The airplane descended into a river during the initial climb from a private airstrip. In a written statement, the pilot reported that prior to departure he performed a practice takeoff run. The mock departure encompassed the entire length of the runway, which he performed in an effort to assess the runway surface condition and airplane's performance. He noted that the terrain was soft with mounds of uneven dirt and sand-like material. He taxied back to the approach end and began the takeoff roll. The airplane's landing gear followed contours in the soft terrain, resulting in a slower acceleration. The pilot further stated that as the airplane began to gain airspeed he realized that he would be unable to abort the takeoff, as brush and low trees were located adjacent to the end of the runway surface. The airplane became airborne and remained in ground effect. The pilot maneuvered the airplane in a right turn in an attempt to avoid the brush and proceed toward the lower terrain near the river. The pilot physically detected a stall buffet and lowered the nose in an effort to arrest the imminent stall. The airplane descended into the river, coming to rest in shallow water adjacent to a sand bar. The pilot reported no preimpact mechanical malfunctions or failures with the airframe or engine. The pilot estimated the density altitude at 7,000 feet.