When cleaning clothes in the backcountry, I either use water only or baking soda. I prefer to keep as many chemicals out of the backcountry as possible and baking soda is useful in many other ways. If you prefer to use a detergent, there are some that are considerably safer for the environment, and safer ways to use them.
First, there are no detergents safe enough to dump directly into a water source, even if the detergent's label has a bright blue sky, green leaves, and the word natural printed on it. Not even biodegradable soaps are intended to be used directly in a water source.
Although you should only use biodegradable soap in the backcountry, remember that no soap is biodegradable in water. Biodegradable soaps are only biodegradable when buried in the soil.
Spend enough time on the trails and you'll hear something like, “I have biodegradable soap, so I just jump in the lake to bathe.” If that is how you’re using it, it’s not biodegradable soap. If biodegradable soap accumulates in water sources, as with any soap, it can lead to excessive plant and algae growth and decrease dissolved oxygen in the water.
To print biodegradable on a label, the product just has to be “capable of being decomposed by biological agents, like bacteria, fungi, or algae, and break down into carbon dioxide, water, and biomass in a reasonable amount of time in the natural environment.” Further, it could take up to six months to biodegrade in the soil, and still be deemed biodegradable. By then, if not properly buried at least 6" in the soil and 200 feet from a water source, it could work itself into the aquatic ecosystem.
Biodegradable soap is a good example of the cobra effect, when an attempted solution to a problem actually makes the problem worse. Overall, biodegradable soaps are a good thing. They are technically much better for the environment, but because the term is often misunderstood, the product is often used in an environmentally unfriendly way. So, a product with good intentions can actually end up being worse for the environment.
It’s like being okay with producing more garbage, because you recycle, or leaving an energy-efficient light bulb on more often because it uses less energy.
Also, since there is an assumption that it is safe for the environment, some people may end up using more of it than they would otherwise. With the method above, a couple drops is all you need.
In addition to only using biodegradable soaps, look for those that are phosphate-free, chlorine-free, fragrance-free, dye-free, and ones that are plant-based and contain no petroleum solvents. Soap companies don't have to disclose all ingredients on their labels, so choose a detergent based on what they claim they don’t add, because false advertising is still illegal.