The pilot reported that he was departing from a rough, 700-foot-long dirt-covered off-airport site with a load of Caribou meat, on a Title 14, CFR Part 91 personal flight. The pilot said that during the takeoff roll the airplane did not become airborne before reaching the end of the site, and the main wheels struck a pool of water at the departure end. The airplane nosed over, resulting in substantial damage to the wings and fuselage. The pilot said there were no preaccident mechanical problems with the airplane, and noted in his report to the NTSB that the accident might have been avoided if he had weighed the meat before takeoff.