It's running about 1500 right now and at that level it's rocking Class III+/IV and definitely a Class IV swim. use the little view area on the hill off Hwy 9 and you can overlook the Boulder Creek Rapid, then imagine swimming that stretch. The water is a supercooled fluid coming off the bottom of Lake Dillon - if it weren't moving, it would crystallize. But seriously, even if it's sunny out, make sure you've got head to toe splash gear and neoprene because the water is really cold.
The Upper Blue's a blast, R2 would be great in a mini-max.
Have fun & be safe!
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse