On April 13, 1996, at 2210 hours mountain standard time, a Maule, M-5-235C, N9177E, owned and operated by the pilot, experienced a partial loss of engine power during cruise flight. The pilot made a forced landing on a river bank near Lake Havasu City, Arizona. The float equipped airplane was substantially damaged. Neither the commercial pilot nor the two passengers were injured. Visual meteorological conditions prevailed during the dark nighttime personal flight, and no flight plan was filed. The flight originated from the Colorado River, near Bullhead, Arizona, at 2150. According to the pilot, when he was about 12 minutes from reaching Lake Havasu, his airplane's engine began surging, and then gradually lost power. The pilot planned to make a forced landing on the bank of the river. However, upon touchdown the pilot discovered that he had landed so close to the shoreline that the airplane slid on a grass bank and then impacted a 3-foot-high tree trunk. The airplane was subsequently examined by the Federal Aviation Administration and by an independent airplane mechanic. The following observations were made in verbal and written inspection reports: (1) Sediment was found in the fuel filter and carburetor; (2) The number 3 cylinder intake valve was found open and stretched in excess of 0.200 inches; (3) No compression was obtained from the number 3 cylinder; (4) The engine oil was black in color; (5) The magneto's drive end seal bearing was found leaking oil into the magneto; (6) The left set of points were oil fouled, and the right set of points had oil on the contacts and were coked up; and (7) The left distributor gear was missing a gear tooth.