Rodeo Lagoon is a coastal

lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') a ...

located in the

Marin Headlands The Marin Headlands is a hilly peninsula at the southernmost end of Marin County, California, United States, located just north of San Francisco across the Golden Gate Bridge, which connects the two counties and peninsulas. The entire area is pa ...

division of the

Golden Gate National Recreation Area The Golden Gate National Recreation Area (GGNRA) is a U.S. National Recreation Area protecting of ecologically and historically significant landscapes surrounding the San Francisco Bay Area. Much of the park is land formerly used by the United ...

, which is in southern

Marin County Marin County is a county located in the northwestern part of the San Francisco Bay Area of the U.S. state of California. As of the 2020 census, the population was 262,231. Its county seat and largest city is San Rafael. Marin County is acros ...

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

.USGS
/ref> This

brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...

water body is separated from the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

by a sand bar that forms Rodeo Beach. Rodeo Lagoon stretches approximately by , and is about deep at its maximum depth. It covers a surface area of about .

Inflows and outflows

Rodeo Lagoon empties into the Pacific Ocean when the water level reaches a high enough level to erode through the sand bar. This high water level usually occurs in the winter months. The outlet channel, shown at left looking southeast towards Rodeo Lagoon, is spanned by a pedestrian bridge that provides access to Rodeo Beach. Like many other coastal lagoons in California, the outlet becomes completely blocked by a sand bar during the dry summer months, resulting in negligible exchange between the lagoon and ocean during that time. The depth, surface area, and volume of the lagoon all vary depending on the configuration of the sand berm (high or low) and rainfall (wet or dry). Depths in the center range from less than in a dry summer to nearly in a wet winter. Tides do not have a significant effect on the circulation of water in Rodeo Lagoon. Instead, the circulation of water in Rodeo Lagoon is driven almost entirely by the wind. Since the Marin Headlands are often quite windy, it is not unusual to see

Langmuir circulation In physical oceanography, Langmuir circulation consists of a series of shallow, slow, counter-rotating vortices at the ocean's surface aligned with the wind. These circulations are developed when wind blows steadily over the sea surface. Irvin ...

form

windrow A windrow is a row of cut (mown) hay or small grain crop. It is allowed to dry before being baled, combined, or rolled. For hay, the windrow is often formed by a hay rake, which rakes hay that has been cut by a mowing machine or by scythe into a ...

s of foam on the surface of the lagoon.A. J. Szeri
1996, Langmuir circulations on Rodeo Lagoon, Monthly Weather Review 124(2), 341-342. Rodeo Lagoon is a brackish water body, with

salinities Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...

in most of the lagoon ranging from 2 to 10 practical salinity units (psu) over the course of the year. However, water near the bottom of the lagoon is often much saltier, reaching levels as high as 25 psu. The bowl-shaped

bathymetry Bathymetry (; ) is the study of underwater depth of ocean floors (''seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water de ...

of the lagoon traps this dense, salty water and prevents it from quickly draining back out in the ocean. Pelican In Rodeo Lagoon

Wildlife

Rodeo Lagoon provides habitat to a protected species, the

tidewater goby ''Eucyclogobius newberryi'', the Northern tidewater goby, is a species of goby native to lagoons of streams, marshes, and creeks along the coast of California, United States. The Northern tidewater goby is one of six native goby species to Califo ...

. It also provides habitat to many species of migrating waterfowl. Resident fish include the threespine stickleback and prickly sculpin in addition to the tidewater goby. A family of river otters also makes frequent use of the lagoon, and includes

brown pelican The brown pelican (''Pelecanus occidentalis'') is a bird of the pelican family, Pelecanidae, one of three species found in the Americas and one of two that feed by diving into water. It is found on the Atlantic Coast from New Jersey to the mout ...

s in their summer dietJ. Laws
Otter Chaos at Rodeo Lagoon, Bay Nature, July–September 2007 The lagoon is ringed by emergent aquatic vegetation, such as willows and cattail, and has submerged aquatic vegetation like sago

pondweed Pondweed refers to many species and genera of aquatic plants and green algae: *''Potamogeton'', a diverse and worldwide genus *''Elodea'', found in North America *''Aponogeton'', in Africa, Asia and Australasia *''Groenlandia'', a genus of aquatic ...

and widgeon grass in the shallow areas. The

phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. Ph ...

community includes

diatom A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...

chlorophyte Chlorophyta or Prasinophyta is a taxon of green algae informally called chlorophytes. The name is used in two very different senses, so care is needed to determine the use by a particular author. In older classification systems, it refers to a ...

s, flagellated

protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...

, ''Microcystis aeruginosa'', and '' Nodularia spumigena''. ''Microcystis sp.'' and ''Nodularia sp.'' are types of toxic

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

, capable of releasing the toxins microcystin and nodularin into the water.

Eutrophication

The population of phytoplankton described above is extremely high in the summer, a condition known as

eutrophication Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...

. At times, the algae form a visible surface scum, and their high concentration results in large swings in the dissolved oxygen content of the water, culminating in a depletion of oxygen when the population finally crashes. This annual algae bloom, because of its size and potential toxicity, is considered problematic for water quality and fish survival. Cyanobacterial Scum

Rodeo Lagoon typically suffers from an oxygen deficit or

hypoxia Hypoxia means a lower than normal level of oxygen, and may refer to: Reduced or insufficient oxygen * Hypoxia (environmental), abnormally low oxygen content of the specific environment * Hypoxia (medical), abnormally low level of oxygen in the tis ...

in the summer and fall. This deficit is caused by the high oxygen consumption of decaying algae that occurs during that time of year. The large algae population, which withdraws carbon dioxide from the water, also raises the pH to very high levels. The pH is typically greater than 9 throughout the summer

algae bloom An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in Fresh water, freshwater or Ocean, marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term ...

Geology

The lagoon is located within the geologically complex Marin Headlands and fills a valley drowned by recent

sea level rise Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cry ...

following the last glacial period.National Park Service
Geology of the Golden Gate Headlands Field Guide The bed of the lagoon is covered by viscous black mud that is high in organic content, except at the east and west ends, where non-organic sediment can be found.

History

Rodeo Lagoon was mostly unaltered until the area was developed by the U.S. military into

Fort Cronkhite Fort Cronkhite is one of the components of California's Golden Gate National Recreation Area. Today part of the National Park Service, Fort Cronkhite is a former US Army post that served as part of the coastal artillery defenses of the San Francisc ...

and

Fort Barry Fort Barry is a former United States Army installation on the West Coast of the United States, West Coast of the United States, located in the Marin Headlands of Marin County, California, north of San Francisco. Opened in 1908, the fort was part ...

San Francisco Estuary Institute
Wetland Habitat Changes in the Rodeo Lagoon Watershed, Marin County, CA, October 2004. In 1937, the Army constructed a road crossing at the east of the lagoon, which resulted in a reduction in the lagoon's size as the area upstream of the crossing gradually turned into a freshwater marsh. The lagoon is now about 80% of its original size.

Recreation

Recreation is not allowed on or in Rodeo Lagoon because of water quality and wildlife concerns. However, the lagoon and surrounding area is an excellent place for wildlife viewing, particularly birding
/ref> and watching the playful river otters.