On August 12, 2001, at 1905 hours Pacific daylight time, a Douglas DC-3C, N376AS, encountered turbulence during cruise flight over mountainous terrain following an aerial deployment of smoke jumpers near Alturas, California. The airplane was not damaged. Neither the two airline transport pilots nor the third crewmember was injured. The fourth crewmember was seriously injured. Visual meteorological conditions prevailed for the flight operated by Rhoades Aviation Inc. under a United States Forest Service contract. The public-use flight was performed under 14 CFR Part 91. The airplane departed Redding, California, at 1730, and was returning to Redding when the accident occurred. A company flight plan was filed. The operator stated that their mission was to drop smoke jumpers on the Dusenbury fire incident in the South Warner Wilderness area of the Modoc National Forest. The air was smooth as the airplane descended over the fire. The 9 smoke jumpers were let out over the fire at 8,500 feet mean sea level (msl) in five passes, followed by four more passes to drop cargo. When the jump operations were completed, 360-degree circuits were made to gain altitude for the flight back to Redding. The "fasten seatbelt" sign was turned off for jump operations and cabin securing. There were two cabin crewmembers (spotters) on board who were securing the cabin for their return flight to Redding. As the airplane crossed over a ridge line at 9,000 feet msl (800 feet above ground level), it encountered a sudden wind shear, or downdraft, but then immediately back into smooth air. One spotter hit his helmet on the ceiling, but was not injured. The second spotter, who was in mid-cabin reported, "I just broke my leg." The pilot diverted the flight to Alturas. No other turbulence was encountered prior to or after the accident event. Following the accident, the pilot filed a "Safecon Aviation Safety Communique" through the U.S. Forest Service aviation management website. Additionally, the pilot made the following recommendations to prevent future accidents of this nature: 1) Re-emphasize to flight crews the importance of cabin safety awareness. 2) Re-emphasize to flight crews the potential for hidden turbulence over mountainous terrain. 3) Emphasize the importance of remaining in stable air while cabin personnel are securing cargo and moving about the cabin at the completion of drop missions. 4) Ensure all personnel aboard the aircraft are seated with seatbelts fastened whenever turbulence is anticipated.