In 1856, having accompanied the General W. S. Harney's Sioux Expedition up the Missouri River, 27-year-old Lt. Gouverneur Kemble Warren,1 of the Army's elite Department of Topographical Engineers, was ordered to map the Missouri through Dakota Territory—from the mouth of the Kansas to the mouth of the Milk, and the Yellowstone as far as the Tongue. At Fort Union, on the Missouri opposite the mouth of the Yellowstone, Warren hired the famous mountain man and trader, Jim Bridger (1804–1881), as a guide. He purchased wagons and mules from the notorious British "sportsman" Sir St. George Gore,2 who happened to be at Fort Union, then proceeded by steamboat 100 miles up the Yellowstone, the wagons following the river on the north bank, to a point about ten miles above Glendive's Creek, where the steamboat left supplies and returned to the Missouri River, while Warren and his crew rode mules to the mouth of the Powder River. They floated back down the Yellowstone to Fort Union in bull boats.
Most of the tributaries have already lost the names Clark gave them, and some new ones have been added. Glendive's Creek, which Clark did not record, is thought to be a corruption of the name Glendale, given it by Gore, who camped in that vicinity the year before Warren got there. O'Fallon's Creek, previously Oak-tar-pon-er, Oahtaroup, or Coal, Creek (July 31, 1805), was renamed by an unknown person for the trader, Indian agent, and nephew of William Clark, Benjamin O'Fallon (1793–1842). "Braseau's Houses" was a trading post established by John Brazeau, a clerk with the American Fur Company, in 1833.
Two important elements have been added. One is the location of each of the major rapids mentioned by Clark in the 1814 edition of the journals, but not shown on his 1814 map. The other is a sequence of topographic elevations—which Clark could not have reckoned at all—that are reasonably close to those determined by 20th-century means. At the mouth of the Powder River, for instance, Warren's elevation of 2360 feet above mean sea level is 2234 ft MSL on modern maps.
1. He was not a lieutenant governor. He was a lieutenant whose given name was Gouverneur.
2. See Dave Walter, "The Unsaintly Sir St. George Gore, Slop Hunter Extraordinaire," in Dave Walter, ed., Speaking Ill of the Dead: Jerks in Montana History (Guilford, Connecticut: Globe Pequot Press, 2000), pp. 7–23, Roberta Carkeek Cheney, Names on the Face of Montana: The Story of Montana's Place Names (rev. ed., Missoula, Montana: Mountain Press, 1984), p. 120. Gore didn't quite single-handedly eliminate the vast numbers of bison, bear, antelope and other wildlife that had so impressed Lewis and Clark, but he definitely put a dent in it. And the memory lingers on in the misspelled name of a creek and a town.