At about 1200 Alaska time, a Piper PA-18-160 struck a frozen river while maneuvering at low altitude about 80 miles northwest of Skwentna. The pilot and passenger were not injured. The pilot said he had been flying about 100 feet above the ground at a reduced power setting. He said he had initiated a 90 degree downwind turn, and hit a sinker, which pushed the airplane toward the ground. He immediately applied full power, but the engine did not respond with maximum power. The pilot said it is possible the engine had accumulated carburetor ice.