I have a buddy that has a duck marsh at a lower elevation than his main 2-acre pond.
He gets his duck grains started and then floods the marsh at the optimum time. (I can't remember the date.) Sometimes it pulls his main pond down for most of the fall if there are no rains, but his fish population doesn't seem to mind.
In your situation, I would build a "patch quilt" of duck marshes. Scrape up dirt to build a berm about 10' wide and install a large pipe and gate valve. Make each one a 1/4-1/3 acre in size. (Whatever size you think would pull ducks.)
Flood one from your main pond. If you get more rain, then open that valve to the next marsh and flood #2 and #3. Some years you will have a massive duck marsh, some years a small one.
If you make the bottoms of your marshes perfectly flat, then you can flood them deep enough for ducks with a surprisingly low volume of water.
