Bolinas Lagoon is a tidal estuary situated between the communities of Bolinas to the west and Stinson Beach to the east, about 35 miles (56 km) south-southeast of Bodega Bay and 17 miles (27 km) northwest of San Francisco, California. The lagoon opens to Bolinas Bay, a small bight between Duxbury Point and Rocky Point. The San Andreas fault zone forms the valley between Tomales Bay and Bolinas Bay and the trough is aligned with the long axis of Bolinas Lagoon. The lagoon formed at least 7,700 years ago when rising sea levels flooded the trough. The lagoon now forms a large inner harbor protected from the main bay by a sandspit known as Stinson Beach. The lagoon has a watershed of 16.7 square miles (43 sq km), with streams and canyons including Audubon Canyon, McKinnan Gulch, Morses Gulch, Picher Canyon, Pike County Gulch, Stinson Gulch, Volunteer Canyon, and Wilkins Gulch. Kent Island is located in the lagoon and is surrounded by mudflats at low tide.
Prior to the European colonization of California, the Coast Miwok lived here and the name Bolinas may have originated from their word for this area. In 1834, Rafael Garcia was a corporal stationed at Mission San Rafael Arcángel and became the first settler to establish a cattle ranch around Bolinas Lagoon. In 1836, Rancho Tomales y Baulines, a Mexican land grant of 9,468 acres (38.32 km2) located in the Olema Valley was given by Governor Nicolás Gutiérrez to Garcia and he moved north to allow Gregorio Briones, his brother in law, to work the ranch at Bolinas Lagoon. In 1846, a land grant of 8,911 acres (36.06 km2) called Rancho Las Baulines was given to Gregorio Briones by Governor Pío Pico. The grant extended around Bolinas Lagoon and encompassed present-day Stinson Beach and Bolinas. In 1848, following the Mexican-American War, California was ceded to the United States, and the Treaty of Guadalupe Hidalgo provided that the land grants would be honored. As required by the California Land Act of 1851, a claim for Rancho Las Baulines was filed with the Public Land Commission in 1853 and the grant was patented to Gregorio Briones in 1866. In the late 1800s, Bolinas was a little town growing around an embarcadero with an economy that was dependent on coastal schooner traffic between Bolinas and the markets of San Francisco. Bolinas offered hotels and restaurants for intrepid vacationers from San Francisco who wanted to escape the summer fog in the city. In 1906, the California earthquake caused an offset of about 20 feet (6 m) near Bolinas and many buildings were severely damaged. Today, Bolinas Lagoon is internationally recognized as an important wetland habitat and is part of the Gulf of the Farallones National Marine Sanctuary and borders on the Point Reyes National Seashore to the north.