definition of waterfall far broader than in common parlance.

According to this, Phelps can swim at about 4.7 MPH. Running through the equations for freefall, I find that a waterfall with a height of just 0.74 feet results in water (at the bottom of the fall) traveling at 4.7 MPH.

So the tallest honest-to-god freefall waterfall that Phelps could ascend is less than a foot.

Well, it’s got a DC of 80, so it’s impossible for non-epic humans.

On Earth, no. But it might be possible for a human to swim up a waterfall on a very small planet with low gravity. Good luck finding one of those that also has liquid water.