The Middle Fork Eel River is a major tributary of the Eel River of northwestern California in the United States. It drains a rugged and sparsely populated region of the Yolla Bolly Mountains, part of the California Coast Range, in Trinity and Mendocino Counties. Its watershed comprises roughly 745 square miles (1,930 km2) of land, or 20% of the entire Eel River basin. The river provides groundwater recharge and is used for recreation and for industrial, agricultural and municipal water supply by residents.

The Eel River is a major river, about 196 miles (315 km) long, in northwestern California. The river and its tributaries form the third-largest watershed entirely in California, draining a rugged area of 3,684 square miles (9,540 km2) in five counties. The river flows generally northward through the Coast Ranges west of the Sacramento Valley, emptying into the Pacific Ocean about 10 miles (16 km) downstream from Fortuna and just south of Humboldt Bay. The river provides groundwater recharge, recreation, and industrial, agricultural and municipal water supply.