On June 27, 2008, about 1730 Pacific daylight time, a Cessna A185F, N5403E, nosed over on landing touchdown in the Columbia River near Kelso-Longview Airport, Rainier, Oregon. Armstrong Marine, Inc., was operating the amphibious airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The pilot held a private pilot certificate, and received a minor injury. The airplane sustained substantial damage to the wings. The personal flight departed Blue Ribbon Airport, Sequim, Washington, about 1620. Visual meteorological conditions prevailed, and no flight plan had been filed. In a written statement, the pilot reported that he was not familiar with the river in which he intended to land. After flying over the landing spot, he made a normal approach and performed a pre-landing check of fuel, propeller, mixture, and landing gear selector. He landed at 60 knots, and the airplane immediately became inverted. He exited the airplane through a window. Post accident investigation determined that the landing gear was extended, but the gear selector was in the retracted position. The pilot believes that a hydraulic leak in the left float prevented the landing gear from retracting. A National Transportation Safety Board investigator examined the airplane on July 17, 2008, at the pilot's hangar in Sequim. He noted no anomalies with the right float. The investigator observed hydraulic fluid within the number seven and eight bulkheads of the left float. Both hydraulic lines for extending from the left float to the fuselage were removed at the fuselage attach point. Compressed air was applied to one of the lines and hydraulic fluid was observed leaking from the tension actuator. The tension actuator was secure to both the forward and aft mounts. The actuator was removed from the left float. During removal, the gasket surrounding the actuator was cut to facilitate removal. The internal structure of the bulkhead where the gasket was located exhibited a slight tear/crack that originated from the gasket mount. The hydraulic accumulator remained attached to the engine firewall and was undamaged. The hydraulic fluid quantity sight gauge showed no sustainable level of fluid. On September 22, 2008, at the facilities of Kenmore Air, Kenmore, Washington, the Safety Board investigator supervised examination of the left float tension actuator. Both the forward and aft pins were intact and safety wired. The actuator was installed on a test rig, and technicians applied hydraulic pressure to the forward hydraulic port. There was no evidence of leaks when the technicians applied 1,000 pounds per square inch (psi) of hydraulic pressure. When hydraulic pressure was applied to the aft hydraulic port, there was extensive leaking of hydraulic fluid around the rear seal. The securing pin safety wire was cut and removed. Multiple attempts to remove the securing pin using a punch and hammer were unsuccessful. The investigator observed corrosion around and on the aft securing pin.