LARAMIE - Meriwether Lewis and William Clark changed the course of American history when they steered their boats up the Missouri River instead of the Yellowstone, according to an expert on the Lewis and Clark expedition and its geography.

John Logan Allen, Lewis and Clark scholar and chairman of the University of Wyoming Geography Department, presented his "alternate history" of the expedition in a lecture on the UW campus Friday.

Allen's theory is that President Thomas Jefferson wanted the explorers to avoid the northern reaches of the Missouri because they would pass too close to the 49th parallel, established by the Treaty of Paris as the southern limit of British dominance in North America.

The explorers, he argued, took a "wrong turn" when they headed up the Missouri on the advice of Mandan and Hidatsa Indians with whom they spent the winter of 1804-05. Jefferson, he believes, would have wanted them to take the Yellowstone, regarding it as the southern branch of the Missouri.

Had they done so, he argued, they would have found a relatively easy passage from the Yellowstone's headwaters to the Snake River and thence to the Columbia, avoiding a nearly month-long portage around the Great Falls of the Missouri and an arduous crossing of the Bitterroot Mountains. They would have had to portage around the Grand Canyon of the Yellowstone, but it would not have taken them nearly as long as it did around the Great Falls, he said.

After reaching the Snake, they would have followed it downstream until it became too treacherous for their canoes after passing through Hells Canyon, and then crossed the Blue Mountains to the Columbia by the easy route followed today by Interstate 84.

On the return trip, Lewis would have retraced their Missouri River route, as he did, but Clark, after going up the Snake to Jackson Hole, would have followed the Green River, discovered South Pass and then gone down to the Missouri on the Platte.

Whereas the expedition actually failed in its effort to find an easy water passage to the Pacific and "Lewis and Clark were forgotten persons," his scenario would have provided such a passage and made them instant heroes, the geographer said. Americans would have quickly controlled the fur trade in the West, driving the British and Spanish from the field.

The transcontinental railroad would have been built in the 1840s, years ahead of its actual construction, with St. Louis as its eastern terminus. Casper, Boise and other cities in the fur trade area would have become major metropolises, with Portland and Seattle coming along later as less important cities.

Lewis would have gone on to a successful career and led further explorations in the West, instead of committing suicide in depression while struggling with a political post for which he was ill-suited.

"It did not, of course, happen that way, but it could have, except for the actuality of a wrong turn," said Allen, whose book, "Passage through the Garden," is an honored study of Lewis and Clark and the Western landscape.