Delineation of Watersheds in Northwestern Mississippi to the Sub-Watershed Level

Author(s): Rose, C.

In 2009, river systems in Mississippi were delineated to the watershed level, and most were delineated to the subwatershed level. One of the exceptions was the northwestern part of Mississippi in the lower part of the Yazoo River Basin (locally referred to as the “Delta”) where only watershed-level delineations were completed. Watersheds and sub-watersheds previously delineated in Mississippi were based on elevation change and water body locations as observed from topographic maps, digital elevation models, and aerial photographs. Previous attempts to delineate watersheds in the Mississippi Delta region to the sub-watershed boundaries were problematic due to the following issues: topographically-uniform, low relief land elevations (less than 100 feet of rise in land-surface elevation from Vicksburg to Memphis); land management practices employed by land owners and growers who frequently change drainage patterns; and insufficient scale of available elevation to allow delineation of subtle topographic features. Therefore, the Mississippi Delta region was not delineated to the sub-watershed level until more precise digital elevation tools were available for use. Delineation of watershed boundaries and designation of watershed numbering and naming is an important first step for resource managers that are concerned with ecosystem and water body health and remediation of point and non-point source pollution. Previous scientific studies have implicated the Delta region as a contributor to the hypoxic zone in the Gulf of Mexico, and a large percentage of Delta waters are listed as impaired on the section 303(d) List of Impaired Waters. For Delta streams, ecosystem health and mitigation of nonpoint source pollution is a primary concern for resource managers, and delineation of watersheds in this region is a critical and necessary step for future planning and mitigation activities. Since the publication of the Mississippi Watershed Boundary Dataset in 2009, the entire Yazoo River Basin (including the Mississippi Delta region) has been mapped using Light Detection and Ranging (LiDAR) technology (funded by the U.S. Army Corps of Engineers, Vicksburg District), which has a root mean square error vertical land surface bare-earth accuracy of 9 centimeters. With the availability of LiDAR data, it is now more reasonable to delineate the Mississippi Delta region to the sub-watershed level. The U.S. Geological Survey, in cooperation with the Mississippi Department of Environmental Quality, has completed the delineation of river systems in the Mississippi Delta region to the subwatershed level.

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