Most of the Senate District 18 population is in southern San Diego County, primarily the South Bay area south of the Coronado Bay Bridge including part of the City of San Diego, all of National City,  Chula Vista, Bonita, Imperial Beach, Lincoln Acres, San Ysidro, Otay Mesa, the Tijuana River Valley, and the southeastern side of San Diego stretching from the Pacific Ocean along the Mexico–United States border eastward, including all of Imperial Valley, and its several cities (El Centro, Brawley, Calexico, Salipatria, Holtville, Imperial, Westmorland) all the way to the Arizona border and stretching northward, including areas of California along the Colorado River, parts of Riverside and San Bernardino Counties, including the cities of Blythe, Needles, and Indio.