In 1921, on the southern tip of Brannan Island, was a swampy area of approximately 335 acres. The property was part of the Peter Cook holdings. The land was acquired by the Sacramento San Joaquin Drainage District for the State Reclamation Board for use by the United States Army Corps of Engineers. The area, was pumped full of sand and silt from the river during the dredging and widening of the Sacramento River channel between 1926 and 1929. The dredging operation filled the area with spoils to a level between twenty to forty feet above water level. After the operations were over the land sat idle, except for sales of sand by the State to private contractors.
In 1950 the Rio Vista Chamber of Commerce spearheaded a drive to have the area become a State Park. A transfer of the title to the property to the State Division of Beaches and Parks, set the stage for the creation of the park. Between 1952 and 1954, the area again sat idle. The Division of Beaches and Parks granted permission to Travis Air Force Base to establish a boat harbor on Seven Mile Slough for Air Force Personnel.
Over the years improvements and additions to park facilities have made Brannan Island State Recreation Area an important recreational facility strategically located between the Sacramento and San Joaquin Rivers. The dredging that created the area for the park also made it the highest spot east of the Sacramento River for a radius of 10 to 25 miles.
Brannan Island was named after Samuel Brannan who led a group of Mormons to California in 1846. In 1846, Brannan rented a ship's long-boat named the Gusanita and sailed across San Francisco Bay and enter the San Joaquin River. He was looking for suitable land on which to establish a community farm. The site of New Hope was chosen and thus the first Mormon farm in the San Joaquin Valley was started.
In May, 1848, Brannan spread the news of gold discoveries elsewhere along the American River. The result was a business boom in California. By February, 1848, Brannan owned a store on the Sacramento waterfront. In just a few months he had made over $100,000. By the end of 1849, Brannan owned one-fourth of all the land in Sacramento and would have eventually owned one-fifth of San Francisco. In 1859 Brannan purchased a tract of land surrounding a hot spring at the foot of Mt. St. Helena, with the intention of creating a summer resort. At a promotional dinner Brannan toasted his proposed resort as the "Saratoga of California" but unfortunately it came out as the "Calistoga of Sarafornia" and the name Calistoga stuck.
Brannan set out to interest people in the resort by financing a branch of the railroad to Calistoga Hot Springs. He also grew grapes and was successful in the wine making business. Although he amassed a fortune during his lifetime, Samuel Brannan died in poverty on May 14, 1889. He contributed his name to a street in San Francisco and the island and this park in the Delta.