A Chico-ER photographer was able to catch the right moment where the plunge pool erosion found a direct pipe inlet opening to a longitudinal drain. If these drains are near the 16 inch diameter estimates that have been tossed about, this is what a "full on" pressurized longitudinal pipe can do.
Pretty impressive flow. Surprised the pipe sections didn't separate from the pressure.
Looks like a large bilge pump dumping out there! :)