Mad River is a Census County Division (CCD) in southern Trinity County, CA. The population was 838 at the 2000 census. Most of the region is covered by the Six Rivers National Forest and the Shasta-Trinity National Forest. This small community is near the western extent of Ruth Lake, the major water reservoir for the populated area around Humboldt Bay within Humboldt County located to the northwest. The ZIP Code is 95552 and this CCD is inside area code 530.