Duxbury Reef ASBS

An Area Of Biological Significance In Our Own Backyard

This page provides resources for residents and visitors to better understand the Area of Special Biological Significance (ASBS) at Duxbury Reef, and the special protections that seek to maintain natural water quality and protect the sensitive biological community. Browse the panels that follow for more information on Duxbury Reef.

What is an Area Of Special Biological Significance?

The concept of “special biological significance” recognizes certain biological communities, because of their value or fragility, deserve special protection that consists of preservation and maintenance of natural water quality conditions as much as possible.

Duxbury Reef is a State designated Area of Special Biological Significance (ASBS) with 3.8 miles of coastline near the town of Bolinas in Marin County. It is the largest exposed shale reef in California and is formed entirely of rocks of the Monterey Formation. The area contains rich intertidal life including seastars, mussels, barnacles, sea cucumbers, chitons, nudibranchs (sea slugs), a rare burrowing anemone, and a unique acorn worm.

Where is Duxbury Reef?

Duxbury Reef ASBS lies entirely within the Gulf of the Farallones National Marine Sanctuary. Offshore of this area is the Duxbury Reef State Marine Conservation Area and Extension, which is managed by the California Department of Fish and Game.

Duxbury Reef ASBS located in Bolinas, Marin County

The watershed draining to the ASBS is dominated by federal park lands to the north on which there is some livestock grazing, and a few trails that lead to an expansive beach, but low visitor use. At the southern end of the watershed, there are urban and rural residential areas in the town of Bolinas.

Ways You Can Help Duxbury Reef

If you visit, live or work in the area, you can help protect water quality in the ASBS. Remember that everything that touches the ground can wash down roadside ditches and storm drains to the ocean.Once in the water, these become pollutants whose impact can be significant. For example, pet waste contributes bacteria and nutrients which can negatively impact water quality.

Pollution Prevention Resources and Conservation Tips:

Key Regulations Affecting ASBS
In 1972, the State Water Board adopted the California Ocean Plan (Ocean Plan) as the State’s water quality control plan for ocean waters. Shortly after adoption of the 1972 Ocean Plan, the State Water Board designated 34 Areas of Special Biological Significance (ASBS), comprising approximately one-third of the State’s coastline, including the Duxbury Reef ASBS. For more detail about the State’s ASBS program and a map of the 34 protected areas, visit the State Water Resources Board ASBS website.

An ASBS supports an unusual variety of aquatic life, and often host unique individual species, therefore they are considered the basic building blocks for a sustainable, resilient coastal environment and economy. Since 1983 The Ocean Plan has prohibited waste discharges to an ASBS and states that “discharges shall be located a sufficient distance from such designated areas to assure maintenance of natural water quality conditions in these areas.” This absolute waste discharge prohibition applies unless an “exception” is granted.

As of January 2005, ASBS areas were re-designated as a subset of “State Water Quality Protection Areas” (SWQPAs) that require special protection.

Section 36700(f) of the Public Resources Code defines a state water quality protection area as “a non-terrestrial marine or estuarine area designated to protect marine species or biological communities from an undesirable alteration of natural water quality, including but not limited to, areas of special biological significance that have been designated by the State Water Board through its water quality control planning process.”

The State issued Marin a Stormwater Permit which includes references to ASBS. In response, the County of Marin updated their County Stormwater Ordinance – Chapter 23.18 Urban Runoff Pollution Prevention.

Duxbury Reef ASBS Compliance Plan

On October 18, 2004, the State Water Board notified the County that they must cease storm water and non-storm water waste discharges into the Duxbury Reef ASBS, or apply for an exception to the Ocean Plan. The County was one of 27 applicants requesting an exception to discharge to various Areas of Biological Significance throughout California.

An exception is a special permission, granted by the State Water Board, to discharge into the ASBS. It is not a discharge permit and only applies to point and nonpoint source discharges (e.g., stormwater runoff, which can be either a point or nonpoint discharge) provided they are covered under an appropriate authorization, such as an NPDES permit. Stringent Special Protections were adopted by the State Water Board as conditions for the Ocean Plan Exception.
The Special Protections require development of a Compliance Plan that describes the measures by which the Special Protections will be achieved.

This ASBS Compliance Plan describes how the County, a point source (storm drain system) discharger permitted under the NPDES Phase II Permit, will comply with the Special Protections.

County Projects to Protect Water Quality

In the summer of 2013, using Proposition 84 grant funds, the County implemented improvements to the Agate Beach County Park parking lot to address potential stormwater pollution to the ASBS. The parking lot was retrofitted
with pervious pavement and integrated storm water retention and infiltration structures. Improvements were also made to the adjacent drainage swale to create a series of rock check dams to slow and filter storm water during periods of heavy runoff when the parking lot BMPs may not be able to absorb the full volume of runoff.

The paved trailhead of the path from the parking lot to the beach was also replaced with decomposed granite to allow runoff to sink in, and the portable toilets were relocated away from the top of the stream bank and placed in a covered enclosure to prevent spills.

agate beach parking lot design schematic

During the 2013-14 and 2014-15 winter seasons, the County of Marin is engaged in a regional water quality monitoring effort to characterize the quality of the water from a storm drain outfall on Agate Beach and to measure its impact on the ocean water at Duxbury Reef. Water quality monitoring of the parking lot project demonstrated a nearly 60% reduction in runoff volume as well as reductions in the concentrations of sediment, oil and grease and some trace metals resulting from the parking lot improvements.

The County is also active in a regional monitoring program to characterize the quality of the ocean water at Duxbury Reef and identify if specific pollution prevention actions are needed.

Contact information and Special Events

For more information about Duxbury Reef Area of Special Biological Significance,  please contact:

Rob Carson
Marin County Public Works Department
E-mail Rob Carson with questions or suggestions.