What happened
The pilot reported that he was conducting a landing operation at a remote gravel bar. Upon touchdown, the aircraft traveled further down the gravel surface than originally intended. As the airplane continued forward, the pilot identified a depression in the terrain directly ahead of the flight path.
In an effort to avoid the depression, the pilot attempted to initiate a takeoff. However, the rough and uneven terrain caused the airframe to bounce or jolt violently. This impact knocked the pilot's hand off the throttle control. With the throttle no longer being managed by the pilot, the airplane lost forward momentum and entered the depression. The main landing gear dug into the soft ground, causing the aircraft to nosed over.
The investigation
The investigation focused on the sequence of events leading to the nosedown impact. Examination of the site confirmed that the terrain was rough and contained depressions consistent with the pilot's description. No mechanical failures were reported prior to the loss of control during the takeoff attempt.