Oro = "gold" ville = "town"

California had a gold rush - part of the gold mining was done using "hydraulic mining", where you use water to scrub away rock and direct it into a channel for the runoff to be run through mining equipment.

Be sure, there are plenty of gold prospectors downstream panning or perhaps even using metal detectors in the areas where the waters have receded, looking for golden flakes ("flour") and nuggets.

I don't know the exact law in CA but in general you can use human-powered methods without mechanical assistance, to pan for gold (basically what looks like a pie-pan which has ridges inside to catch the heavier gold).