The pilot of the tailwheel-equipped airplane departed from a dirt airstrip in mountainous terrain earlier during the day, and was preparing to land at another mountain dirt airstrip. He reported that the wind was about 7 to 10 knots, and that he was flying a stabilized approach during the downwind, base, and final segments. As he initiated the landing flare, the wind changed to a right crosswind, and the right wing lifted. He was unable to keep the airplane on the runway centerline, and the airplane descended down an embankment into an adjacent creek. The airplane came to rest nose down in the creek bed, sustaining substantial damage to the firewall and forward fuselage. The pilot reported no preimpact mechanical malfunctions or failures with the airframe or engine that would have precluded normal operation.