High Quality Content by WIKIPEDIA articles! The Mad River is a river in upper Northern California. It flows for 113 miles (182 km) in a roughly northwest direction through Trinity County and then Humboldt County, draining a 497-square-mile (1,290 km2) watershed into the Pacific Ocean north of the college town of Arcata near Arcata-Eureka Airport in McKinleyville. The river`s headwaters are in the Coast Range near South Kelsey Ridge.