On June 16, 1997, at 1740 hours Pacific daylight time, a Schempp-Hirth Nimbus 3/24.5, N4427M, collided with power lines while attempting a forced landing near Macdoel, California. The glider was substantially damaged, and the private pilot/owner sustained minor injuries. The personal flight originated at the Siskiyou County, California, airport at 1240. Visual meteorological conditions existed for the flight and no flight plan was filed. The pilot reported that the purpose of the flight was a closed circuit contest. The departure and destination points were the Siskiyou airport via Callahan Dry Lake, Butte Valley Airport, and Tenant South, all in California. The pilot said that in order to stay clear of high cirrus clouds over the Butte Valley airport, he flew south instead of direct to Tennant South, where he expected a thermal lift. He stated that approximately 7 miles south of Butte Valley airport he thermaled in a weak lift and it disappeared. At this point, the glider's altitude was 5,900 feet mean sea level (msl). The pilot stated that because there was not a sufficient gain in altitude he elected to fly north again to land at the Butte Valley airport. Approximately 4 miles from the Butte Valley airport, the pilot encountered an unexpected heavy sink rate which precluded landing at the airport. The field that he chose for an off-airport landing was a dirt strip approximately 1,000 feet long by 120 feet wide. The pilot reported that on final approach to the field he noticed a different set of power lines than the ones he had seen during the initial look at the area. These lines ran from east to west at the northern edge of the dirt strip on which he was landing to the south. In order to avoid the obstructions, the pilot attempted to fly over the lines. The pilot stated that after the right wing caught the wires, the glider fell to the ground, hit tail first, then bounced back into the air again before contacting the ground.