I am sorry but I don't think there's a really good answer to that... At least I don't have it. I will tell you that some days, my ducks stay really close to their pen, some days close to me, wherever I'm at in the yard, and then some days they're off doing the wandering duck thing. So it probly depends on your ducks. Mine roam probly 3 acres usually. Now if something spooks them, they might go further. Either way, you really don't want them in too much water too early, because they aren't mature enough to be totally waterproof til they've got *all* their feathers (unless they're with mom) ...And oh ya, that looks river-like to me, and honestly I think my ducks might be frightened of it. I guess perhaps in the slow and low season you may be just fine with adults, buried vote against any fast moving body of water.