A significant amount of water is going down the remnants of the bottom of the spillway on March 17th. On March 21st almost no water is going down the remnants.
If the release rate is approximately the same each day then the channel must have grown enough to handle all of the flow that was previously spilling over onto the remnant of the spillway.
I am certainly not an expert on dams. Another possibility, of course, is that flow has decreased enough to account for the difference. It would have to be a pretty big change.
Interesting observation.
The captions on both pictures say 40,000 cfs.
I think the caption is wrong and the 1st pic on the 17th is 50,000
you can tell by the spray (more of it) plus on the 17th total out flow was 50,000 in the afternoon according to the data and they turned the power plant off at first until that evening......not 100% sure though
in any case, assuming a steady 40000 out of the main spillway the outflow from the power plant has been slowly decreasing over time and is now around 5000 cfs ....a few days ago it was 6800...
I suspect more debris is being washed in to the pool and they have to cut back to prevent the water backing up