Coast Guard Island is an island in Embarcadero Cove (Brooklyn Basin) which is home to a Coast Guard base. It is within Alameda city limits, but only reachable by land via a bridge from Dennison Street in Oakland. It gets its power and other utilities from Alameda.
The 1877 map of Oakland shows two smaller, marshy islands near where Coast Guard Island is now, but they were no longer there in the 1912 map, presumably having been dredged away for better access to Brooklyn Basin. The current island was created in 1913 by the dredging project which extended the estuary to San Leandro Bay (thereby making Alameda an island), and was originally called Government Island as there were several government agencies based there.
The Coast Guard arrived in 1926 and established a base, and in 1931 they began constructing facilities for a permanent base. A training center was created in 1940 and was the major training center for the Coast Guard from 1942 until training was moved to New Jersey in 1982.
Support Center Alameda was established June 1, 1982 and the island was renamed Coast Guard Island. Now the base (referred to as Base Alameda) is home to six Coast Guard cutters as well as various support commands. A few employees work in office space nearby in Oakland, but are due to move into the Federal Building.