The pilot and two passengers were departing from a remote gravel bar located along a river in the tailwheel-equipped airplane. After loading the airplane to 2,100 pounds, which was 165 pounds over the maximum allowable gross weight for the airplane, the pilot began a westerly takeoff since the prevailing wind above the treeline was coming from the west. Once the airplane cleared the treeline, the wind pushed it to the left, he continued the left turn, and the airspeed began to decrease. In effort to correct for the decreasing airspeed, he pushed forward on the airplane’s control stick and pitched the nose of the airplane down. Realizing he would be unable to regain sufficient airspeed, he reduced the engine power to idle, pulled back on the control stick and impacted the river in a nose high attitude. The airplane sustained substantial damage to the wings and fuselage. The pilot reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.