BEFORE TAKING OFF FROM AN UNNAMED LAKE, THE PLT NOTED FUEL HAD SIPHONED FROM THE FUEL TANKS, BUT HE THOUGHT HE HAD ENOUGH FUEL TO GET TO PORT ALSWORTH, AK. EN ROUTE, THE ENG LOST POWER & A FORCE LANDING WAS MADE ON A SWIFT MOVING RIVER. AFTER LANDING, THE RIVER SWEPT THE FLOATPLANE REARWARED INTO AN OVER HANGING TREE, WHICH CAUSED IT TO NOSE OVER. A PERSON AT THE DEPARTURE POINT STATED THAT JUST PRIOR TO THE FLT, HE SAW FUEL DRIPPING FROM THE WING AND A SLICK ON THE WATER.