Since the time of Columbus, explorers dreamed of a water passage across the North American continent. President Thomas Jefferson shared this dream. He conceived the Corps of Discovery to travel up the Missouri River to the Rocky Mountains and westward along possible river routes to the Pacific Ocean. Meriwether Lewis and William Clark led this expedition of 1804-6. Along the way they filled hundreds of notebook pages with observations of the geography, Indian tribes, and natural history of the trans-Mississippi West. In April 1805, Lewis and Clark and their party set out from Fort Mandan following the Missouri River westward. This volume recounts their travels through country never before explored by white people. With new personnel including the Shoshone Indian woman Sacagawea, her husband Toussaint Charbonneau, and their baby, nicknamed Pomp, the party spent the rest of the spring and early summer toiling up the Missouri. Along the way they portaged the difficult Great Falls, encountered grizzly bears, cataloged new species of plants and animals, and mapped rivers and streams. Gary E. Moulton is Thomas C. Sorensen Professor of American History at the University of Nebraska and the recipient of the J. Franklin Jameson Award of the American Historical Association for the editing of these journals.