On May 25, 2006, at 2115 eastern daylight time, an amphibian equipped Maule MT-7-235, N3010Q, sustained substantial damage during landing roll on runway 9 (1,330 feet by 150 feet, grass) at the Dalton Airport (3DA), Flushing, Michigan. The pilot and one passenger were not injured. The 14 Code of Federal Regulations Part 91 personal flight departed Clare Municipal Airport (48D), Clare, Michigan, at 1955. Visual meteorological conditions prevailed at the time of the accident. No flight plan was filed. The pilot reported that he checked the weather at Flint (FNT), Michigan, located about 6 nautical miles to the southeast. The pilot reported that the Automatic Terminal Information System (ATIS) indicated that the winds were from the east at 7 - 9 knots. The pilot elected to land on runway 9. Runway 18/36 at 3DA is an asphalt runway that is 2,510 feet by 50 feet. The pilot reported that the landing was normal until about 150 feet into the landing rollout. The pilot reported that the small castoring wheels on the amphibian floats sank into the soft ground and "caused our plane to pole-vault onto its tail." The pilot reported that the inspection of the runway revealed that there was about 3/4 inch of standing water on the runway. The amphibian airplane was equipped with model BF2750A Baumann floats. The left and right landing gear drag braces and the left and right nose wheel castor housings were sent to the Materials Laboratory at the National Transportation Safety Board for inspection. The metallurgical inspection revealed that the fracture surfaces of the weld between the forward end of the struts and the castor bodies exhibited deformation consistent with overstress with the nose wheel moving aft relative to the strut.