The local sightseeing flight departed Prescott to tour the Jerome, Cottonwood, and Verde Rivers areas in Arizona. The pilot was maneuvering through a dry river bed in a hilly area. He did not see the power lines until they hit the windshield about level with the top of the instrument panel. The nose immediately pitched up and then quickly pitched forward. The pilot estimated that the airspeed was 70 knots, and the altitude was 150 feet above ground level (agl). He lowered the collective to enter an autorotation, but delayed the flare until passing over a ridgeline. He was able to bleed off most of the airspeed, and leveled the skids prior to touchdown; however, the skids dug into soft dirt, and the helicopter pitched forward. The main rotor blades were still at 100 percent revolutions per minute (rpm) when they struck the ground and sustained substantial damage. The helicopter rocked back, and came to rest upright on the skids. The main rotor blades, mast, firewall, tail boom, and tail rotor drive shaft sustained substantial damage.