US Army Corps of Engineers
St. Louis District

Rivers Project


Environmental Significance of the Mississippi and Illinois River

The Mississippi is one of the world’s great rivers and one of its most complex ecosystems. It is a critical migration corridor for millions of birds and is essential to the ecological health of the North American continent.  The river environment is home to an incredible array of fish, wildlife and plants. In turn, millions of people use and enjoy these diverse resources. The river, its floodplain, and its adjacent upland corridor are essential to the survival and dispersal of a great portion of the vertebrate species (particularly birds, amphibians, and fish) and aquatic invertebrate species that inhabit this continent. The Mississippi more than any other natural feature, is a globally recognized symbol of this nation.

The Mississippi River is the largest and longest river in North America. Its tributaries spread throughout the central United States, comprising a drainage basin encompassing 40 percent of the continental United States—an area totaling 1.9 million square miles. The drainage basin, the fourth largest in the world, is defined on the east by the Appalachian Mountains and on the west by the Rocky Mountains. 

The bottomlands for the entire Mississippi make up the largest wetland area in the United States, and its bottomland hardwood forests are the most extensive in North America. More than half of the wetlands existing at the time of European colonization have been lost. The annual net loss of wetlands from the mid 1950s to the mid 1970s was mostly due to conversion to agriculture. In the following decade, there was a rapid decline in agricultural conversion and between 1982 and 1992, non-agricultural conversion accounted for over half of conversions. Wetland loss has been slowed considerably due to a governmental ‘no net loss’ policy.

More than 47 percent of the nation’s duck population migrates along the river, and one-third of the freshwater fish species in North America live in the river. The river flows through eleven different eco-regions; this accounts for the great biological diversity of the river valley.

Since many of the characteristic birds of the river valley are migratory, the study area is of national and international significance. In addition, the Mississippi River and its tributary valleys form a natural route over which the non-migratory or semi-migratory species may expand their ranges. The river valley forms a wildlife corridor between the Gulf of Mexico and the Great Lakes Region. The mammalian species are generally representative of eastern (Alleghenian) types, with some influence of southern (Carolinian) and northern (Canadian) species.

Plant species in the river valley also enjoy conditions that are not generally associated with the geographic location of the river.  Overlapping of eastern and western species and subspecies of plants as well as animals occurs in the river valley. In disturbed sites without previous growth where species of plants are beginning to grow, known as pioneer sites, found along sandbars, mud flats, and other open places of recent soil disturbances, the usual forest is dominated by black willow and cottonwood. In forested areas on the floodplain, silver maple, cottonwood, elm, hackberry, green ash, oaks, willows and box elder are the usual dominants.