If they aren't having any other symptoms, they might just have eaten something weird. I would still try to get the fecal culture done though, since birds are great at hiding illness until they are really ill. Now is the best time to start treatment if there is something going on, so it's good that you are concerned, even if it turns out to be nothing.
I would also really look into the fermented feed. It will help give them firm, less smelly droppings. Which reminds me, may sound weird, but have you smelled their droppings? If there is something bacterial going on, there will probably be a funky smell, not like normal chicken poop smell but more offensive. I had an African Grey that had a clostridium infection and it smelled pretty terrible and looked a lot like that picture. He was very ill from something else (PDD) and so he had a compromised immune system. I haven't heard of any chickens with it, although that doesn't mean it doesn't happen.
A running creek shouldn't be an issue until there is something nearby contaminating it (something industrial or maybe the carcass of an animal). Do you know what is upstream from the creek?
