The Mare Island Strait is a channel

Strait

A strait or straits is a narrow, typically navigable channel of water that connects two larger, navigable bodies of water. It most commonly refers to a channel of water that lies between two land masses, but it may also refer to a navigable channel through a body of water that is otherwise not...

 at 38.10°N 122.265°W in the San Pablo Bay

San Pablo Bay

San Pablo Bay is a tidal estuary that forms the northern extension of San Francisco Bay in northern California in the United States. Most of the Bay is shallow; however, there is a deep water channel approximately in mid bay, which allows access to Sacramento, Stockton, Benicia, Martinez, and...

 separating Mare Island

Mare Island

Mare Island is a peninsula in the United States alongside the city of Vallejo, California, about northeast of San Francisco. The Napa River forms its eastern side as it enters the Carquinez Strait juncture with the east side of San Pablo Bay. Mare Island is considered a peninsula because no full...

 and the mainland in Vallejo, California

Vallejo, California

Vallejo is the largest city in Solano County, California, United States. The population was 115,942 at the 2010 census. It is located in the San Francisco Bay Area on the northeastern shore of San Pablo Bay...

 in Solano County. The strait was formerly used by the Mare Island Naval Shipyard

Mare Island Naval Shipyard

The Mare Island Naval Shipyard was the first United States Navy base established on the Pacific Ocean. It is located 25 miles northeast of San Francisco in Vallejo, California. The Napa River goes through the Mare Island Strait and separates the peninsula shipyard from the main portion of the...

 until its closure in 1995. The strait is the mouth of the Napa River

Napa River

The Napa River, approximately 55 miles long, is a river in the U.S. state of California. It drains a famous wine-growing region, called the Napa Valley, in the mountains northeast of San Francisco. Milliken Creek is a tributary of the Napa River....

 and is used for both recreational and freight boating. The Vallejo Ferry Terminal

Vallejo Ferry Terminal

The Vallejo Ferry Terminal is a ferry terminal in Vallejo, California. It is the northernmost terminal in the San Francisco Bay Area, which serves as the northern destination point for Baylink Ferry. It connects with Vallejo Transit's local and Baylink bus services, as well as Napa VINE.-External...

 and its commuter ferry service to San Francisco are located on the strait.