Christian's got a good suggestion but the shuttle's going to be a PITA with the construction work in Glenwood Canyon. You may want to float from Two Rivers Park in Glenwood Springs to either New Castle or go a little further to Silt. I was out there this last weekend and floated the entire stretch from Grizzley to Rifle over two different days. There are boat ramps at New Castle, the I-70 parking area at about Mile marker 108 on the eastbound side, Silt, and at Rifle. The section below New Castle has some riffles with small waves and is all flatwater.
The water's big and pushy at this level but any holes or waves that could flip a boat are easy to avoid.
Have a good trip,
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