That’s right: usually. Sometimes no. Or sometimes the volume of water only slowly drains away (like some rivers move extremely slowly and it’s almost as if it’s not moving at all). If it takes 3 days for the water in the normally filled river to move 1 mile, even if it takes 2 days with the flooded valley to drain instead of 3, that’s still 2 days of floods.

Imagine you drop a bunched up shirt onto the floor. If you look, you’ll see that there are lower spots surrounded on all sides of high spots. Terrain irl is not so different from that in spots. Hope this helps explain :)