|Smith Jr, Sammie|
Submitted to: Mississippi Water Resources Research Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: February 22, 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. 36th Annual Mississippi Water Resources Conference 2006 Program and Abstracts, page 6. Mississippi Water Resources Research Institute, Mississippi State University, Mississippi State, MS. Technical Abstract: We examined sedimentation rates, current watershed contamination contributions and potential impacts of long-term 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. Although reservoir life expectancy was originally estimated at 25 years because of high erosion rates in the watershed, our study revealed that the reservoir continues to function 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 fed by the two river inflows (Yalobusha and Skuna rivers) showed little or no sedimentation. Sedimentation rates near the two river inflows were also generally low. 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, a triazine herbicide, was routinely found in stream water and sediment. Atrazine was also detected in reservoir water samples but at only one fifth of contributing stream concentrations. Naturally-occurring aluminum and iron were found in high concentrations. Residual pesticides were generally not detected in water but were detected in stream and reservoir sediments. Sediments within the debris jam contained concentrations of arsenic and mercury lower than watershed and reservoir locations. Debris jam sediments held highest observed concentrations of 'DDT, beta-BHC and Endosulfan II but did not contain detectable amounts of several legacy pesticides that were found in both the watershed and reservoir samples. 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.