Guessing by the suggested flows you must be talking about Bloomington gorge( south of St George). We did last fall, on rain, at between 3500 and 4000 cfs and it was great. Great play, big swirlies, and a couple 4-4+ rapids(mile ten rapid? & the last one of the run)
It didnt wash out at that flow, I think it would take alot more water to truly wash out. Maybe some Southern Utah boaters can chim in (it ran at 20,000 cfs in this winters flood). I would guess it would just get bigger and more fun the higher it got.
Haven't done that run at lower flows, I think 1000-2000 would be fun.
Flows are good for there right now. Other runs to consider are:
Narrows (a 2-3 box gorge, in Zion park)
Staircase (3+-5 depending on flow, in zion park)
Timpoweap Gorge(4-5, & 5+/portage)
If your comming from CO, the fremont is running, Haven't done it but heard its sweet!