What obstacles did Lewis and Clark face?

In their epic journey into unexplored America in the first decade of the 19th century, Lewis and Clark faced many serious obstacles to success. Foremost among these were the threat from hostile Native tribes, the severity of winter and summer weather in the wilds, the possibility of injury or illness far from adequate medical care, and of course the simple fact of "the unknown": they did not know how far they would need to travel, nor even if they stood any real chance of success at finding a viable passage to the Pacific Ocean.