The local soaring flight departed via air-tow. Upon entering two back-to-back thermals, the glider climbed to an altitude of 9,000 feet mean sea level (msl). The pilot maneuvered the glider toward marker clouds for additional thermaling; however, he encountered what he described as severe turbulence and decided to head back to the airport. Just abeam the northwest end of a lake near the airport, the glider encountered turbulence along with a strong sink rate. The pilot reported the sink rate to be from 500 to 800 feet per minute (fpm), with extremes up to 1,200 fpm to 1,500 fpm. The pilot attempted to maneuver the glider to minimize its time in the sink condition. He realized that he was not going to make it back to the airport and started to look for a place to land. The pilot reported that due to busy roads and full parking lots, he chose to land along the shoreline of the lake. The glider touched down about 30 yards from the shore. The pilot stated that in retrospect, he understands that the glider was caught on the leeward side of a ridge or the downdraft of a wave. He reported no mechanical anomalies with the glider prior to the accident. The right side of the v-tail separated from the glider during the ditching and damage occurred to the spoilers and ailerons.