On July 2, 1998, at 1415 central daylight time (cdt), a Robinson R-44, N8329B, piloted by a private pilot, received substantial damage when it contacted wires and impacted the ground during a precautionary landing, in the Roaring River State Park, near Cassville, Missouri. The pilot stated that the precautionary landing was attempted due to deteriorating weather in the area, resulting from thunderstorms, lightning, and lowered visibility. Visual meteorological conditions prevailed at the time of the accident. The personal 14 CFR Part 91 flight was not operating on a flight plan. The pilot and one passenger reported minor injuries. The flight departed from a private helipad, near Tulsa, Oklahoma, at time unknown, with the intended destination of M. Graham Clark Airport, near Point Lookout, Missouri. According to the pilot's written statement, the pilot and his passenger were enroute to their destination when they became aware that they were approaching a line of thunderstorms from behind. The pilot stated that he decided to make a precautionary landing due to the convective weather in the area and wait for the line of thunderstorms to move further from their route. The pilot decided to land on a gravel bar in a river that ran through the Roaring River State Park. During approach to the landing area, the aircraft struck a power line, approximately 35' above ground level (agl), and the pilot executed a run-on landing. The aircraft impacted the ground resulting in substantial damage to the landing skids, rotor blades, and tail boom. In a telephone conversation with a Federal Aviation Administration (FAA) inspector, the pilot stated that he had asked his passenger to help him watch for wires, but he did not see them until it was too late to avoid them. The pilot stated that there were no aircraft mechanical problems related to the accident. The pilot stated, in a phone conversation with the Investigator in Charge (IIC), that he was aware of the thunderstorms from watching a television weather broadcast, but thought the line of storms had moved past his route of flight when he had made his departure. There were no Flight Service Station (FSS) or Direct User Access Terminal System (Duats) pilot briefings given for the aircraft in question.