On July 12, 1998, at 0820 hours Pacific daylight time, a Piper PA-18-150, N6248R, impacted terrain and came to rest inverted after experiencing a loss of engine power during cruise near Hyampom, California. The aircraft, operated by the pilot/owner under 14 CFR Par 91, sustained substantial damage. The private pilot was not injured. Visual meteorological conditions existed for the personal flight and no flight plan was filed. The flight originated at 0800. The pilot reported that he had just taken off from the accident airport and made a climbout over mountainous terrain. After passing over the terrain, he reduced power, and was executing descending "S" turns into an open valley. At 1,800 feet above ground level, he engaged the throttle to increase power, but there was no corresponding power increase. The pilot declared an emergency and chose a narrow gravel bar next to a river. He stated that due to the short length of the landing area, he was attempting to touchdown at the waters edge and roll out on the gravel bar. The aircraft touched down approximately 30 feet short and the main landing gear skimmed over the water and up onto the gravel bar edge. The left main landing gear sunk into the soft gravel at the water's edge and the aircraft came to rest inverted on the intended landing area. In an interview with a Federal Aviation Administration (FAA) inspector from the Sacramento, California, Flight Standards District Office, the pilot said he landed on a sand bar in a river. He reported that the right main landing gear touched down on a sand bar and the left main landing gear touched down in the water. The throttle cable came off of the carburetor throttle arm, and the bolt, nut, and cotter pin were missing from the throttle cable. In the pilot's written report, an annual inspection had been performed on the airplane approximately 11 days prior to the accident and had flown for 3 hours. The FAA was unable to verify that the bolt, nut, and cotter pin were missing from the throttle cable, as the pilot had already repaired and ferried the aircraft to another location. The FAA reported that the pilot did not have a ferry permit.