On August 21, about 0910 Alaska daylight time, a Grumman G-44 amphibious airplane, N139F, sustained substantial damage during a runway excursion and collision with terrain, while landing at the Dillingham Airport, Dillingham, Alaska. The airplane was operated by Fresh Water Adventures, Dillingham, as a visual flight rules (VFR) passenger flight under the provisions of 14 Code of Federal Regulations, Part 135, when the accident occurred. The commercial pilot, and the three passengers, were not injured. Visual meteorological conditions prevailed, and a VFR flight plan was in effect. The flight departed Nerka Lake, Alaska, bound for Dillingham, about 0850. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on August 21, the Dillingham airport manager said when the airplane touched down it immediately veered off the runway to the left, and collided with terrain. He said the pilot told him the left brake locked up, and he lost directional control. The airplane received substantial damage to the left wing and the left aileron. In a written statement dated August 22, the pilot reported that he was unaware that the left wheel brake system had picked up a rock while departing from a beach at Nerka Lake, which resulted in the left brake locking up on landing. In a written statement dated August 28, an FAA air safety inspector, who had examined the airplane at the accident site, confirmed that a small rock was found lodged in the left brake assembly.