The Mandan spoke a Siouan language and, like the Arikara, traded food crops to nomadic hunting Indians, especially the powerful Assiniboin and Cree tribes from Canada.  Lewis and Clark described the1250 remaining Mandan in 1804 as "brave, humane, and hospitable" – "the most friendly Indians" along the Upper Missouri River.  That favorable impression strengthened the belief of many white people that the Mandan were the descendants of ancient Europeans from Wales.

The traditional friendliness of the Mandan, who had traded with the French for seven decades prior to the Louisiana Purchase, convinced the Corps of Discovery to spend the winter of 1804-05 with them.  On Saturday, 27 October, four days after the first snowfall, Lewis and Clark arrived at the Mandan village of Mitutanka, four miles south of where the Knife River flowed into the Missouri.  Located 1600 miles by water from St. Louis, that was the most distant fur-trading site that Missouri's French merchants knew much about.  Two miles from this village and less than four miles from the second town of Rooptahee, the Americans built Fort Mandan out of logs to serve as their winter quarters. 

Waiting out the bitterly cold, five-month winter between November 1804 and April 1805 in their small fortress, Lewis and Clark drew maps and wrote reports describing what the Expedition had discovered up to that time.  But those many months also gave the Americans a rare opportunity to get to know the Mandan better than any other Indians east of the Rocky Mountains.   The Corpsmen welcomed dozens of curious Mandan people into their quarters on a regular basis, sharing food, warm fires, and many stories of hunting, warfare, and other adventures.  On one such occasion, "Doctor" Clark saved the life of a young Mandan boy by amputating his frost-bitten toes to prevent deadly blood poisoning.  American medicines helped cure other Mandan of many diseases.

The Corps of Discovery shared in many village activities and celebrated New Year's Day of 1805 with the Mandan.  York and other Corpsmen entertained the Indians with their dancing, singing, and fiddle playing.  One of the French boatmen, Francois Rivet, even danced upside down on his hands!  When winter temperatures reached 40 degrees below zero, the meat from joint Mandan-American hunts and the Indians' stored corn kept everyone alive.  To show their gratitude for Mandan hospitality, Lewis and Clark gave the tribe a large iron corn mill for grinding grain into meal.  In addition, the Expedition blacksmith, John Shields, made or fixed metal tomahawks and hoes in exchange for Mandan maize.

Especially important were the personal friendships that developed between Mandan and American leaders.  In several parleys and visits, Lewis and Clark got to know Black Cat, "Grand Chief" of the Mandan, and Chief Raven Man from the same village of Rooptahee.  No leader trusted the Americans more than Sheheke, or "Big White," head chief at the village of Mitutanka.  When Lewis and Clark returned to the Mandan on their way home in August 1806, Sheheke and his entire family joined them for the voyage back to St. Louis.  There the chief was given new clothes and presents at Fort Bellefontaine before beginning his journey of discovery to meet President Jefferson in Washington, D.C. 

Many months later, Sheheke finally returned to his North Dakota home, telling his people about the amazing things he had seen in the eastern United States.

On Sunday, 7 April 1805, the Expedition divided into two groups to accomplish different goals.  The large "barge" (keelboat), with 10 men aboard, and a two-man canoe departed for St. Louis.  They carried back reports, maps, Indian objects, and natural history specimens of unusual plants and animals Lewis and Clark wanted President Jefferson to see.  At the same time, the main Corps of Discovery continued the voyage to the Pacific in the two pirogues and six dugout canoes.  The memorable winter with the Mandan was over.