The watershed has provided habitat for a wide array of native plants and animals and has historically supplied humans with water, fish, and fertile farmland.

The dam was holding a full reservoir of 12.4×109 US gal (4.7×1010 L) of water that surged down San Francisquito Canyon and emptied into the river.

The Santa Clara River then bends southwest, passing the Saticoy Oil Field on the north bank where South Mountain marks its entrance onto the broad Oxnard Plain.

A sand bar usually stands across the mouth at the Santa Clara Estuary Natural Preserve[6] that lies within McGrath State Beach in Oxnard and bounded on the north by the city of Ventura wastewater treatment plant.

Although located just north of the heavily populated Los Angeles Basin, the 1,600 sq mi (4,100 km2) Santa Clara River watershed remains one of the most natural on the South Coast.

Piru, Castaic and Sespe Creeks, each over 50 mi (80 km) long, are the primary tributaries of the Santa Clara River.

When the river watershed has an exceptionally dry year, the berm acts as a dam, allowing the water level to rise with the discharge.

[9] In 2012, the Los Angeles Regional Water Quality Control Board required the Counties of Ventura and Los Angeles together with cities along the river to limit the total maximum daily load of bacteria potentially harmful to human health that discharges from stormwater outfalls into the Santa Clara River, primarily during the dry season.

Examples of ways they will improve water quality include increased frequencies of street sweeping and stormwater catch basin cleaning; field surveys to locate and eliminate both dry season street runoff and leaks from the sanitary sewer systems; and enhanced public education.

[15] The river is habitat for threatened species such as the unarmored three-spined stickleback, steelhead, southwestern pond turtle, and least Bell's vireo.

[16] Historic documentation of an important recreational steelhead trout (Oncorhynchus mykiss) fishery occurs for the Santa Clara River into the mid 1900s.

A wild rainbow trout population still exists in the headwaters of the Santa Paula, Sespe, Hopper, and Piru Creek tributaries and is producing out-migrating steelhead smolts bound for the Pacific.

[17] However, challenges to outgoing smolt migration include low to no stream flows downstream of the dam or predation in the coastal estuary.

[22] There were beaver (Castor canadensis) historically in the Santa Clara River until Europeans arrived, according to oral Ventureño history taken by ethnolinguist John Peabody Harrington in the early twentieth century.

"[23] This historical observer record is consistent with a beaver skull collected in 1906 in the Sespe Creek tributary by Dr. John Hornung, a zoologist at the Los Angeles Museum of Natural History.

As the landfill aged and its contents decomposed, the release of gas became intermittent and the gases from the recovery system are burned off in a flare.

[31] The riverbed was mined extensively for sand and gravel throughout the post–World War II building boom for the construction of homes and highways.

Mining the riverbed for sand and gravel impacts the riparian zones by destroying habitat and changes sediment flow regimes.

The District owns Lake Piru and key facilities along the Santa Clara River that are used to manage groundwater supplies.

For decades before the structure was built, earthen dams were constructed in the river to divert water to farmers and replenished the aquifer.

The National Marine Fisheries Service determined in 2015 that fixing this was a high priority since it is the first structure the steelhead encounter when attempting to migrate from the ocean.

[40] The required permits for the project describe how the work will fill in and alter more than 82 acres (33 ha) of flood plain and tributaries.

[44] The landmark California Environmental Quality Act (CEQA) used to challenge the development, may have led to a better-designed project while saving crucial habitat.

The ruling also supported the agency's determination that storm-drain runoff from the project's 2,587 acres (1,047 ha) into the Santa Clara River would not harm juvenile steelhead trout downstream in Ventura County.