This is a classic debris flow — the front end is almost entirely dry (note the dust in the image above) and mostly large boulders. The tail of the debris flow has more water and finer material. Note that the debris flow goes through a series of surges.

The weather appears to be dry and sunny. The very small debris flow that the hikers are crossing at the start is also quite intriguing. Even this appears to be debris rich. I wonder if this is the tail of an earlier surge? The walls of the gully appear to be wet?