I use free little packets of shampoo from hotel rooms!

Whatever you use, be sure to keep the shampoo out of streams (even if it is biodegradable). Here is the routine: 1) find a location that is away (at least 100-200 feet) from the stream or lake and that does not drain towards the water body, 2) fill large collapsable water bottle with water and set it at your wash location, 3) dunk your head in the stream to get it totally wet, 4) go back to your wash location and apply shampoo, 5) rinse 6) if not rinsed enough, go back and get more water. Be sure all soap is out of your hair before you ever dunk your head back in the stream. Obviously, this process is a lot easier wtih a buddy to be the water hauler.