Southern California’s largest water district has signed a multimillion deal to buy several sprawling islands far north in the Sacramento-San Joaquin River Delta, drawing criticism for giving the powerful district a stronghold in the vital water hub, officials said Monday.

The deal worth $175 million will put the Metropolitan Water District of Southern California in ownership of 20,000 acres in the delta, a source of water for millions of residents throughout the state and some of the nation’s most valuable farmland.