The airline transport certificated pilot flew a tailwheel-equipped airplane to a remote gravel bar to pick up a friend who was rafting in the area. The area of departure was about 1,000 feet long. The pilot said that during the takeoff roll, the tailwheel struck a large rock. He continued the takeoff and landed at Coldfoot, Alaska, without incident. A postaccident examination of the tailwheel area revealed damage to the tailwheel casting, and damage to fuselage bulkheads and a stringer near the tailwheel attach point.