The area of the stream I use moves quickly on it's own.  With some help it turns into the perfect set up for a sluice box.  The material runs smoothly and quickly.  Not quick enough to force valuable material off the back.  But just enough to be efficient.

Before I dam it up, the water is only 3 - 5", after damming it is 5 - 6".  When I finish I leave the stream bed as I found it except for a few holes.  That fill in with the next storm.  I set the stones I use for my dam in a higher place than the stream bed to use the next time I am there.  If they get washed away…..There are always more.