The pilot of the helicopter stated that he was conducting an aerial application flight over a river. He completed a site survey, noted wires in close proximity to the site, and began application at an altitude of 40 feet. The pilot stated that approximately three quarters of the way across the river, he "momentarily lost sight" of the power lines. The main rotor blades struck the power lines, and the pilot maneuvered the helicopter over a shallow water area. The helicopter lost anti-torque control and the pilot performed a hovering autorotation to the river, where it impacted rocks and rolled to the right. The helicopter sustained substantial damage to the main rotor system, fuselage and tail boom. The pilot stated that there were no mechanical malfunctions or anomalies with the helicopter, and that the accident could have been prevented with a more thorough examination of the spray site and adjustment of the spray pattern.