Submitted to: Journal of Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2009
Publication Date: 9/25/2009
Citation: Bennett, S.J., Rhoton, F.E. 2009. Linking Upstream Channel Instability to Downstream Degradation: Grenada Lake and the Skuna and Yalobusha River Basins, Mississippi. Journal of Ecohydrology. 2: 235-247. Interpretive Summary: The Yalobusha River drains approximately 80,000 hectares of highly erodible soils and unstable streambanks. Consequently, its discharge has the potential to impact the sediment quality in Grenada Lake which serves as a sink for the wateshed. We characterized the sediment contained within a debris plug upstream, and within the bottom sediment environment of Grenada Lake for several chemical and physical properties that determine sediment and water quality. The results indicated that only 3% of Grenada lake's storage has been lost to sedimentation, and that most of the sediments accumulating in the reservoir were clay size or smaller, which has contributed to higher concentrations of agrichemicals and trace metals than exist in sediments upstream. Thus, it is apparent that the quality of sediment entering Grenada lake is more critical to its water quality than sediment quantity.
Technical Abstract: In north-central Mississippi, unstable, deeply incised streams with actively calving banks can dominate total sediment loads within these watersheds. The Yalobusha River is one such system having a long history of channelization, excessive stream channel and bank erosion, and large woody debris recruitment and accumulation, and this degradation has not gone unrequited. The physical and chemical characteristics of two downstream locations are examined to explore these geomorphic linkages between upstream instability and downstream degradation. For the current large woody debris plug, it is found that: (1) the deposit is nearly entirely composed of sand covered with a thin veneer of clay, (2) agrichemicals and relatively high concentrations of trace elements are observed, and (3) river flow has been forced entirely out-of-bank. For Grenada Lake, it is found that: (1) the impounded sediment is predominantly clay, (2) agrichemicals and relatively high concentrations of trace elements are observed, and (3) only 3% of the reservoir’s storage capacity has been lost due to sedimentation. While excessive sedimentation and large woody debris recruitment has had a marked affect on stream corridor function in the area of the debris plug, it is the caliber and quality of the sediment loadings to Grenada Lake, rather than its quantity, that has affected adversely water quality and ecologic indices within this water body.