I would throw millet out on the edges I the mud about twice as it dries up. depending on the water clarity and quality, wigeon grass may or may not work you transplant it onto the bottom and actually mash it into the mud. Sunlight must be able to penetrate through the water to the actual plant. In wooded river bottom sloughs, with rotten/decaying organic matter like logs, I would check the ph and haul in a few buckets or water troughs full of duckweed. if the water is right, and most river bottom sloughs are, it will take off and be covered up with gadwalls and wigeons next winter. Under optimum conditions(good ph and water temp), duckweed will double in size every 36-72 hours. very fast growing, but they are still plants that take time...and its almost august.