The pilot reported that during departure, he proceeded east along the Alsek River and along the south side of the Alsek Glacier while climbing to 6,200 ft. He then decided that the intended route was closed due to low clouds and he initiated a turn to reverse course. Reportedly, a left turn was commenced and "one notch of flaps set." The pilot stated that shortly after that, the engine began to lose power. He reported that the mixture, props, and throttle were advanced, but there was no increase in power. He switched the fuel selector from the right to the left main tank and turned on the boost pump. He stated the plane continued to lose altitude and continuation of the turn was impossible due to terrain. He decided to try and cross over the glacier and land on a solid stretch of ice, but there was insufficient altitude and the plane crashed. A subsequent examination and test run of the engine revealed no discrepancies.