If you follow those red roots back to the plant, I think you will find that they actually "turn into" more traditional looking roots. Could easily be from the grass right there. Roots can change their morphology, depending on the conditions they grow in. Roots tend to color when exposed to light and grow thick in diameter. Growing in water (rather than in soil) also produces a thicker root. The fast movement of the water likely has a similar impetus. If this is the first time you've seen this (and you have lived there for a few years already), my guess is there is a change in the quality of the the creek water that has stimulated this growth.
