I’ve surfed the internet looking for the answer to that exact question for almost five years to no avail, but the way I do it to avoid potential trouble is to prospect with a pan. Once I find a spot where I think has good dirt, I dig a couple of buckets of dirt, well maybe four or five, then setup my recirculating sluice at home, or at my campsite far away from the river.
At home is tough for me because I live In North Texas, but after being introduced to prospecting by a coworker in 2015, I make the trip to Idaho to camp and play in the dirt a couple times per year. My wife thinks I’m nuts to drive 26 hours to camp, but she only camps at the Holiday Inn. Lol
I know this probably doesn’t help, but it works for me.