The 2,530-hour private pilot/owner was flying his experimental single-engine, amphibious airplane about 10-feet-high above the Mississippi River when he realized that the landing gear was extended. When the pilot attempted to retract the landing gear, he accidentally pulled the throttle back to the idle position, instead of the manual landing gear release lever. The pilot immediately realized what he had done and pushed the throttle to full power; however, the nose of the airplane had already dropped and the airplane collided with the water while at an airspeed of 70 to 75 miles per hour. Upon impact with the water, the airplane flipped-over. The pilot and his passenger were able to exit the airplane unassisted and swam to shore. The airplane sank in the river and attempts to locate it were unsuccessful. The airplane is still missing and presumed to be destroyed.