The Mad River Watershed is approximately 100 miles long and drains about 500 square miles of coastal hills within Humboldt County.  Mountain range elevations are from 6,000 feet at the headwaters to 3,000 feet along the western ridge.  The Mad River flows through, around and over a mélange of Franciscan bedrock geology, composed of an assemblage of radiolarian cherts, greywacke sandstone, limestone, serpentinite, shale, metamorphic and sedimentary rocks that are geologically unstable.

Due to the nature of the background geology, historic and present land practices un-natural amounts of sediment enter the river every year.  “In 1992, the Environmental Protection Agency added the Mad River to California’s Clean Water Act Section 303(d) impaired water list due to elevated sedimentation/siltation and turbidity. The North Coast Regional Water Quality Control Board (RWQCB) identified water temperature as an additional impairment to the watershed in 2006.” (Mad River Watershed Assessment)

As a result of historic over harvest and cumulative impacts several species of fish and other animal populations are suffering and listed by Endangered Species Act as threatened with Extinction, fish species include; Coho salmon were listed under the ESA as threatened on 6 May 1997, Chinook salmon were listed under the ESA as threatened on 16 September 1999, NC Steelhead were listed as threatened under the ESA on 7 June 2000, On March 18, 2010, the southern Distinct Population Segment of Eulachon (Thaleichthys pacificus) was listed, and California Fish and Game Commission determined that longfin smelt should be listed as threatened throughout their range in California which includes Humboldt bay and nearby river/ estuary systems.  Only non-native hatchery steelhead are catchable, in season, as determined by California Department of Fish and Wildlife, in the Mad River.