The biggest rapids, outside the lowhead dam that you will encounter are Squaw Creek and Bear Creek, both easy III's.
Squaw Creek is just upstream from Independent Whitewater and Kokopelli Rafting which are at the Big Bend recreation site. You can see Squaw Creek from highway 285 where the river makes a hard left turn.
Bear Creek is 3.5 miles from the Salida boat ramp and .5 mile from the Salida East access. If you watch for CR 101 just as you enter the Bighorn Sheep Canyon outside Salida pull over on the left side of the highway at fishing access to scout (the rapid is long, be sure and drive down to the next curve and take another look). It is fun read and run, but I swam in there my first couple seasons....
If your a guide book kind of person there are excellent descriptions of the Bighorn Canyon sections in this book: Arkansas River Guide: Granite To Canon City, Colorado at REI.com