Farther upstream, the San Juan River flows into the Colorado River. In these narrower upstream portions of the rivers, more exposed riverbed appears in the later images.
While the other series of images at this page do not show very much difference between the 2005 and 2018, upstream areas of the reservoir have shrunk or gotten much shallower. Follow the San Juan River upstream, and it becomes less dark in 2015. Shallower water has more sediment and that appears lighter.