According to the pilot of the experimental, amateur-built amphibious airplane, he took off from the river behind his home, and proceeded to a local airport where he lowered the landing gear and performed three touch-and-go landings. The pilot said he was preoccupied during the return flight for a water landing and "forgot" to retract the landing gear for the pontoon-hull landing. At touchdown, the airplane "flipped immediately" and came to rest inverted. The airplane's fuselage, forward of the landing gear was fractured. The pilot did not report any preaccident deficiencies with the performance and handling of the airplane.