The Yampa
Me at Wagon Wheel The Yampa/Green run that I'm doing is a 70+ mile stretch of river starting at Deerlodge Park in Colorado, and ending near Jensen, Utah. It runs through Dinosaur National Park, and is pretty spectacular.
Usually, the water is too low by July, but it comes up again in the fall after the farmers upstream finish diverting the water. The recommended flows are 800 - 6000. I've run it at 2200 and about 9000.
After putting in at Dearlodge Park, it's a easy float down to Warm Springs (on day 4 or 5). At 2200, this is about a Class II+ rapid. At 7200, it's a III-III+. There was a clear side stream here, which is much easier to filter than the water from the Yampa. The Yampa water clogs even the best filters pretty quickly.
After Warm Springs, you've got another couple of days of easy floating up through Island Park on the Green. The last day you get to run Split Mountain Gorge. This is a good solid class III-III+ run, and the commercials run it as a one day trip. Take out is a boat ramp at the top of the reservoir.

Here are some links that may be of some use: