That's a lot of rain. Tons and tons of water. That's about 22 inches of snow, for comparison's sake.

Now understand that if you put out a bucket to capture rain water and got 1.8", everywhere got 1.8". The river got 1.8". All of the land next to it got 1.8". All of the land higher up got 1.8". All of the land further upriver got 1.8". All of the land at higher elevation upriver got 1.8".

Now do you get it?