THE CESSNA 180 WAS IN CRUISE FLIGHT A FEW MINUTES AFTER DEPARTURE WHEN THE ENGINE BEGAN RUNNING ROUGH AND LOST POWER. THE PILOT WAS UNABLE TO RETURN TO A SAFE LANDING SITE, SO ELECTED TO ATTEMPT A FORCED LANDING NEAR A RIVER. AN ENGINE TEARDOWN REVEALED THAT THE EXHAUST VALVE ROCKER SHAFT BOSS ON ONE CYLINDER HAD FAILED AND THE BROKEN PIECE MIGRATED UNDER THE INTAKE ROCKER SHAFT, WHERE IT CAUSED THE VALVE PUSH ROD TO BEND.