Lewis and Clark carried some instruments for calculating distances–a chronometer, or clock, a surveyor's compass, and an instrument for determining latitude and longitude. Odometers or "trip meters" were unknown to them. Instead, as experienced soldiers and frontiersmen they had a strong sense of pace, time, and distance that is impossible for most modern civilian travelers to replicate, much less appreciate. So, how far did they travel, really?
When Clark arrived at the mouth of the Columbia River on November 16, 1805, he calculated that he had covered 4,142 miles from the mouth of the Missouri River. The National Park Service, which oversees the Lewis and Clark National Historic Trail, states that it is actually about 3,700 miles long. A leading student of the expedition has calculated that Clark's estimate was about 15 percent longer than actual. Why the discrepancies?
One answer is that while traveling on water, or when afoot in rough terrain such as the Bitterroot Mountains, Clark tended to overestimate distances by as much as one-third. Only when he was on horseback were his mileage figures more accurate. Nevertheless, his feat of measuring more than half a continent was an almost Herculean task, and one can't imagine anyone doing as well today, given the same circumstances.
But what would the final "correct" figure mean, after all? In general, measured distance and felt distance can be miles apart, according to the character of the water, land or air covered, the means of travel, the weather, the time available, and even the health and attitude of the traveler. We've all experienced this to one extent or another. According to the latest Rand-McNally road atlas, it's exactly 2,151 real miles from St. Louis to Fort Clatsop via state and interstate highways. By jet plane, nonstop to Portland, then by commuter plane to Astoria, it's only 1,800 air miles. But given an unforseen delay, combined with some personal reason for urgency, the felt distance takes on a new dimension.
There's another factor that prevents us from saying how far the expedition "really" travelled, and that is that rivers are, by their nature, dynamic. Their lengths change from season to season, and from year to year. Left untrammeled by levees, dikes, or dams, they are continually bending and straightening, coiling and uncoiling, never pausing for our rulers, never posing for their portraits.
For example, in 1805 Clark figured the distance from the mouth of the Missouri to the mouth of the Knife River, near Fort Mandan, at 1,616 miles. In 1890 the U.S. Corps of Engineers calculated it was then just 1,514 miles between the same two points. In 1941, measured along the thalweg, or deepest part of the riverbed, the distance appeared to be 1,441 miles. In 1960, with only two dams in place, it was 1,376 miles. Yet nearby St. Louis, Missouri and nearby Bismarck, North Dakota haven't moved an inch.
To look at it another way, if the Lewis and Clark expedition could be repeated today under precisely the same travel conditions, including the same equipment and corresponding delays enroute, it would take 22 days less than it took them in 1804.