Before Euro-American settlers arrived in the mid-1800s, the native peoples occupying the lower Mad River watershed were the Wiyot (from approximately Blue Lake to its mouth, plus the greater Humboldt Bay region) who spoke a dialect affiliated with the Algonquian language family, with upriver reaches controlled by three different groups whose languages are related to the Athabascan family, the Whilkut, Nongatl and Lassik (Baumhoff 1958).

The dam is owned by Humboldt Bay Municipal Water District, which provides water from Ranney collectors near Essex for municipal and industrial use in Eureka, Arcata, Blue Lake and numerous unincorporated communities in the Humboldt Bay area.

The proposed blockage would have flooded the Maple Creek/Butler Valley area and adversely affected the health of the Mad River watershed.

In addition, the removal of riparian vegetation (primarily due to conversion of natural lands to ranching purposes) increases erosion and urbanization causes decreased water quality.

This bend denotes the usual upper limit of the estuary, although brackish waters can extend as far upstream as Highway 101 during king tides.

Although small, this estuary provides nursery habitat for juvenile rockfish (Sebastes melanops, S. rastrelliger) and several species of flatfish, including starry flounder (Platichthys stellatus), English sole (Pleuronectes vetulus), and speckled sanddab (Citharichthys stigmaeus).

The estuary also serves as a migration corridor for salmonids and Pacific lamprey (Lampetra tridentata), and as a summer feeding ground for several marine species, including topsmelt (Atherinops affinis) and surf smelt (Hypomesus pretiosis).

[11] Above the estuary, the Mad River is home to resident coastal cutthroat trout (O. clarki clarki), rainbow trout (O. mykiss irideus), Sacramento sucker (Catostomas occidentalis), and Humboldt sucker (Catostomas occidentalis humboldtianus).