During the droughts back in '02 - 03 we were running it down to 400. At 550 you may still have to drag the boat off a rock or two. If you go anytime soon it may be ice bridges/dams you have to worry about more than rocks. If you really want to get on the water now, check out Glenwood Canyon, launching at either Shoshone or if you're just getting the hang of the oars, from Grizzley Creek to Two Rivers Park just below the Roaring Fork confluence or float to South Canyon.
Have fun and stay warm!
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