But that's because the region was wetter than it had been just four years earlier, thus the bison and deer and other animals needed for the explorers to survive were more available than they would have been during a drought. Even at that, the expedition had to struggle to come up with enough food.
Food became so scarce at times that they had to eat some of their horses, and even dogs belonging to Native Americans. Their diet "alternated between periods of feasting and starvation," historian R. D. Burroughs contends.
The expedition's log notes that in September of 1805 "the men are becoming lean and debilitated, on account of the scarcity and poor quality of provisions on which we exist." Some were clearly in the early stages of starvation.
Knapp stops short of saying the expedition would have failed if it had been attempted during a drought, but he has no doubt that the weather played a key role in its success.
What would have happened if they had turned back, or encountered conditions that were hopelessly inhospitable? They might have reported back to the president that it wasn't worth it. Literally, don't go there.
And if they had starved to death because the green hills that attracted wildlife had remained brown and lifeless? Would the westward expansion have continued?
Maybe, but probably at a different pace. The Lewis and Clark journey is widely considered the most important expedition in the history of the nation, because it did help set the country on a new expansionist course.
If they had failed, perhaps the United States would have been less fierce in claiming those lands, and more of us would be speaking Spanish today.
Lewis and Clark succeeded, forever carving their names in the history books. Sometimes, you just can't beat being lucky.
Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.