Prior to departure, the pilot visually observed approximately 12 gallons of fuel in the left wing tank and 8 gallons in the right fuel tank. His examination of the fuel gauges confirmed the same amount. He performed a run-up inspection, departed from the airport, and after approximately 45 minutes he noticed the engine oil temperature began to rise. Shortly after, the engine rpm decreased to 1800 rpm, and the pilot initiated a descent and looked for a place to land. He proceeded toward a grass landing strip and when the airplane was at an altitude of 500 feet the engine lost power. He realized he would not be able to make it to the landing site, and performed a forced landing to the river ahead of him. The airplane came to rest upright and floated for several minutes, while the occupants egressed and swam to shore. After the airplane was recovered from the river, 10-12 gallons of fuel were drained from the right wing fuel tank. The left fuel tank was empty, and the fuel selector was selected to the left tank.