On July 25, 1999, approximately 0630 Pacific daylight time, a Piper PA-18A-150, N3992P, was damaged during a hard landing on rough terrain about 15 miles northwest of Jordan Valley, Oregon. The private pilot and his passenger were not injured. The aircraft, which was owned and operated by a friend of the pilot, sustained substantial damage. The 14 CFR Part 91 personal pleasure flight departed a private airstrip located near Rome, Oregon, about 30 minutes prior to the accident. No flight plan had been filed and there was no report of an ELT activation. The pilot stated that, while cruising at approximately 100 feet above the ground (agl), power was applied to out-climb rising terrain. Just after the power application, the engine started to run rough, so the pilot applied carburetor heat. The application of carburetor heat resulted in a further loss of power, so the pilot returned the carburetor heat to the off position. He then used a combination of on-off-on settings in an attempt to remedy the suspected carburetor icing. Because his actions did not result in the engine returning to full power, he elected to attempt an emergency landing in a nearby clearing. While on final approach to the clearing, and still about 20 feet above the ground, the aircraft stalled/mushed into the terrain. The closest weather information available to the accident site was the Boise Air Terminal, Boise, Idaho, 0656 METAR observation. The temperature was 16 degrees C and the dewpoint was 7 degrees C. According to the DOT/FAA Carburetor Icing Probability Chart, carburetor icing could be expected at glide and cruise power under these temperature/dewpoint conditions.