The Whitewater River Run Mainstream from the Confluence with South Fork Little Rock Creek to Little Rock Creek Reservoir in California is a popular destination for whitewater enthusiasts. The ideal streamflow range for this segment is between 300-800 cfs, and the class rating for this section is a mix of Class III-IV rapids. The segment mileage is approximately 6.5 miles and takes about 3-4 hours to complete.

Some of the notable rapids and obstacles on this run include Leap of Faith, Roller Coaster, and S-turn. These rapids can prove challenging, with narrow chutes, steep drops, and strong currents. Therefore, this run is recommended for experienced paddlers only.

Specific regulations to the area include a Forest Service permit, which is required for all visitors entering the San Bernardino National Forest. Additionally, all boaters must wear a personal flotation device (PFD) at all times on the river and respect the "pack it in, pack it out" policy to help preserve the natural beauty of the area.