About 2 miles north on Trident Road is the confluence of the Jefferson and Madison Rivers, and a mile farther downstream the Gallatin River merges with them to form the headwaters of the Missouri River, the longest U.S. river at approximately 2500 miles. In 1804, at the bidding of President Jefferson, Lewis and Clark and several men came up the Missouri River from St. Louis, Missouri. In Mandan, now North Dakota, a young Shoshone girl named Sacajawea, her husband Charbonneau, and baby Pomp joined the group. The expedition continued along the Missouri River and arrived at its headwaters in July 1805. For many years Indian tribes traveled through the Missouri headwaters and the Gallatin valley to and from buffalo hunts. A chert mine across the Missouri River at the mouth of the Gallatin River provided material Indians used for making arrows, tools, and for trading with other tribes.