The helicopter was involved in firefighting operations. The pilot flying was in the left seat, but the senior pilot was in the right seat. It had made numerous water drops on the fire during a 2-hour period. The helicopter was making its second drop in an area of steep terrain, which was approximately 1,000 feet lower in elevation than the lake it was using for a water source. Due to smoke, the visibility was about 1/2-mile. All five main rotor blades made contact with a snag (typically the remnants of a tree such as the trunk and/or main branches without most of its foliage); the operator pointed out that the snag was similar in color to the smoke. The senior pilot took control of the helicopter, climbed, and made an uneventful landing near the fill point.