The Carquinez Strait (; Spanish: Estrecho de Carquinez) is a narrow tidal strait located in the Bay Area of Northern California, United States. 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 is long and connects Suisun Bay, which receives the waters of the combined rivers, with San Pablo Bay, a northern extension of the San Francisco Bay.
The strait formed in prehistoric times, near the close of one of the past ice ages, when the Central Valley was a vast inland lake. Melting ice from the Sierra Nevada raised the water level while seismic activity created a new outlet to the Pacific Ocean, draining the lake into the ocean and exposing the valley floors.
Etymology
The strait is named after the Karkin people ( in Spanish), a linguistic division of the Ohlone indigenous peoples who once resided on both sides of the strait.
History
thumb|left|[[Suisun Bay (bottom), Carquinez Strait (with bridges crossing it), and San Pablo Bay (upper center), with Point Reyes in the background]]
Andrei Sarna-Wojcicki, a geologist emeritus of the US Geological Survey (USGS), estimates that the Carquinez Strait was likely formed about 640,000 to 700,000 years ago, while much of modern California was emerging from an ice age. The present Sacramento Valley and San Joaquin Valley were covered by a huge lake (now extinct), which has been called Lake Corcoran. Initially, this lake drained into the ocean through a valley near present-day Monterey. However, ongoing seismic activity raised the coastal mountains sufficiently to plug this outlet. Concurrently, ice melting off the Sierras raised the water level in Lake Corcoran until the lake began to carve a new outlet to the ocean. At some point, the coastal barrier collapsed between today's cities of Crockett and Benicia, releasing lake water in a cataclysmic flood.
The strait forms part of the county border between Solano (to the north) and Contra Costa (to the south), and it 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. Its watershed covers , approximately 40 percent of California's total surface.
Tall pylons carrying power lines cross the strait as well. The Carquinez Strait Powerline Crossing was the world's first powerline crossing of a large river.
Ship traffic
The channel is navigable and is used for commercial and military shipping. Deep water ship traffic bound for both the Port of Sacramento and the Port of Stockton traverse the strait through the Stockton Deepwater Shipping Channel and Sacramento Deep Water Ship Channel. Carquinez Strait is part of the Baldwin Deepwater Shipping Channel.
Formation of delta
right|thumb|Annotated satellite image of the San Francisco Bay Area, featuring San Pablo Bay and the Carquinez Strait, 2004
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 with heavy rainfall or snowmelt events. Because the Delta area is the first to fill and last to drain in a flood event, silt and soil have more time to drop out of suspension, creating the inverted river delta feature.
Saltwater intrusion
Seawater is more dense than fresh water because of its higher concentration of salts. Under stable conditions, this means that an invisible boundary forms where two such streams meet, as where the fresh water from Sacramento and San Joaquin Rivers meet the sea water contained in the San Francisco Bay.
By the early 20th century, farmers in the Sacramento-San Joaquin Delta, who depended on irrigating their fields with fresh water from the two rivers noticed an increase in salinity farther inland than before. It became obvious that fresh water was being pumped out of the Delta faster than it could be replenished by rain and snow during the wet season. Farmers, businessmen and politicians complained that allowing fresh water to flow to the ocean was wasteful. While many solutions were proposed, few appeared practical. A political consensus formed that damming the Carquinez Strait should solve the seawater intrusion problem.
In September 1923, the California Legislature appropriated $10,000 (equivalent to $ today) for a saltwater dam survey. The Federal government added a $20,000 contribution ($ today) through the US Reclamation Service. Under Reclamation Service rules, another $10,000 needed to be raised from local supporters of the project. The necessary money was raised by March 1924, and the first of three site surveys was announced.