With a friend in the White House and their party in control of both chambers of Congress, House Republicans have embarked on their most ambitious effort yet to change the way water flows in California. Legislation that the House sent to the Senate last week outlines a bold effort to build big new dams and shift water from fish, birds and other wildlife to farms in the San Joaquin Valley. The legislation would dry up long stretches of the state’s second-longest river, the San Joaquin, and end efforts to restore its obliterated salmon runs.