The pilot was engaged in a stream sampling operation. He reported that he was flying at an airspeed of about 20 knots at about 100 feet above the ground when sudden sink was encountered. Power was increased and the descent was arrested, but the main rotor blades struck a dead branch protruding from a tree. The low rotor rpm warning sounded, and the pilot reduced the power and autorotated to a clear area for a safe touchdown. The sink had been experienced in gusty crosswind conditions, while the pilot had been operating at a relatively low power setting. Post flight inspection revealed that the tail rotor had also received damage, probably caused by debris from the branch struck by the main rotor blades.
