Klondike here...
Back in the day... the old boys made dams... with spill ways made of piping or maybe a flume....with a gate....
They'd stock pile their material while waiting for the "lake" to fill... then they'd pull the "plug" as it were and washed their material as long as the water supply lasted...
Maybe 35 years ago or so... I had a small 2 in dredge.. I went to the desert, where we had some good results dry washing.... which isn't very effective...
A couple of us guys headed out to our favorite dry washing place with a weather report showing possible thunder storms and flash floods....
We had built a dam in our favorite wash of large rocks and filling the holes with small stones and rocks.... it wasn't perfect, but felt it would work okay... Sure enough, before the weekend was over, we had a real good shower.... it was perfect in that the wash flowed with water, but wasn't a real gully washer....
Soon the water was backing up and we threw our small dredge in the pond and began sucking...with the discharge over or on top of the dam....and we dredged about 45 minutes before we were using more water than what was coming in....and we had to stop....
In a entire weekend of dry washing, we might get 1/2 an ounce on a good weekend.... in that 45 minutes, we got around 2 plus ounces... don't remember exactly now....but we were impressed... just shows how ineffective a dry washer really is...
A year later or so.. this whole area for as far as you could see was removed from entry to protect a desert tortoise
There are two ways you can overcome the lack of water.... I am sure there are other ways too..
Good luck and hope this helps...
Klondike...