California Gov. Jerry Brown on Wednesday endorsed a $14 billion plan to sharply increase water supplies to parched Southern California, in a move that would remake the water map in the semi-arid state.

The plan, which Mr. Brown backed alongside U.S. Interior Secretary Ken Salazar, involves two massive tunnels to divert water from wet Northern California to the south. It was immediately assailed by a broad range of interest groups, which denounced it as too expensive and potentially detrimental to the environment and to the...