To further clarify the gauge problem (since it seems to be a source of much confusion):
NSV is split into 2 sections by Buttonrock res. The class V section above Buttonrock has a gauge on it. The section below Buttonrock no longer has a functioning gauge on it. The Proving Grounds section you're talking about starts just below the reservoir. There's a gauge in Lyons that includes both the north & south branches of the St. Vrain. Unfortunately, the south branch (SSV) no longer has a functioning gauge on it either. SSV does have a foot gauge on it (visual only) and there's a Facebook page where, in theory, people report the current reading. You can kind of correlate that foot gauge to cfs and get an idea of what's coming in on NSV below Buttonrock. You can also guess that what they're releasing out of Buttonrock will be similar to what's flowing into it, but you might be wrong!
Ian just calls the water board there and asks how much they're releasing from the reservoir - that seems to be the more straightforward approach.
As for the run itself, I did it around 400 cfs. I thought that was a good *minimum* and not because I'm some hardcore bad ass, but because the riverbed is not particularly channelized and as noted above, the main gorge is a sieved out boulder garden that just doesn't really go with low water. I've heard many reports of people running this section and saying it was absolutely terrible. That's usually because they went in with 250 cfs or something too low.
Also realize the run is short. There are two gorges right at the beginning of the run and they're a really fun set of drops. However, after that it just turns into continuous nondescript creeky III-IV boogie water for a short distance and that's the run.
Lastly, the main gorge in there isn't a cake walk. "Proving Grounds" is more descriptive in that it gives you a short sampling of a class V creeky gorge without fully committing to long sections of those rapids. It's not like running Black Rock rapid on Clear Creek. When I ran it, I saw two solid paddlers blow one of the main lines. It wasn't consequential, but they had to wheelchair down the bank and get back into the main flow. A third paddler blew the line up top and just barely averted disaster. That said, I really enjoyed paddling through the 2 main gorges and we saw two owls fly just overhead as we were waiting in an eddy between them.
- It's worth doing the run.
- Don't go until it really has a good flow (~ > 400 cfs). The Lyons Outdoor Games is a great time to go as they usually are able to get a release out of Buttonrock during the event (consider volunteering for safety on SSV in the morning).
- Don't expect too much fun (it's short)
- Don't go because you're not quite ready for class V creeking and think this is the ticket
- If it's running, something better quality is probably also running.