The Mississippi-Missouri Basin

Course changes

Over geologic time, the Mississippi River has experienced numerous large and small changes to its main course, as well as additions, deletions, and other changes among its numerous tributaries, and the lower Mississippi River has used different pathways as its main channel to the Gulf of Mexico across the delta region.

As Pangaea began to break up about 95 million years ago, North America passed over a volcanic "hotspot" in the Earth's mantle (specifically, the Bermuda hotspot) that was undergoing a period of intense activity. The upwelling of magma from the hotspot forced the further uplift to a height of perhaps 2–3 km of part of the Appalachian-Ouachita range, forming an arch that blocked southbound water flows.

Cultural geography

The Mississippi River runs through or along 10 states, from Minnesota to Louisiana, and is used to define portions of these states' borders, with Wisconsin, Illinois, Kentucky, Tennessee, and Mississippi along the east side of the river, and Iowa, Missouri, and Arkansas along its west side. Substantial parts of both Minnesota and Louisiana are on either side of the river, although the Mississippi defines part of the boundary of each of these states.

In all of these cases, the middle of the riverbed at the time the borders were established was used as the line to define the borders between adjacent states.

History

Approximately 50,000 years ago, the Central United States was covered by an inland sea, which was drained by the Mississippi and its tributaries into the Gulf of Mexico—creating large floodplains and extending the continent further to the south in the process. The soil in areas such as Louisiana was thereafter found to be very rich.

The area of the Mississippi River basin was first settled by hunting and gathering Native American peoples and is considered one of the few independent centers of plant domestication in human history. Evidence of early cultivation of sunflower, a goosefoot, a marsh elder and an indigenous squash dates to the 4th millennium BC.

Ecology

The Mississippi basin is home to a highly diverse aquatic fauna and has been called the "mother fauna" of North American freshwater.

About 375 fish species are known from the Mississippi basin, far exceeding other North Hemisphere river basins exclusively within temperate/subtropical regions, except the Yangtze. Within the Mississippi basin, streams that have their source in the Appalachian and Ozark highlands contain especially many species. Among the fish species in the basin are numerous endemics, as well as relicts such as paddlefish, sturgeon, gar and bowfin.

Because of its size and high species diversity, the Mississippi basin is often divided into subregions.

Course

The longest source stream begins near Brower's Spring in southwest Montana, 9,100 feet (2,800 m) above sea level on the southeastern slopes of Mount Jefferson in the Centennial Mountains. From there it flows west then north; runs first in Hell Roaring Creek then west into the Red Rock; swings northeast to become the Beaverhead River; and finally joins with the Big Hole to form the Jefferson River.

The Firehole River, which originates in northwest Wyoming at Yellowstone National Park's Madison Lake, joins with the Gibbon River to form the Madison River.

The Gallatin River flows out of Gallatin Lake which is also in Yellowstone National Park.

Ecology

Historically, the thousands of square miles that comprised the floodplain of the Missouri River supported a wide range of plant and animal species. Biodiversity generally increased proceeding downstream from the cold, subalpine headwaters in Montana to the temperate, moist climate of Missouri. Today, the river's riparian zone consists primarily of cottonwoods, willows and sycamores, with several other types of trees such as maple and ash. Average tree height generally increases farther from the riverbanks for a limited distance, as land next to the river is vulnerable to soil erosion during floods. Because of its large sediment concentrations, the Missouri does not support many aquatic invertebrates.

Geology

The Rocky Mountains of southwestern Montana at the headwaters of the Missouri River first rose in the Laramide Orogeny, a mountain-building episode that occurred from around 70 to 45 million years ago (the end of the Mesozoic through the early Cenozoic). This orogeny uplifted Cretaceous rocks along the western side of the Western Interior Seaway, a vast shallow sea that stretched from the Arctic Ocean to the Gulf of Mexico, and deposited the sediments that now underlie much of the drainage basin of the Missouri River.

This Laramide uplift caused the sea to retreat and laid the framework for a vast drainage system of rivers flowing from the Rocky and Appalachian Mountains, the predecessor of the modern-day Mississippi watershed.

Tourism and recreation

With over 1,500 mi2 (3,900 km2) of open water, the six reservoirs of the Missouri River Mainstem System provide some of the main recreational areas within the basin. Visitation has increased from 10 million visitor-hours in the mid-1960s to over 60 million visitor-hours in 1990. Development of visitor facilities was spurred by the Federal Water Project Recreation Act of 1965, which required the USACE to build and maintain boat ramps, campgrounds and other public facilities along major reservoirs. Recreational use of Missouri River reservoirs is estimated to contribute $85–100 million to the regional economy each year.