Rain is surprisingly wet. Like, yeah it makes everything kinda wet and fills in puddles, but also...
1mm of rain is like a Lieter per square meter. 1 million lieters per square kilometer (about 684,202 gallons per mile I think; an inch of rain is ~25.4mm so, 25.4x that for an inch of rain.)
Rivers make a lot more sense when you run some of the numbers to get a sense for the *volume* of water involved.
Where I live, we average around 40" in a year.