This is secondhand information so do with it what you will. A friend of mine once conversed with an aquatic biologist about backcountry sanitation, and he was adamant that soap (yes, even Dr. Bronner's!) should only be used well away from running water sources, and preferably rinsed into a sump hole. His reasoning was that fish have an epidermal mucus layer that is necessary for them to thermoregulate. Soap in the water, even small amounts, can disrupt or scrub off that layer.

It sounds reasonable to me, and I like fish, so now I use soap well away from streams.