The pilot reported that he had landed on a creek bed, and unloaded the passenger and supplies. He said he was taxiing for takeoff en route to retrieve another load of supplies, when his right wing struck a tree. He said the airplane spun sharply to the right, the left main landing gear collapsed, and the left wing struck the ground. The pilot said he was probably taxiing too fast for the terrain conditions, and he had a tailwind. The left wing spar and landing gear of the airplane were damaged.