|Smith Jr, Sammie|
Submitted to: Mississippi Water Resources Research Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: April 25, 2006
Publication Date: April 25, 2006
Citation: Cooper, C.M., Smith Jr, S., Ritchie, J.C. 2006. The yalobusha river - grenada reservoir watershed: sediment movement, accumulation and quality in a mississippi intensive agricultural landscape. Proceedings of the 36th Mississippi Water Resources Research Conference, April 25-26, 2006, Jackson, MS. Mississippi Water Research Institute, Mississippi State University, Mississippi State, MS. pp. 6-12. Interpretive Summary: The upper Yalobusha River watershed and Grenada Reservoir are located in north central Mississippi, USA. The reservoir was impounded in 1954. Long-term watershed erosion, agriculture, and channelization created concern, and a large erosion control demonstration project has potential for solving many problems. With the exception of approximately 21 km (13 miles) in the Yalobusha River upstream of Grenada Reservoir, all of the river and major tributaries of the watershed have been channelized. We studied accumulated sediments and agriculturally-related contaminants to better understand how the two are tied together, to obtain the current contaminant picture, and to provide base-line data for comparison with post demonstration project conditions. With increased watershed stability over the next several decades, movement of sediment and associated contaminants from field soil, urban areas, and streambeds should be minimized. Metals and pesticides were detected in 12-15% of water and sediment samples in all habitats. Overall, pesticide and metal concentrations were well below levels of concern. Results of this study will assist state environmental protection agencies, reservoir managers, and watershed planners in understanding and planning erosion control plans.
Technical Abstract: We examined sedimentation rates, watershed contamination contributions and potential impacts of row cropping (cotton, corn, soybeans, and sweet potato) on a small river and a large downstream flood control reservoir in the loess hills of Mississippi, USA. Grenada Reservoir has a total watershed drainage area of ~3,419 square kilometers. Sediment accumulation rates revealed that the reservoir functions with only slightly reduced storage capacity. Sediment delivery to the reservoir by the Yalobusha River at the most downstream measured site during our study, from 1996 to 2002, averaged 126 mg/L (range 12 to 767 mg/L, S.D.=136). Long-term sediment accumulation within the permanent pool adjacent to the dam was <1cm yr-1 except for a depositional area near tributary inflow that accumulated sediment at about 5 cm yr-1. The central area of the permanent pool experienced sediment accumulation rates that averaged <1.5 cm yr-1. Sites within the two reservoir arms near river inflows showed little or no sedimentation. A large debris jam which formed a river plug southwest of Calhoun City accumulated sediment from the upper portion of the watershed. From 1996 to 2002 analyses were conducted in water and sediment for 8 metals and 48 pesticides/contaminants at 25 stream/river locations and 9 locations within the reservoir. In spite of long-term historical use of residual pesticides in the watershed and widespread use of currently applied agricultural compounds, concentrations in stream or reservoir sediments and overlying water were generally low and sporadic or not detectable. Conversely, several metals (arsenic, lead, copper, iron, aluminum and zinc) were abundant in stream and reservoir sediments. Atrazine was routinely found in stream water and sediment. It was also detected in reservoir water samples at one fifth of contributing stream concentrations. Residual pesticides were generally not detected in water but were found in stream and reservoir sediments. Sediments within the debris jam contained low concentrations of arsenic and mercury. Debris jam sediments held highest observed concentrations of 'DDT, beta-BHC and Endosulfan II. A dredged channel through the debris jam was completed in late 2003. It may affect future sediment and contaminant accumulation in the natural river channel, its floodplain, and Grenada Reservoir. Because of processes associated with transitioning from a channelized stream to a natural one, it is likely that the plug phenomena will reoccur.