The Carquinez Strait is a narrow tidal strait in northern California. It is part of the tidal estuary of the Sacramento and the San Joaquin rivers as they drain into the San Francisco Bay. The strait connects Suisun Bay, which receives the waters of the combined rivers, with San Pablo Bay, a northern extension of the San Francisco Bay.
In 1985, an errant humpback whale swam up the Carquinez Strait as far as Rio Vista. Named Humphrey the whale by the media, he was successfully turned around and rescued by the Marine Mammal Center and other volunteers.
The USNS Glomar Explorer (T-AG-193) was moored for many years at the eastern end of the strait.
Carquinez Strait Powerline Crossing was the world's first powerline crossing of a large river.
The strait forms part of the border between Solano (to the north) and Contra Costa (to the south) counties, and is approximately 15 mi (25 km) north of Oakland. The cities of Benicia and Vallejo lie on the north side of the strait, while Martinez, Port Costa, and Crockett sit on the southern coast. The Napa River joins the strait, via the short Mare Island Strait, near its entrance into San Pablo Bay.
A rail bridge is just east of the Benicia-Martinez bridge, which is used by the Capitol Corridor, California Zephyr, and Coast Starlight trains. A rail ferry, with the ferries Contra Costa and Solano provided service across the strait, near the location of the current rail bridge, until the rail bridge was built in 1930.
Ship traffic 
Formation of delta 
The narrow gap in the Coast Range that forms the strait has led to the formation of the San Joaquin-Sacramento River Delta, an inverted river delta, upstream of it, a rare geological feature. The strait is too small to allow the passage of huge amounts of floodwaters created during years that have tropical storms lined up, one after another, dropping torrential rainfall and melting snowpack in the Sierra Nevada Mountains simultaneously. These events have been documented since the Gold Rush period, when pioneers wrote about the "Inland Sea" upon the wet pages of their personal diaries. The Delta area is the first to fill up and the last to drain out after a flood event, causing silt and soil to drop out of the flow and creating the inverted river delta design.
- Coffey, Geoffrey. "Going in for natural solitude on Mt. Wanda". The San Francisco Chronicle. Retrieved 2008-04-08.