The Lewis and Clark expedition, led by Meriwether Lewis and William Clark, was ordered by President Thomas Jefferson to explore the region now in the northwestern United States. Lewis and Clark traveled from St. Louis, up the Missouri River, across the Rocky Mountains and eventually down the Snake and Columbia Rivers to the Pacific. Immediately before the journey, Congress approved the Louisiana Purchase, which meant that the expedition was exploring new United States territory, at least until the reached the Rockies. At that point, they entered territory disputed by the United States, Spain, Great Britain, and Russia, and of course occupied by a number of Indian peoples. Along the way, Lewis and Clark documented their journey, including descriptions of the territory they crossed, the animals they saw, and their encounters with Native Americans. The journey had several implications for westward expansion. On the one hand, it established almost beyond a shadow of a doubt that no Northwest Passage existed, meaning that there was no water route to the Pacific Ocean. It also staked a more serious American claim to the Pacific Northwest, as Americans were the only ones to have explored the region's interior (though a handful of British expeditions from Canada would follow shortly thereafter.) The trip did blaze a trail to the Northwest, and when people began to settle in the region that would later become Oregon Territory, they took a path that followed that of Lewis and Clark very closely.