On July 8, 2017, about 1045 mountain daylight time, a Piper L-21B, N99031, was substantially damaged during a wire strike and subsequent impact with the Yellowstone River in Big Timber, Montana. The private pilot and his passenger were seriously injured. The airplane was owned by a private individual and operated by the pilot as a 14 Code of Federal Regulations Part 91 personal flight. Visual meteorological conditions prevailed and no flight plan was filed for the local flight. The flight departed Big Timber Airport (6S0), Big Timber, Montana about 1030. According to the pilot, he departed the airport to the north with a family member onboard. The pilot then turned east to follow the river and look for his sister who was participating in a water sport. The airplane was about at tree level when it collided with a set of power lines and then immediately impacted the water. The pilot reported to his father that he observed sparks and wires before the airplane entered a nose-down dive. The 1055 recorded weather observation at 6S0 included wind 220° true at 11 knots, visibility 10 statute miles, clear skies, temperature 25° C, dew point 11° C, and an altimeter setting of 30.20 inches of mercury. According to the U.S. Naval Observatory, the sun azimuth at the time of the accident would have been 116°. The airplane came to rest near an eyot approximately 4 nm northeast of 6S0. Photographs from the Federal Aviation Administration revealed that the airplane sustained substantial damage to the wings, empennage, and fuselage. The pilot reported that the airplane was on a southeast course at the time of the accident. He noted that the sun was on the horizon, but did not obstruct his vision. The pilot further remarked that he was aware of the presence of these power lines from a previous experience flying over the Yellowstone River, several years ago. A law enforcement representative stated that he observed a broken residential power line near the accident site and that he received reports of a disruption in power service from nearby residents around the time of the accident. The silver colored power line was one of a three phase power configuration that ran between two distribution towers on both sides of the river from a height of approximately 90 feet. The towers were about 4 nm east of the airport. According to the pilot's father, who is also the 6S0 airport manager, transient pilots frequently fly at low altitudes over this particular portion of the Yellowstone River to avoid turbulent air in the summertime. The airport manager further reported that the power lines were not equipped with aerial marker balls for identification. At the request of the NTSB Investigator-in-Charge, the airport manager agreed to install signage in the local fixed based operator office at 6S0 to caution pilots of the presence of power lines over the Yellowstone River.