On twitter and in blog comments, I've heard about a range of ideas:
- wind roughing the surface of the water
- minerals, sediments, glacial flour in water influencing the color
- angle of the sun and the viewer changing what gets reflected to viewer, and
- those same angles changing whether the light mostly reflects off the water or refracts into the water.
- Update: The influence of a polarizing filter
In this third photo, the water shows its many different faces in a single shot.