On May 1, 2004, approximately 1245 mountain daylight time, a Piper PA-18-105, N5483H, registered to and being operated/flown by a private pilot sustained substantial damage during inadvertent flight into terrain while maneuvering approximately two nautical miles south of Laurel, Montana. Neither the pilot nor the passenger was injured. Visual meteorological conditions prevailed and no flight plan had been filed. The local flight, which was personal, was operated under 14 CFR 91, and originated from Billings, Montana, at 1157. The pilot reported that he and his wife were sightseeing along the Yellowstone River just south of Laurel, Montana, when he spotted what he described as unusual animal tracks on a gravel sand bar adjacent to the river. He applied one notch of flaps and executed a 180 degree course reversal for a "low and slow look." He further reported "...In a moment I was hitting the water and then flipped forward and onto the back of the plane..." (refer to attached NTSB Form 6120.1/2. The pilot reported that there was no mechanical malfunction or failure with the aircraft or its powerplant/systems.