The powerful Metropolitan Water District of Southern California agreed Tuesday to buy four islands and a portion of a fifth in the Sacramento-San Joaquin River Delta, a bid that some conservationists believe is a blatant water grab by California’s largest water agency.

The district’s 37-member board of directors, representing 26 agencies in Southern California, authorized Metropolitan’s general manager to enter into a purchase agreement to buy 20,369 acres of land encompassing Webb Tract, Bacon Island, Bouldin Island, most of Holland Tract and a portion of Chipps Island, in Contra Costa, San Joaquin and Solano counties.