Since you don't mind cold or flatwater...
San Juan river might be a good bet for likely okay flows, and pretty sweet camping and hiking.
I've been wanting to try Cataract this time of year, but haven't yet.
Gates of Lodore might be okay, but kind of short.
If you can take a few extra days for travel, you might be able to squeeze in a Grand Canyon trip, if you can find a cancellation permit.