On July 6, 2008, about 1245 mountain daylight time, a Piper PA-28-140, N7360J, was substantially damaged during an off-airport landing about 24 miles northeast of East Carbon, Utah. The commercial pilot and his passenger were seriously injured. The airplane was registered to Sports Flying Inc. of Northwood, North Dakota and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no flight plan was filed for the cross-country flight. The personal flight originated from the Canyonlands Field Airport (CNY), Moab, Utah, about 1145 with an intended destination of Vernal, Utah. In a written statement, the pilot reported he was traveling from Prescott, Arizona, to Alaska for a camping trip with a few intermediate stops planned. Prior to the accident flight, he refueled the airplane with 20 gallons of 100 low-led fuel. After departing CNY, he intended to climb to a cruise altitude of 9,500 feet mean sea level (msl), which he used previously on the flight from Prescott to CNY. The pilot stated that he planned to continue northbound if he was able to climb to an altitude of 6,500 feet msl prior to entering the Green River Canyon. The pilot entered the southern part of the Green River canyon at an altitude of 6,500 feet msl. A few minutes later, he noted that the climb rate started to decay and it was difficult to maintain altitude. The pilot further reported that he was committed to flying through the canyon and it was not possible to execute a 180-degree turn. The airplane started a steady slow rate of descent below the canyon rim. The pilot added that as he was making "fairly steep turns" around the corners within the canyon, each time noting the stall warning light illuminating. As the pilot continued progressing through the canyon, he noted that during each turn, he was losing 50 to 100 feet of altitude and was unable to regain any altitude. At an altitude of approximately 200 feet above ground level, he decided to initiate a precautionary landing in an open area near the Green River. During the landing sequence, the airplane impacted several large rocks before it came to rest upright near the river where a post crash fire ensued. Examination of the airplane by the Carbon County Sheriff Department revealed that a post crash fire consumed the airplane. The fire reportedly ignited a 200-acre wild fire in the area. The pilot reported that at the time of the accident, there were no mechanical anomalies with the airplane or engine. Using the recovered weather observations from a weather recording station located about 41 miles east of the accident site and the accident site elevation of 4,280 feet msl; the NTSB investigator-in-charge calculated the density altitude to be 6,708 feet msl. The pilot reported that at the time of the accident, he estimated the weight of the airplane to be 2,040 pounds. The certified maximum gross weight of the airplane is 2,150 pounds.