I took a canyon day tour from San Rafael. In 2003, it cost $10 US, and paid another $5 for the optional one hour whitewater rafting. The canyon was great for the variety of different landscapes and colorful geology. Although it is a national park, it is also a major hydroelectric project with many man-made dams and lakes.
The whitewater rafting was great when I was there. It was only class 3 and maybe 4, but no pauses for slow water. The river flowed swiftly for the whole hour.