California is building walls at its borders — they’re just not the kind President Trump has in mind. As the Trump administration continues its assault on environmental regulation, state officials are throwing up legal barriers to some high-stakes attacks. They are preparing to strengthen safeguards for waterways that are about to lose federal protections in a major rollback of the Clean Water Act. They are refusing to issue permits the federal government needs to build a controversial dam project that would drown portions of a Northern California river renowned for its wild trout fishery.