The alluvial soil depth exceeds 400 feet in areas. Test wells have been drilled miles from the river that are impossible to pump down. Reports I have read indicate that the visible river represents 10% or less of the total river. One test well array I am familiar with was a 12" cased well surrounded by 6" monitoring wells. A 300HP pump on the 12" well drew down the 6" wells less than a foot. The instant the pump stopped, the water returned to the original depth in the 6" wells.