i got down about 3feet in same old piles of dirt and hit the damp stuff,i mixed some of it with dry,but mostly just ran the dryer stuff.i think you would be good down to 2 feet most places now
brad
iam not sure about the river beds,this was just some piles of dirt. also i used some black tape to make some 5/16 to 3/8 dead zones and found more black sand than i was getting before,so i liked the way it worked.
brad