Perhaps you’ve noticed the tranquility that defines Rodeo Lagoon. It is a coastal lagoon within the Golden Gate National Recreation Area. More specifically, it is in the Marin Headlands.
Rodeo Lagoon offers a serene escape. This brackish water body is set apart from the Pacific Ocean. A sand bar creates Rodeo Beach and separates it from the ocean.
Let’s explore its dimensions. Rodeo Lagoon stretches about 900 metres long. It is 250 metres wide. Its maximum depth reaches approximately 2 metres. The lagoon covers a surface area of about 15 hectares.
How does Rodeo Lagoon connect to the ocean? When the water level rises enough to erode the sand bar, it empties into the Pacific. This usually happens in the winter months. A pedestrian bridge spans the outlet channel. It gives access to Rodeo Beach. During the dry summer months, the outlet is often blocked by a sand bar. This results in minimal exchange between the lagoon and the ocean.
The lagoon’s depth, surface area, and volume change. It depends on the sand berm and rainfall. Depths in the center can be less than 1.5 metres in a dry summer. They can reach almost 3 metres in a wet winter.
Tides do not greatly affect the water flow in Rodeo Lagoon. Wind mainly drives its circulation. The Marin Headlands are often windy. This often causes Langmuir circulation to form foam windrows on the lagoon’s surface.
Rodeo Lagoon is a brackish water body. Most of the lagoon’s salinity ranges from 2 to 10 psu throughout the year. However, the water near the bottom can be much saltier. It can reach levels as high as 25 psu. The lagoon’s bowl shape traps this dense, salty water. This prevents it from quickly draining back into the ocean.
Rodeo Lagoon provides a habitat for the tidewater goby. This is a protected species. Many migrating waterfowl species also live here. Resident fish include the threespine stickleback and prickly sculpin. River otters also frequently use the lagoon. They even include brown pelicans in their summer diet.
The lagoon is ringed by aquatic vegetation. Willows and cattails grow here. Submerged aquatic vegetation includes sago pondweed and widgeon grass in the shallow areas. The phytoplankton community includes diatoms and chlorophytes. Flagellated protozoa, Microcystis aeruginosa, and Nodularia spumigena can also be found here. Microcystis sp. and Nodularia sp. are toxic cyanobacteria. They can release microcystin and nodularin toxins into the water.
The phytoplankton population is very high in the summer. This condition is known as eutrophication. Algae sometimes form a visible surface scum. Their high concentration causes large swings in the water’s dissolved oxygen content. This results in oxygen depletion when the population crashes. This annual algae bloom is considered problematic for water quality. It can also impact fish survival.
Rodeo Lagoon often suffers from an oxygen deficit in the summer and fall. This deficit is due to the high oxygen consumption of decaying algae. The large algae population withdraws carbon dioxide from the water. This raises the pH to very high levels. The pH is typically greater than 9 during the summer algae bloom.
The lagoon fills a valley drowned by recent sea level rise. This happened after the last glacial period. The lagoon bed is covered by viscous black mud. It is high in organic content. Non-organic sediment is at the east and west ends.
Rodeo Lagoon remained mostly unchanged. Later, the U.S. military developed the area into Fort Cronkhite and Fort Barry. In 1937, the Army built a road crossing at the lagoon’s east end. This reduced the lagoon’s size. The area upstream of the crossing gradually turned into a freshwater marsh. The lagoon is now about 80% of its original size.
Recreation is not allowed in Rodeo Lagoon. This is due to water quality and wildlife concerns. However, the lagoon is an excellent place for wildlife viewing. Birding and watching the playful river otters are popular activities. The Parks Conservancy supports wetland habitat protection at Rodeo Beach. They control the invasive cape ivy. This protects habitats for the endangered marsh sandwort and threatened California red-legged frogs.
Don’t miss the chance to see Bird Island. It is just off Rodeo Beach. Its white, guano-covered crags are full of cormorants, gulls, and pelicans. The cliff-top trail offers the best pelican watching. As many as 1,200 of these great birds may settle on the rocks at once.
Inland, look for egrets and ducks. Other birds feed and shelter in the shallow wetlands.
Remember to protect the endangered tidewater goby. Please do not enter any part of Rodeo Lagoon.