Physical Characteristics

Bolinas Lagoon is a small, shallow estuary located directly along the San Andreas Fault, 15 miles northwest of the Golden Gate Bridge.

Covering less than 1500 acres, the triangular lagoon is 3.5 miles long and a mile across at its widest. It is protected from the Pacific Ocean by a long spit which terminates at the lagoon's mouth, its lifeline to the sea. Like an enormous aquatic lung, the lagoon breathes in sea water on rising tides, and exhales a mixture of fresh and sea water at low tides. An average of three million cubic yards of water are exchanged between the lagoon and the ocean with each tide.

Most lagoons are born when a river valley floods during a period of rising sea levels. Their life span is usually determined by the balance of two opposing processes: sedimentation and scouring.

Due to its location along an active fault, Bolinas Lagoon is unusual in being influenced by a third process as well -- tectonic subsidence -- which assists scouring to counteract the force of sedimentation.

As sediments fill a lagoon, the amount of water exchanged between the ocean and the estuary is reduced. Without tidal influence, the lagoon is no longer able to support estuarine habitats.

Opposing the process of sedimentation is lagoon scouring. Scouring is the re-suspension of bottom sediments into the water column. Water carrying the suspended sediment flows out of the lagoon with the ebb tide and into the ocean. The better the scouring action within a lagoon, the longer its life span. Storms, with their high winds that churn the water, and rains which boost the speed of creek flow, help to scour lagoons and remove sediments to the ocean.

The life span of a most lagoons depends on the balance between the processes of sedimentation and scouring. Because sedimentation is usually stronger than scouring, it is the fate of most lagoons to become upland habitat with in a few hundred to a few thousand years.

Bolinas Lagoon, on the other hand, has maintained a healthy estuarine ecosystem for many thousands of years. This longevity is due to the third force assisting the scouring process, subsidence, the same force that created the lagoon.

A Clash of Continents

Bolinas Lagoon is the result of a conflict of continents that continues today to maintain the lagoon's estuarine characteristics. While most estuaries along the Pacific Coast were formed at river mouths, two owe their origins are to tectonic forces; they are classified as fault bays.

The story of the birth of these two, Bolinas Lagoon and Tomales Bay, has its roots in one of Earth's most fundamental and unique processes -- continental drift.

A study of the rock formations on either side of the Bolinas Lagoon reveals a tale of migration on a massive scale, and the meeting of lands formed millions of years ago and thousands of miles apart.

To the west of Bolinas Lagoon the Point Reyes Peninsula rides upon the Pacific Plate, which for millennia has been creeping northwest at the rate of about 2 cm per year.

East of the lagoon, the North American Plate carrying the Coast Range has been slowly slipping to the southeast. Strained by friction between the two sliding continents, the underlying rocks are stretched to the breaking point - an earthquake occurs and the intercontinental sediments subside, creating a valley. The San Andreas Fault Valley, in which Bolinas Lagoon and Tomales Bay lie, was created in this way.

Over 7000 years ago, rising ocean levels filled the southern end of the valley. Sea water flooded in and mixed with fresh water from the creeks. When a sandy spit formed across the southern reach of the valley, trapping much of the saltwater behind, Bolinas Lagoon was born.

As the continents continue on their paths, a quake cycle is repeated. After a quake, the rock under the fault valley is released from strain, but only temporarily. Stress soon builds up again as the plate's part, the rocks break, a quake occurs, and the cycle begins again. With each quake along the San Andreas Fault, Bolinas Lagoon subsides a little, allowing more seawater to pour in.

The subsidence counteracts the affects of sedimentation in the lagoon by increasing the volume of water that can be exchanged with each tide (tidal prism). When sediments are flushed out more efficiently the lagoon's estuarine nature is reinvigorated. Thus, the cycle of quakes has worked to prolong the life span of Bolinas lagoon, and for millennia, the processes of scouring and tectonic subsidence have balanced the process of sedimentation.

Geographical Uniqueness

Bolinas Lagoon is unique geographically as well as geologically. Coastal wetlands are a rare habitat on the West Coast, and of those that still occur, Bolinas Lagoon is one of the most pristine. Wetlands characterize the East Coast of North America, geologically a partially submerged plain. Tidal marshes occur in most low-lying areas, and the many rivers, fed by abundant rainfall, generate estuaries both large and small up and down the coast. Though most wetlands on the East Coast have been destroyed or heavily modified by man, many remnant marshes remain.

In contrast, the arid west produces few rivers to meet the sea. And, unlike the soft sediments typical of much of the East Coast, the Pacific Coast is largely rocky. As a result, the few estuaries that have formed at river mouths are contained within a small space, with the notable exception of San Francisco Bay. Of the fifty or so estuaries that have formed along the Pacific Coast, Bolinas Lagoon is one of only 13 that sustains large numbers of migratory shorebirds, a testament to its vitality.

Biological Characteristics

Bolinas Lagoon boasts a healthy though fragile ecosystem, the vitality of which stems from the diversity of habitats it supports. These include subtidal channels and eelgrass beds, rocky and mudflat intertidal substrates, salt marsh, and upland marsh.

