The old timers typically washed these areas down to bedrock, and some areas appear terraced. I would imagine this is because these hydraulic mines were generally where the miners found old Tertiary river channels on the sides of mountains that were gold-bearing.

There were iron stains all over and even a few places where I could see iron trash sticking out of the bedrock. These would be ideal spots to start with as the gold travels with the iron and lead.

So, how much rain does it take to cause a major movement of gravels in a stream and redistribute the gold? The technical answer is enough water to move the bed load of the river.