The pilot was departing, uphill, from a rough and uneven, tundra-covered off-airport site, which required a correction for a variable 10 knot left crosswind. The site was situated atop a mountain ridgeline. The pilot reported that during the takeoff roll the airplane hit a series of bumps and ruts, and it became airborne prematurely. He added that as the airplane continued to bounce, he braced himself against the rudder pedals, and he inadvertently depressed the brakes. When the airplane settled to the ground and the wheels touched down, the airplane slowed. In an effort to avoid rising terrain ahead, and in an attempt to gain airspeed, the pilot turned the airplane to the left and downslope. The airplane continued to descend and it subsequently collided with brush-covered terrain, sustaining substantial damage to the wings, fuselage, and empennage. The pilot reported no preimpact mechanical malfunction or failure with the airplane that would have precluded normal operation. The pilot also noted that he failed to use the predeparture checklist, and a postaccident inspection revealed that the propeller pitch control was found to be in a cruise power setting.