On July 21, 2001, at 1230 hours Pacific daylight time, a Cessna R172K, N736YR, was destroyed when it collided with trees and mountainous terrain during an attempt to reverse direction in a box canyon 11 nautical miles southwest of the Mammoth Yosemite Airport, Mammoth Lakes, California. The accident site is at 9,500 feet mean sea level (msl) in the Sierra Nevada Mountains. The airplane, operated by American Aviation of Van Nuys, California, and rented by the pilot, was destroyed. The private pilot and one passenger were seriously injured. Visual meteorological conditions prevailed for the personal flight conducted by the pilot under 14 CFR Part 91. The airplane departed Van Nuys approximately 1000 for the flight to the Mammoth Yosemite airport. No flight plan was filed. An accident report was submitted by the pilot. He stated that he had received a weather briefing by computer prior to the flight. He departed the Van Nuys airport approximately 1000 and proceeded to the Friant VOR. Upon reaching Friant, he tracked the 023-degree radial outbound until he was over a lake. When he saw the lake, he turned east toward the Mammoth Yosemite airport. He entered a canyon and later realized that he had entered the wrong one. He said that the canyon was closed and above his altitude. He also stated that he was experiencing moderate turbulence from the time he entered the canyon up to the accident. The pilot began a "full power climb" at 75 knots in an attempt to clear the ridgeline at the end of the canyon. When he realized that he would not clear the terrain, he began a steep left turn. During the turn, the stall warning was activated and the pilot lowered the nose and decreased the bank angle. The airplane struck trees on the top of the canyon edge, tearing off the left wing with the airplane striking the ground in a level attitude. He stated that, "The engine was working properly and all the gauges were in the green." According to the San Francisco sectional chart, the Mammoth Pool Reservoir, which is 55 nautical miles southwest of the Mammoth Yosemite airport on a magnetic bearing of 213 degrees, is under the 023-degree radial of the Friant VOR. The pilot stated that when he saw the reservoir, he turned east toward the Mammoth Yosemite airport. The magnetic course from the reservoir to the airport was 033 degrees, or a northeasterly direction. There was a canyon leading northeast from the Mammoth Pool Reservoir along the Friant 023-degree radial, which is also a Low Altitude Federal Airway (Victor 230). The canyon terminated at Mammoth Lakes, which is 11 miles due west of the Mammoth Yosemite airport. The sectional chart indicated that no mountain pass was present in the area. The closest mountain passes depicted were the Tioga Pass, 25 miles north-northwest, and the Piute Pass, 25 miles south east of the accident site, respectively. A review of the pilot's operating handbook (POH) for this aircraft was conducted. At 10,000 feet msl, with an outside air temperature of 20 degrees Celsius, the published best rate of climb (Vy) speed was 76 knots indicated airspeed (IAS), which would result in a climb rate of 320 feet per minute. The temperature at the Mammoth Yosemite airport was 24 degrees Celsius at the time of the accident.