The Mississippi Alluvial Valley (Region 21*) is the historic floodplain of the Mississippi River formed by melt water as glaciers receded approximately 12,000 years ago. The MAV is approximately 800 km long and covers portions of 7 states from southern Illinois to Louisiana.
The most recent climax plant community in the MAV consisted of approximately 10 million ha of bottomland hardwood forest dominated by hard and soft mast-producing trees including several species of oak (e.g., Nuttall, overcup, willow, water, etc.), hackberry, and green ash. Over 70 species of trees occur in the region. Elevation interacts with hydrology, especially the frequency, duration, and periodicity of flooding, to determine plant community composition and species distribution (Fredrickson 1978, Larson et al. 1981, Reinecke et al. 1989). For example, cherrybark and willow oaks occur on higher, less flood-prone sites, while overcup oak occurs on low sites that flood frequently and for long duration. Cypress and tupelo dominate permanently flooded sloughs (Reinecke et al. 1989).
Flooding in the MAV historically was driven by winter and spring precipitation. Winter rains in combination with greatly reduced plant evapotranspiration resulting from winter dormancy, typically resulted in winter flooding that made mast and other foods available to migrating and wintering waterfowl. Annual variation in duration and extent of flooding likely was great, but inundation of much of the MAV probably occurred each winter (Heitmeyer and Fredrickson 1981).
The landscape in the MAV has changed dramatically during the last 200 years, with the most rapid change occurring within the last 75 years. Today, only about 2 million ha or 20% of the original forest remains in the MAV, the rest having been converted to agricultural production. Initially higher elevation areas were cleared and placed into agriculture. However, even these relatively high sites were prone to flooding, which led to attempts to more or less successfully control hydrological events that sustained and were the very basis for development of this system. Flood control projects have reduced the extent of flooding in some parts of the MAV by as much as 88%, while simultaneously altering the ecologically important effects of flood periodicity, duration, and frequency (Reinecke et al. 1989).
* Region 21 - NABCI Bird Conservation Region 26