I ran into this while taking geology courses at university, and the lack of attention to detail by the developers is frankly insulting.
EDIT: The red line is what the developers did. The green line is what they should have done for realism. Explanation in the comments.
Sure! I tried to indicate with the red and green lines hastily added in MS Paint, but the gist of it is when water goes around a curve, it doesn't flow perfectly in the middle. The majority of the water hugs the outer wall (the cutbank) and is traveling faster. As it's faster, it takes more sediment with it, thus deepening that part of the river. The deepest point in a river is part of a line called a thalweg. You can see it all summarized in the image below.
tl;dr: Water moves through a river kind of like a race car goes around a race track
It actually makes a lot of sense when you think about it! Thanks for explaining so thoroughly.
Thank you for the explanation! I would however say it's the exact opposite of how a racecar goes around a track, because they try to take the inside of corners rather than the outside.
Makes sense now, thanks!