The best thing you can do to both ward off illness, and catch any illness so it can be treated early, is to schedule an appointment with your doctor, soon. When you request the appointment, make sure you mention drinking untreated creek water as the reason for the appointment. That will give your doctor time to look up references to the appropriate information they will need to advise you. Follow their advice.
Prior to seeing your doctor, practice good self care. Get your regular 8 hours of sleep a night, stay well hydrated, eat a reasonably balanced diet, get a little bit of exercise every day, and stay sober. Basically, do the things you would do to try to stay healthy if you thought you might have a cold coming on right before a big vacation and you really don't want to get sick and have to cancel.
Also, practice good hand-washing hygiene, especially around using the bathroom and preparing food for others. Some water-borne illnesses such as cholera are caused by exposure to waste in the water (either human or animal), and can be spread by preparing food with poorly washed hands. Again, it's nothing very special, just being diligent about actually doing the good habits that we all know we should do but sometimes get lazy about.