The pilot stated that he was conducting firefighting operations in a helicopter equipped with a 50-foot long-line and a 120-gallon Bambi bucket. Earlier in the day he performed 29 water pickups and returned to a nearby helibase to refuel. The pilot went back to the lake where he had performed the previous fills and completed 13 water pickups noting that the wind was increasing. He went to perform the 14th water pickup and entered a left downwind approach to the lake as he had done all day. On final approach, the helicopter encountered a wind shift, which resulted in a rapid rate of descent. The pilot attempted to arrest the descent by raising the collective which led to a settling with power event. The bucket impacted the water that resulted in a dynamic rollover and the helicopter nosed-in to the lake. The pilot stated that there were no pre impact mechanical failures or malfunctions that would have precluded normal operation.