On September 3, 2019, about 1315 Alaska daylight time, an amphibious Lake LA-4-200 airplane, N6672L, sustained substantial damage following a runway excursion at the Girdwood Airport (AQY), Girdwood, Alaska. The private pilot was not injured. The airplane was registered to, and operated by, a private individual, under the provisions of 14 Code of Federal Regulations Part 91 when the accident occurred. Visual meteorological conditions prevailed and no flight plan had been filed. The flight departed Merrill Field (MRI), Anchorage, Alaska at 1230 destined for AQY. According to the pilot, after departing PAMR the landing gear and flaps failed to retract, so he verified that the hydraulic pump circuit breaker had not opened and noted that the hydraulic pressure gauge indicated zero. He utilized the emergency hydraulic hand pump and was able to successfully retract both the landing gear and wing flaps. Upon arrival at AQY, the hand pump was once again utilized to extend both the landing gear and wing flaps. After touchdown, in an effort to slow and steer the airplane, the wheel brakes were applied but did not work properly. The airplane exited the runway, proceeded down an embankment and came to rest on its left wing and hull, sustaining substantial damage to the left wing and fuselage. According to the Lake LA-4 Owner's Manual, the airplane is equipped with a castering nose wheel, which requires differential braking for directional control. The hydraulic system is used to operate the landing gear, flaps, and longitudinal trim, and is composed of an integral pump and electric motor, pressure limit switch, accumulator, and reservoir, interconnected by necessary piping, check valves, and restrictors. The brake system is independent of the hydraulic system. An examination of the brake and primary flight control systems revealed no preaccident mechanical malfunctions or anomalies that would have precluded normal operation. The closest official weather observation station to the accident site was Portage Glacier (PATO), Whittier, Alaska, located about 20 miles southeast of the accident site. At 1253, a METAR was reporting, in part, wind, variable at 6 knots; visibility, 10 statute miles; clouds and sky condition, overcast clouds at 5,000 ft; temperature, 55°F; dew point, 52°F; and an altimeter setting of 30.08 inches of mercury.