The pilot reported that he was departing from a soft, off-airport site, and just after takeoff, the airplane settled back to the runway. During the touchdown, the left wheel hit a soft spot, and the airplane veered to the left and into a creek bed where it collided with terrain. Upon exiting the airplane, the pilot noticed flames in the engine compartment. Unable to extinguish the flames, the airplane was consumed by a postcrash fire. The pilot reported that there were no preaccident mechanical anomalies with the airplane.