It's also a question of time and space, a larger volume of water either fresh or salt water needs to move through the same space in the same time the level has to get higher, it will also get faster.
I used to live ner the river wear and during heavy rain and higher tides the water level rose a lot, there was also a good deal of mixing so the ideal laminar flow mentioned earlier may not have been happening. It wasnt the sort of river to swim in with your mouth open so a proper test was not conducted.
Also went out of Whitby an a diver cx'n assessment after torential rain and the amount of water and debris coming down ws impressive, 3500rpm just stand still good experience though.
New boat is here, very happy!