Two days ago, I asked a [question how to stop a waterfall](https://worldbuilding.stackexchange.com/questions/25634/how-to-stop-a-waterfall). I got great replies, and had to reconsider some of my numbers. Now I came up with this calculations, and I would like to check 'm with all the geniuses here. Thanks in advance.

I have to figure out where all the water for my huge waterfall comes from, and rain seems to be the best (only) option. I limited the width of my waterfall from 1km to about 300m, kinda like the American Falls of the Niagara Falls, who have an estimated water flow of 75,000 gallons/second. This way, all the water would fit through a tunnel with a diameter of 5m, where it was 18m before (thx @hde-226868). This thing should be possible to block.

A smaller waterfall also needs fewer water, so I could go with 6,000 km² in stead of the 50,000 km² @RobWatts measured my 1km width waterfall would need. That's a surface of 75 and 80 km, but I would guess less when we have a cone like a mountain is.

6,000 km² on a mountain. What would be a realistic height here for the mountain, with an average grade of 7-10, to make it big enough to collect all the water needed for my waterfall? And how big would the ground surface be?

Small follow-up on my previous question: would it be possible for this amount of water to disappear in a cave construction beneath the water, after rerouting it like @mikey said, if there's an underground passage to the sea?