WATER TRUCK OPERATOR HAD STARTED TO ASCEND A HAUL ROAD GRADE & DID NOT SHIFT TO A LOWER GEAR UNTIL THE MOTOR STARTED TO STALL. WHEN HE DISENGAUGED THE CLUTCH, THE TRUCK ROLLED BACKWARD. THE OPERATOR HIT THE BRAKES AND SLID IN WATERED AREA SIDEWAYS & THE TRUCK ROLLED OVER.