St Francis Basin of Arkansas Potential Natural Vegetation

REVISED JUNE 2013

The St. Francis Basin lies in northeastern Arkansas and southeastern Missouri. This map includes only the Arkansas portion of the basin.  The eastern boundary is defined by the mainstem Mississippi River levee, and the western boundary is Crowley’s Ridge.  More than 2 million acres are included in the mapped area.  The principal streams are the St. Francis, Tyronza, and Little Rivers. 

Within Arkansas, approximately half the St. Francis Basin is made up of recent (Holocene) meander belt deposits of the Mississippi River and smaller streams.  These include poorly-drained backswamps, better-drained point bars, and well-drained natural levees.  Abandoned channel segments form crescent-shaped oxbow lakes and depressions.  The remainder of the area is composed of glacial outwash that flushed into the Mississippi Valley during periods of waning Late Wisconsin continental glaciation.  Sometimes called “valley train” deposits, they are composed of relatively unsorted, coarse materials deposited in a braided-stream environment, and are very different from the later fine-grained, well-sorted deposits of the modern meandering streams. In the St. Francis Basin within Arkansas, they occur as several distinct terraces. The oldest and highest levels are two narrow terraces adjacent to Crowley’s Ridge; the more recent deposits are much more extensive and often lie at approximately the same elevations as the adjacent Holocene meander belt s.  On the lower and younger terraces, the remnant outwash channels are often distinctly visible, and may carry smaller modern streams within them. Relict sand bars and wind- blown sand are apparent on the surface of some valley train deposits.  In the St. Francis basin, there also are numerous more recent deposits known as “sand blows” that were buried outwash sands ejected during the New Madrid earthquakes of 1811 and 1812.  

This complex landscape of old and young deposits of various origins was subject to frequent flooding prior to the early 20 th century. Since then, the St. Francis Basin has been the focus of one of the most comprehensive and effective flood-control and drainage efforts in the world. The mainstem Mississippi River levee and an internal system of levees and floodways—mostly federal projects—prevent regular overbank and backwater flooding. A network of drainage ditches constructed by local interests effectively removes most of the remaining excess water—local precipitation that once ponded on the surface, supporting forested wetlands. While some of these smaller ditches might be plugged or filled as part of a wetland restoration, the large federal projects are assumed here to be permanent features on the landscape. Therefore, the PNV community type descriptions specifically note that some types occur “within floodways,” and there is some uncertainty concerning the long-term persistence of the species identified as dominants in those man-made environments.