SOURCE: “Report from Fort Mandan, March 22-April 6, 1805,” in Undaunted Courage: Meriwether Lewis, Thomas Jefferson, and the Opening of the American West, Simon & Schuster, 1996, pp. 202-10.

In the following essay, Ambrose surveys the almost book-length report Lewis and Clark issued from Fort Mandan, providing a summary of the maps, correspondence, and descriptions of waterways, plants, minerals, and wildlife that the captains included in their final report.

New life was stirring. On the first day of spring, it rained—the first rain since fall. The river ice began to break up. Ducks, swans, and geese sometimes seemed to fill the sky. The Indians set fire to the dry grass to encourage new grass to come up, for the benefit of their horses and to attract the buffalo.

By the end of March, the ice was coming down in great chunks, along with drowned buffalo who had been on the ice...