They'll probably go over to the steam to check it out and drink. If it has a sharp drop off, a klutzy or stupid chicken could possibly fall in. Probably, though, they would just be careful and not fall in. None of mine have ever fallen in my ponds and one has one edge with a sharp drop off. They just drink out of it. Sometimes one of the dogs ends up in it when they're new, before they learn, but I've never had one of the chickens do that. I guess my chickens are smarter than my dogs! If the sides are more gradual, then you shouldn't have any trouble at all. If you do have times of the year where you get flooding and rushing water, you can always keep the chickens in the run during that time.