A joint powers authority has been formed to oversee design and construction of California’s delta water tunnel project, a milestone in the multibillion-dollar plan that has been in the works for more than a decade. The Metropolitan Water District of Southern California, Santa Clara Valley Water District and Zone 7 Water Agency, three agencies that agreed to fund the project, have seats on the board of the newly formed Delta Conveyance Design and Construction Joint Powers Authority. A board slot is reserved for the Kern County Water Agency, whose board will vote May 24 on whether to accept the seat.