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United States Department of Agriculture

Agricultural Research Service

Topashaw Canal Watershed, Mississippi
An ARS Benchmark Research Watershed

Characteristics
Environmental Impacts
Management Practices
Research Objectives
Approaches
Collaborators and cooperating Agencies and Groups

Characteristics

The 168,750 ha (416,800 acres) Yalobusha River Watershed (YRW) in North Central Mississippi was originally selected as a CEAP Benchmark watershed in 2003. The YRW was defined at an arbitrary point in Grenada Lake upstream from the confluence of the Yalobusha and Skuna Rivers in North Central Mississippi. In 2005, the watershed was redefined for CEAP research purposes as the 11,000 ha (27,181 acres) Topashaw Canal Watershed within the YRW. The USGS stream gauge site on Topashaw Canal at Hohenlinden (latitude 33°45'29" and longitude 89°10'43") was chosen to define the Topashaw Canal Watershed (TCW) because it has continuous measurements of both stream discharge and sediment concentration from 2000 to present. The TCW is instrumented with six rain gauges, a weather station, two edge-of-field gully sites without drop pipes, two edge-of-field gullies with NRCS constructed drop pipes, gullies that are routinely surveyed, and a stream gauging station for monitoring of stream flow and sediment concentration on the Little Topashaw Creek subwatershed.

The Topashaw Canal Watershed (TCW) exhibits flashy stream response to precipitation. It experiences an average annual precipitation of 1500 mm (59 inches) with the highest monthly average (137 mm or 5.4 inches per month) occurring from November through February and lowest period (112 mm or 4.4 inches per month) occurring from July through September. Land use consists of 11% cropland, 6% pasture or grassed areas, 80% forested areas, 2% urban and 1% wetland and surface water. The primary crops grown are sweet potatoes (Ipomoea batatus) in rotation with either corn (Zea mays), cotton (Gossypium hirsutum), or to a lesser extent soybean (Glycine max).

The TCW is characterized by relatively flat (<2%) alluvial plains along streams and fairly steep (>12%) forested hillslopes. The watershed lies within the Southern Coastal Plain, Major Land Resource Area 133A and has 25 predominant soil series. Row-crop agriculture is limited to the alluvial plains which are predominantly Falaya silt loam and occupy approximately 11% of the watershed area. The forested hillslopes are predominantly Sweatman loam occupying 46% of the watershed area. The dominant geologic formation is the Midway Group, characterized by dispersive silt soils at the surface (upper meter), overlying layers with high sand content and lenses of increased clay content indicative of the alluvial deposition patterns. These layers overlie consolidated clay materials that generally serve as the stream bed. It is the resistant, consolidated clay bed material that makes the TCW somewhat unique in comparison to other adjusting stream systems in the mid-continent region. Subsurface flow out of streambanks is common, and water perched by clay layers erodes the highly conductive sandy layers above. Undercutting, as a result of the seepage erosion, leaves the streambank susceptible to bank failure.

Environmental Impacts

The TCW experiences deposition and flooding problems in downstream reaches, erosion via headward-progressing knickpoints, and massive bank failures in upper reaches. These general patterns are found throughout the region, and are associated with the consequences of accelerated erosion stemming from land mismanagement and channelization. As a consequence of channel adjustment processes related to channelization in the late 1950's and early 1960's, upstream-migrating knickpoints have caused deepening of upstream reaches and tributary channels. Sufficient deepening occurred to cause significant channel widening by mass failure of channel banks. Currently, the number one problem for which conservation funds are allocated in this region is gully erosion; typically gully inlets and edge-of-field gullies initiated by mass failure of gully banks at entry points of surface runoff into stream channels.

Management Practices

The most common conservation practice by funding in TCW is installation of grade control (i.e. drop-pipe) structures (EQIP practice code 410) to mitigate gully erosion. The other most common practices (program, practice, code) by acreage are:

1. CRP, Pines, CP3
2. CRP, Grass, CP1, CP10
3. EQIP, Grazing, 378, 512, 528, 600, …
4. EQIP, Sedimentation, 342, 350, …
5. CRP, Forest Riparian Buffer, CP4

Research Objectives

General: The overall objective is to evaluate watershed responses to field, edge of field, and channel conservation practices.

Specific:

1. Define the variability of hydrologic and biogeochemical processes that influence the effectiveness of conservation practices processes at different scales within the TCW.

2. Identify and quantify the effects of specific management/conservation practices and systems on contaminant and water transport.

Approaches

Subobjective: 1a. Compile the information collected to date in the TCW on land use, conservation practices, and soil characteristics. Develop a detailed land-use inventory, identify and locate conservation practices, and acquire digitized soils and elevation data.

Subobjective 1b. Compile the information collected to date in the TCW on streamflow and sediment concentrations at baseflow and during storm events.

Subobjective 2a. Determine the effects of specific management / conservation practices and systems on contaminant and water transport processes at different scales within the TCW. The historical hydrologic and water quality data will be used to evaluate correlations with historical land use and conservation practices at the TC watershed scale, the LTC subwatershed, and the field scale.

Subobjective 2b. Determine the effectiveness of drop-pipe structures at controlling the magnitude of sediments and contaminants contributed by surface runoff from agricultural areas to streams.

Subobjective 2c. Evaluate mechanisms of seepage erosion and its contribution to stream bank failure. Subsurface flow and corresponding erosion of bank sediment will be quantified at multiple locations along Little Topashaw Creek and the associated soil physical and hydraulic properties will be characterized. Laboratory experiments will quantify the hydraulic properties controlling the seepage erosion process associated with streambank failure.

Collaborators and Cooperating Agencies and Groups

US Geological Service; Natural Resources Conservation Service; US Army Corps of Engineers, Vicksburg District; Local landowners and drainage districts; Mississippi State University, Cooperative Extension Service; University of Mississippi

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Last Modified: 6/16/2009