Tomales Bay is a long narrow inlet of the Pacific Ocean in Marin County in northern California in the United States. It is approximately 15 mi (25 km) long and 0.5 mi (1 km) wide, effectively separating the Point Reyes Peninsula form the mainland of Marin County. It is located approximately 30 mi (48 km) northwest of San Francisco. The bay forms the eastern boundary of Point Reyes National Seashore. On its northern end it opens out onto Bodega Bay, which shelters it from the direct current of the Pacific. The bay is formed by a submerged portion of the San Andreas Fault.
The area was originally settled by the Coast Miwok, and was visited by Francis Drake. A park was formed to preserve some of it, Tomales Bay State Park. The Marconi Conference Center SHP preserves a small hotel built by Guglielmo Marconi in 1913 to house a ship-to-shore radio station. Ownership of the facility passed to RCA in 1920. The station was closed in 1939. Synanon, a drug rehabilitation cult, owned it from the early 1960s until 1980, when it was purchased by a private foundation and given to the state to operate as a conference center.
TomalesBay is a 6800 acre estuary located on the central California coast approximately 40 miles northwest of San Francisco.
The TomalesBay watershed occupies an area of approximately 219 square miles (140,00 acres, 142 perimeter miles.) The northern end of the bay opens to Bodega Bay and the Pacific Ocean while the southern end is fed by Papermill Creek and marked by wetlands.
Tomales Point and the surrounding hillsides were not traditionally forested, stands of non-native Monterey Cypress trees and "Blue Gum" eucalyptus trees were planted on the Point and along the shores of TomalesBay and on Hog Island to shelter pioneer settlements and summer residences.
TomalesBay ends where wave action from the open ocean wraps around Tomales Point and forms Sand Point at the mouth of the bay, near Lawson's Landing.
TomalesBay is thus an estuarine environment, one of fluctuating salinities.
TomalesBay provides significant habitat for endangered Coho Salmon, is an important area of oyster and clam aquaculture, and hosts recreational fishing and kayaking.