A Cessna 182L experienced a loss of power approximately 1,200 feet above sea level, while 2 miles from its destination airport. The pilot had just applied carburetor heat while entering the base leg of the airport traffic pattern when the loss of power occurred. The pilot attempted to restore power but was unable. After determining that he would not be able to glide to the runway, he ditched the airplane in a river. Post accident examination of the airframe revealed that the wing was substantially damaged. Post accident examination of the engine revealed nesting material and the remains of a rodent in the inlet of the carburetor.