From https://www.history.com/this-day-in-history/lewis-and-clark-depart-fort-mandan

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After a long winter, the Lewis and Clark expedition departs its camp among the Mandan Indians and resumes its journey West along the Missouri River.

The Corps of Discovery had begun its voyage the previous spring, and it arrived at the large Mandan and Minnetaree villages along the upper Missouri River (north of present-day Bismarck, North Dakota) in late October. Once at the villages, Meriwether Lewis and William Clark directed the men to build a sturdy log fort. The following winter was a harsh one, but the expedition had plenty of provisions. The two captains made the best of their enforced halt, making copious notes in their journals and preparing maps of their route. Most importantly, they met frequently with the local Indians, who provided them with valuable information about the mysterious country that lay ahead.

As spring came to the upper Missouri, Lewis and Clark prepared to resume their journey. Lewis penned a long report for President Thomas Jefferson that would be sent back down to St. Louis with 16 men traveling on the expedition’s large keelboat. Although Lewis had yet to explore any truly unknown country, his report provided a good deal of valuable information on the upper Missouri River region and its inhabitants. He optimistically predicted the expedition would be able to reach the Pacific and make a good start on the return journey before the coming winter. “You may therefore expect me to meet you at Monachello

[Monticello] in September 1806,” he told the president.

In fact, the journey was more difficult and slow than Lewis anticipated. The expedition actually spent the winter of 1805-06 along the Pacific Coast, and Lewis did not finally meet with Thomas Jefferson in Washington, D.C., until January 1, 1807. However, as Lewis and Clark prepared to leave Fort Mandan on this day in 1805, they did not know the trials ahead and were likely filled with optimism and excitement. As the keelboat shoved off and started down the Missouri with Lewis’ report to Jefferson, the Corps of Discovery (and their female guide, Sacagawea) resumed the far more difficult task of rowing their small boats upstream.

That night Lewis wrote in his journal that, “Our vessels consisted of six small canoes, and two large pirogues. This little fleet altho’ not quite so rispectable as those of Columbus or Capt. Cook, were still viewed by us with as much pleasure as those deservedly famed adventurers ever beheld theirs.” As Lewis began his journey into a land “on which the foot of civilized man had never trodden,” he proclaimed this day of departure as “among the most happy of my life.”