What happens to the river? (Cities: Skylines)

A common bug. Keep playing it, over time it should subside and go to normal. If not abated, build a dam and Sarosi it after a while.

It somewhere it took a tsunami

I tried to build some dams near the city, then after a while we were drinking a lot of water, I blew them. Happens the flood:)))

Again change the landscape in some places to diminish the earth, to add some and align the bottom of the river. If no mods will come a bit costly in terms of money and time.