Would be nice to know the times of capture as well... if this was while they were reducing the flow rate, would not the flow velocity come down a bit as well? The water flowing off the damaged end of the spillway could be landing further uphill and eroding hillside that was untouched or only slightly impacted at higher flow rates?
>>Would be nice to know the times of capture as well... if this was while they were reducing the flow rate, would not the flow velocity come down a bit as well?<<
Between the two images, the plume volume, shape, outline, and “marker” flow around the rocks indicate a concise match in a volume flow. With a concise match in volume flow, the only way to get a sudden coloration change is through an injection of a material (such as soil).
The best answer to this question likely will be revealed as a possible landslide. The increased waterflow on the left side of the spillway is coming from the blowout area upstream. This left side blowout area is near the electrical transmission tower(s). There are images of landslide erosion on that hillside that is likely going to threaten the closer tower. My best estimate would be that is the place to look for in a before/after hillside comparison.