Is there a gauge in Waterton Canyon? Eddyflower.com uses the following gauge which says the flow rate was 1000cfs this weekend:
But the station description for that gauge says it is "200 ft downstream from bridge on State Highway 75". Highway 75 is in Denver as far as I can tell.
The flow rate on the north fork was 360 cfs:
...and the flow rate on the south fork was 500cfs:
Those add up to 860cfs. Then, to complicate things further, American Whitewater - AW Homepage
says the flow rate in Waterton Canyon was 540cfs.