Aircraft Forced to Land in Creek After Engine RPM Decrease (N35681)

Flight Planning and Initial Conditions The pilot reported obtaining a weather briefing before departure, which indicated headwinds along the planned route. Fuel endurance was calculated using the pilot operating handbook for cruise flight at a 65% power setting. The intended route was estimated to require four hours and 30 minutes, while the manual indicated the aircraft could operate for five hours and 15 minutes at that power setting, including a 45-minute reserve. During the flight, the headwind proved stronger than anticipated. The pilot diverted southwest in an attempt to increase airspeed, but this required climbing several thousand feet for terrain clearance. Airspeed did not improve, and as the flight approached four hours with an estimated 45 minutes to an hour remaining, the pilot selected another airport as an alternate destination. While en route to the alternate, engine RPM began decreasing at approximately four hours and 25 minutes into the flight. The pilot declared an emergency to air traffic control and prepared for an off-airport landing. Upon exiting the cloud layer, the pilot determined the airport could not be reached and initiated a landing in a creek. During descent, the aircraft struck trees before coming to rest in the creek bed. During wreckage recovery, the wing fuel tanks were found intact, and approximately three gallons of fuel were drained from the fuel system. The engine was later prepared for an engine run, started, and operated without anomalies.