On May 12, 2001, about 1940 hours Pacific daylight time, a homebuilt Walters Avid Magnum, N21911, touched down hard on Lake Shasta, California. The seaplane was substantially damaged. Neither the commercial certificated pilot, who owned and operated the experimental airplane, nor the passenger was injured during the personal flight. Visual meteorological conditions prevailed, and no flight plan was filed. The flight was performed under 14 CFR Part 91, and originated from a lake near Oroville, California, about 1850. In the pilot's completed accident report, he indicated that light to moderate turbulence was experienced during his landing approach to the lake. He stated that he encountered a "terrible downdraft." With very little forward ground speed, his airplane descended at 1,000 feet per minute. Despite application of engine power, he was unable to climb out of the downdraft. Suddenly, about 50 feet above the water, the airplane pitched downward and impacted the water at a 45-degree angle. The pilot additionally reported that an acquaintance of his indicated that, at the time of the accident, there were 30-knot wind gusts in the vicinity.