On June 9, 2002, approximately 2030 mountain daylight time, a Beech 58P, N123Z, operated by the U. S. Department of Agriculture (USDA) Forest Service and piloted by an airline transport pilot, was substantially damaged when it encountered severe turbulence, approximately 10 miles east of Grand Junction, Colorado. Visual meteorological conditions prevailed at the time of the accident. The aerial observation flight was conducted in support of wildfire fighting efforts near Glenwood Springs, Colorado, as public use under the provisions of Title 14 CFR Part 91. No flight plan was on file. The pilot reported no injuries. The local flight originated at Grand Junction-Walker Field (GJT), Grand Junction, Colorado, about 1715, and was returning to GJT when the accident occurred. According to the USDA Safety Manager, the pilot departed GJT on fire assignment as a Forest Service lead airplane, to the Coal Seam Fire in Garfield County, Colorado, in the vicinity of Glenwood Springs. The airplane was operating by request of, and under the control of the Garfield County Sheriffs Department. The pilot reported experiencing low-level wind shear and light, moderate, and severe turbulence from just after takeoff through arriving at the area of operation. Following a reconnaissance of the fire, the pilot reported climbing to approximately 5,000 feet agl (above ground level) to avoid the turbulence and wait for wind conditions to improve so that tanker operations (aircraft dropping fire suppressant chemicals on the wildfire) could begin. The pilot reported that wind conditions improved and tanker operations commenced. On conclusion of the tanker operations, the pilot returned to GJT. The pilot reported he was in the vicinity of the Book Cliffs, approximately 10 miles northeast of GJT, approximately 2,000 feet agl in a low power descent, and an airspeed of approximately 150-160 knots, when he encountered one significant event of severe turbulence. The pilot reported the winds were out of the southwest at 20 knots with gusts to 29 knots. The airplane continued to GJT and landed uneventfully. The aircraft damage was noted the following morning during a routine preflight inspection. An examination of the airplane showed the top skin of the airplane's left wing, between the fuselage and the left engine nacelle, was buckled. Flight control continuity was confirmed. An examination of the engines, engines controls, and other airplane systems revealed no anomalies. At 1956, the Aviation Routine Weather Report for GJT was clear skies, visibility 9 statute miles, temperature 91 degrees Fahrenheit (F), dew point 14 degrees F, winds 180 degrees at 20 knots with gusts to 28 knots, and an altimeter setting of 29.55 inches of Mercury. At 2056, the winds at GJT were reported as 170 degrees at 16 knots with gusts to 24 knots.