Mare Island is a peninsula in the United States in the city of Vallejo, California, about 23 miles 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 body of water separates this or several other named "islands" from the mainland. Instead, a series of small sloughs cause seasonal water-flows among the so-called islands. Mare Island is the largest of these at about 3.5 miles long and a mile wide.