Each of these habitats is home to a unique community of plants and animals. The lagoon is a nursery for tiny fish, many of which are important to commercial fishing industries, and which feed nestling herons and egrets in the nearby colony at Audubon Canyon Ranch. Herds of harbor seals haul out onto the mudflats to rest, warm themselves, and give birth. Thousands of wintering shore and waterbirds, including Brown Pelicans and endangered Snowy Plovers, find bountiful forage and roosting areas.

The most productive habitat within the lagoon is, surprisingly, the mud. The unique combination of sun, shallows, tide flow, and sedimentation has furnished the lagoon with over 600 acres of mudflats.

The mudflat food chain begins with photosynthetic bacteria and protists. Hundreds of invertebrate species thrive on these single-celled organisms, planktonic animals, algae, and each other. Marine vertebrates such as fish, shark and rays forage for invertebrates in the soft sediments during high tide.

At low tide, shorebirds peck and probe for abundant worms, clams, and arthropods. And then there are the predators - hawks and harbor seals, osprey and owls, falcons and foxes that hunt day or night, high tide or low. An hour spent along the lagoon shores will reveal at least a few glimpses of the food chain in action - especially in the fall, when raptors send showers of shrieking, sparkling sandpipers shimmering through the air. While Bolinas Lagoon is a sanctuary for the many species which spend most or all of their lives there, it makes its mark on a global scale as a crucial link in the chain of wetlands used by millions of waterbirds which migrate along the Pacific Coast Flyway.

Flyways

Flyways are migration highways for birds. Twice a year, going North in the spring and South in the fall, millions of birds travel between summer breeding grounds and warm winter habitats. There are three major flyway routes in North America. Through large-scale banding and recovery efforts, biologists have defined narrow 'corridors' within each flyway. Bird species rely on one or a few corridors, which connect a series of wetlands upon which they rely for safe feeding and roosting areas along their journey.

The Pacific Coast Flyway is one such corridor. Its scattered wetlands connect the wintering areas of Mexico and Central America to breeding grounds in the Arctic and Sub-Arctic of Alaska and Canada. Because of the scarcity of water in the arid West, the ability to locate dependable, productive wetlands is essential to the survival of tired and hungry migrants. Major stopover areas for migrants are the Copper River Delta in Alaska, Fraser River Delta in British Columbia, and San Francisco Bay in California. Small coastal wetlands such as Morro Bay, Newport Bay, and the estuaries of San Diego County also host large numbers of migrants. Few of these however are protected, healthy habitats. Bolinas Lagoon is a notable exception.

The habitat requirements for migratory and over-wintering birds are very specific. No two wetlands share exactly the same habitat characteristics, and even single wetlands vary greatly with season. So, not only must a migratory bird be able to locate a wetland, but the wetland must also be able to support its feeding, roosting, and protection needs. Any alteration in water depth, roost site, emergent vegetation, invertebrate population, or physical geography may greatly reduce a wetland's ability to support wildlife. In order to support the greatest variety and number of migratory birds, a healthy wetland needs a variety of habitats. Bolinas Lagoon is one of the few remaining wetlands that has this diversity.

In the past, as coastal wetlands gave way to meadows through natural sedimentation processes, dynamic coastal forces allowed new ones to be created elsewhere. Today, because most potential wetlands are diked, filled in, or otherwise unavailable to the creation of new estuarine habitat, any loss of wetlands is a permanent deficit. Fortunately, a groundswell to restore former wetlands and protect existing ones is mounting. Their value as esthetic tourist destinations are more than matched by their value to wildlife.

As wetlands of the Pacific Coast, always a scarce habitat, become rarer, the value of remaining wetlands becomes incalculable. In fact, over 90% of the wetlands that were in existence when Europeans settled the United States have been lost.

Current Trends

Sediments have always entered the lagoon from many natural sources (the highly erodable cliffs of Duxbury Point, sand from the outer beaches, tiny rocks and organic material from the creeks). But beginning in the 1850's and continuing today, practices such as logging, road-building, ranching, farming, and development have contributed to an increase in the sediment load entering the lagoon from the surrounding watershed.

For example, the channelization of creeks for flood control increases stream velocity; fast moving creeks carry sediments all the way to the lagoon system. Before flood control, slower flows allowed sediments to settle in the creek bed.

Similarly, winds that once blew over the sandspit into the lagoon, churning the waters and scouring the sediments, are now blocked by development. Such human-caused increases in sedimentation have overpowered the processes of scouring and subsidence.

Still lovely, still ecologically vital as an estuary, the lagoon is currently undergoing a rapid transformation. As more sediments settle in the lagoon, the volume of water that enters from the sea decreases, the effects of scouring are decreased, and the pace of sedimentation accelerates. Thus, the death of Bolinas Lagoon may occur quite suddenly. Without intervention, it is estimated that the lagoon will turn into a meadow within fifty years. Meadows, valid habitats in their own right are not rare, nor can they support the biomass or match the diversity of species found within coastal wetlands.