On April 11, 2008, about 1515 Pacific daylight time, a Piper PA-18-150, N6845Z, impacted a river while maneuvering about 6 miles from Concrete, Washington. The airplane nosed over in 4 feet of water and was substantially damaged. The private pilot, who owned the airplane, sustained a minor injury. Visual meteorological conditions prevailed at the time of the personal flight, and no flight plan was filed. The flight was performed under the provisions of 14 Code of Federal Regulations Part 91, and it originated from the Concrete Municipal Airport about 1500. The pilot reported to the National Transportation Safety Board investigator that, earlier in the day, he had performed a series of takeoffs and landings during which no anomalies were noted with his airplane. After stopping for 30 to 45 minutes, the pilot resumed flying. According to the pilot, while flying "low and slow" over a river he experienced a "power failure." The pilot reported that the airplane's engine had recently been overhauled. According to the Federal Aviation Administration (FAA) coordinator who examined the engine, no evidence of any preimpact mechanical malfunction or anomaly was noted during his examination. The pilot also asserted that, during the accident flight, the carburetor may have malfunctioned. The FAA coordinator had the carburetor examined and operationally tested. The carburetor functioned in accordance with specifications, according to its manufacturer. The FAA coordinator reported to the Safety Board investigator that he observed cable binding and/or kinking evidence consistent with a possible restriction of movement of the carburetor heat control. However, the evidence was not definitive regarding whether it would result in restricting movement of the carburetor heat control. Additionally, the pilot did not report an inability to activate the heat control. The FAA coordinator also noted that the airplane had been partially disassembled prior to his examination. Entries were observed in the airplane's maintenance logbook that addressed the removal and reinstallation of the carburetor. The page bearing the entries, as presented to the FAA coordinator and the pilot's insurance company agent, were different. The entries were not signed by any FAA certificated mechanic. The pilot's name was found in logbook entries as having performed the carburetor's installation. The pilot does not hold any FAA mechanic certificate.