>>Is there any possibility of that brown water coming from the side of that big @$$ hill that is dividing the water flow?<<
Since the brown water “image capture” is to the left and the right of the spillway, this would indicate that an infusion of a notable amount of soil occurred from the action of the turbulent hydraulic scouring of the big “main spillway” hole.
What makes this interesting is that there seemed to be “good bedrock” just down from the main spillway hole. To cause a break away or infusion of brown material (color matches soil) from this hole is the question. What broke? Did a section of the “good bedrock” finally give way to expose another fractured bedrock/soil pocket a little further? Did the hole go deeper?
It is also possible, that part of a hill landslided into the flow. Only before/after pictures could give a clue.
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?