Unlike on a shallow-water marsh that will see duck activity here and there, moving water will have a more specific spot where the ducks will be. Ducks usually don’t sit randomly on a stream, river, or tidally-influenced wetland. They’re where you find them for a reason. Often, that reason isn’t so much food, though that can indeed be a variable in the overall equation, but rather comfort.
Ducks, like most wild animals, rely on and survive on energy efficiency. They feed and, to the best of their ability, save those calories for when they're most required. In moving water, such as a tiny tributary river, the ducks are unlikely to consistently resist the stream. It is inefficient in terms of energy usage. Rather, they'll seek out areas of calm or calmer water—a quiet eddy, a backwater, the inside of a bend, or the slack water beneath a sandbar or small island—turning the moving water into something that seems like still water. When you combine a food supply, like coontail or other submerged flora, with dabbler-shallow water, you may have discovered a honey hole.
But where do you look for such a utopia? Modern technology, such as Google Maps, is an excellent tool for tracing the path of a stream or a tiny river. An aerial perspective helps you to rule out any lengths of moving water that aren't worth investigating.
However, being on the water is the best method to scout flowing water. In the Midwest, I've often done precisely that in September, combining smallmouth fishing with squirrel hunting and waterfowl reconnaissance. I seek areas of the river where I can see or flush ducks, recalling the cardinal rule that ducks are on the body of moving water for a reason, not just because they can. I also look for things like footprints and geese feces on sandbars to see if there are any potential loafing places. Then I take notes and make plans to go back in October or November.