According to the pilot, he took off from the airport in his amphibious experimental airplane with the intention of flying a closed traffic pattern. Because he planned to land back on the runway, he left the wheels in the down position. During departure, he overflew the Willamette River, located about 3 miles north of the airport, and he observed that the water was calm. The pilot stated that he made a spur of the moment decision to land in the river, and he forgot that the wheels were extended. The airplane is not equipped with an aural gear warning horn or gear position lights; however, it is equipped with mirrors on each wing in which he could have visually determined the position of the gear. The pilot stated that he did not check the position of the gear with the mirrors. He indicated that he had forgotten to raise the gear prior to landing in the water. Consequently, the airplane nosed over as it decelerated. During this event, a portion of the lower engine cowl broke, and most of the front windscreen separated from the cockpit. The pilot did not report having experienced any malfunction with the airplane.