Alluvial Plain (Ecoregion
Streams and rivers have very low gradients and fine-grained substrates. Many reaches have ill-defined stream channels. Ecoregion 73 provides important habitat for fish and wildlife, and includes the largest continuous system of wetlands in North America. It is also a major bird migration corridor used in fall and spring migrations. Potential natural vegetation is largely southern floodplain forest and is unlike the oak-hickory and oak-hickory-pine forests that dominate uplands to the west in Ecoregions 35, 36, 37, 38, and 39; loblolly pine, so common in the South Central Plains (35), is not native to most forests in the Arkansas portion of Ecoregion 73. The Mississippi Alluvial Plain (73) has been widely cleared and drained for cultivation; this widespread loss or degradation of forest and wetland habitat has impacted wildlife and reduced bird populations.
Presently, most of the northern and central sections of Ecoregion 73, including Arkansas, are in cropland and receive heavy treatments of insecticides and herbicides; soybeans, cotton, and rice are the major crops, and aquaculture is also important. Agricultural runoff containing fertilizers, herbicides, pesticides, and livestock waste have degraded surficial water quality. Concentrations of total suspended solids, total dissolved solids, total phosphorus, ammonia nitrogen, sulfates, turbidity, biological oxygen demand, chlorophyll a, and fecal coliform are high in the rivers, streams, and ditches of Ecoregion 73; they are often much greater than elsewhere in Arkansas, increase with increasing watershed size, and are greatest during the spring, high-flow season.
Fish communities in least altered streams typically have an insignificant proportion of sensitive species; sunfishes are dominant followed by minnows. Man-made flood control levees typically flank the Mississippi River and, in effect, separate the river and its adjoining habitat from the remainder of its natural hydrologic system; in so doing, they interfere with sediment transfer within Ecoregion 73 and have reduced available habitat for many species.
Between the levees that parallel the Mississippi River is a corridor known as the" batture lands" . Batture lands are hydrologically linked to the Mississippi River, flood-prone, and contain remnant habitat for " big river" species (e.g., pallid sturgeon) as well as river-front plant communities; they are too narrow to map as a separate level IV ecoregion. Earthquakes in the early nineteenth century offset river courses in Ecoregion 73. Small to medium size earthquakes still occur frequently; their shocks are magnified by the alluvial plain' s unconsolidated deposits, creating regional land management issues.
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