In his written statement the pilot said that after departure on runway 25 he turned in a southwest direction to cross a ridge line at an altitude of about 7,900 feet MSL. He said that near the ridge line they encountered a sudden down draft and he could not arrest the descent prior to colliding with trees. An eye witness to the crash said the aircraft never achieved more than about 300 feet AGL at any time after takeoff. The witness observed the aircraft in a nose high attitude begin a right turn, then it descended below the tops of some trees and out of view. Two other witnesses reported hearing the sounds of a loud and strong engine until the sounds of tree impacts were heard. The airport is in a mountain valley at an elevation of 6,750 feet. Mountain ridges and peaks rise an average of 2,000 feet above the valley floor on all sides of the airport. The airport is at the east end of the valley and a large lake. A dam is at the west end of the valley at a 'V' notch in the surrounding mountains. West of the dam the terrain drops off rapidly to the greater Los Angeles basin. The density altitude was calculated to be about 9,400 feet. Other pilots who departed from the airport reported that turbulence existed over the valley near the surrounding ridges.