Cal. Wat. Code § 81325
Not later than 60 days from the date of the receipt of the resolutions pursuant to Section 81319, the Board of Supervisors of San Mateo County shall determine whether or not resolutions to form and join the agency have been adopted by at least 15 public entities representing collectively at least 60 percent of the total water deliveries of all the public entities identified in Section 81305. In making that determination, the board of supervisors shall use the water delivery quantities set forth in subdivision (b) of Section 81460. If 15 or more public entities representing collectively 60 percent or more of those water deliveries have adopted those resolutions, the board of supervisors, by order entered in its minutes, shall declare that the agency has been formed and list each public entity that is a member of the agency.
Ca. Water Code § 81325