Many thousands of years ago, a great continental ice sheet advancing from the northeast onto the northern plains blocked or reversed the flow of rivers it encountered, creating the glacial river that persists today as the Missouri. The Missouri, the longest waterway in North America, officially begins at the junction of the Madison, the Jefferson, and the Gallatin in southwestern Montana, then meanders more than 3,100 miles to the Mississippi while draining 335 million acres, roughly one sixth of the contiguous United States. In the 19th century, many travelers regarded the Missouri as superior to the Mississippi, with Charles A. Murray arguing that it was broader, longer, and finer, while the Rev. Timothy Flint called it the longest tributary stream in the world. The Missouri River Basin's history in South Dakota stretches from the recession of the last great ice sheet around 10,000 BC and the earliest archeologically recorded human societies around 9,000 BC, through periods of drought, the rise of nomadic hunting groups, the beginning of horticulture, and the appearance of pottery and burial mounds. Over centuries, the Mandan, Arikaree, Sioux or Dakota, and Cheyenne migrated into and occupied the Missouri River valley and surrounding plains, building earth lodge farming villages, cultivating crops, hunting bison, and shifting settlement patterns. European contact and exploration followed with Pierre Gaultier de Varrennes, Sieur de la Verendrye in 1738, the Verendrye Brothers in 1743, expanding French trade, epidemics among the Arikaree, the Lewis and Clark expedition beginning in 1804, and the development of trading posts including Fort Teton, Fort Tecumseh, and Fort Pierre Chouteau. The river valley then saw military campaigns, treaties, gold-driven transportation, statehood, and finally the construction of Fort Randall, Gavins Point, Oahe, and Big Bend dams, which created a major reservoir system, while later federal designations recognized part of the river as a National Recreation River and the Lewis and Clark route as a National Historic Trail.