My sources are... um myself. I wrote a guidebook that will tell which gauge to use and how it interplays with natural flows to determine the Big South flows - you can buy one HERE
. I've run the Big South more than anyone in the past decade. I can say without qualification that the Big South is not running. Those flows are at the canyon mouth, the Big South is at the top of the drainage at over 10,000 ft.
It is not real...
On the other hand from the Narrows down to the canyon mouth is definitely running.