The private pilot was taking off from an off-airport site, ferrying supplies from a hunting camp under Title 14, CFR Part 91. He reported that the takeoff was unremarkable until he started the initial climb. He said the airplane would not accelerate or climb, and descended into willows off the end of the runway. The pilot said that when he examined the airplane after the accident, he found a blue tarp behind the wreckage. He said the tarp had been placed on low brush to mark the departure end of the takeoff area, and he believed that the tarp must have snagged on the tailwheel during takeoff, and kept the airplane from climbing.