Very cool. That’s a flash flood from a very heavy rainstorm up river. They tend to pull all the junk laying in the river bed with them and form a sort of shell on top of the water.
Flash flood. Rocks carried on mud downstream. In SoCal mountains flash floods are quite common.
It’s a flash flood where the velocity of the water has reached over a supercritical flow rate, meaning it begins to scour the channel in which it flows.
This is combined with the slope of the channel, which allows the scoured material to continue with the flow and further scour as it flows downstream.
Those taking the video aren’t cognizant how precarious a situation they have placed themselves.
Once the bank is undermined, nothing would keep the boulders they are standing upon from adding to the flow.
Very cool!