Submitted to: Laboratory Publication
Publication Type: Government Publication
Publication Acceptance Date: 11/1/2007
Publication Date: 12/1/2007
Citation: Simon, A., Klimetz, L. 2007. Characterization of Stream Morphology and Sediment Yield for the Big Black and Tombigbee River Basins, Mississippi. Laboratory Publication. NSL Research Report #54. Interpretive Summary: Sediment is listed as one of the leading causes of water-quality concern in surface waters of the United States, yet little information exists that functionally links sediment-transport rates to aquatic health. The Mississippi Department of Environmental Quality (MDEQ) is in charge of determining water-quality targets for streams within the Big Black and Tombigbee River Basins that are listed as having levels of sediment harmful to aquatic life. To aid MDEQ in determining meaningful targets, historical flow and sediment-transport data from sites in the Big Black and Tombigbee River Basins and the ecoregions in which they lie (the Southeastern Plains –and the Mississippi Valley Loess Plains), were analyzed to develop 'reference' or background suspended-sediment yield values and parameters (metrics) such as frequency and duration of sediment concentrations. It is hoped that aquatic ecologists can use such parameters to develop functional links between sediment and biologic response. Channel stability at MDEQ Index of Biologic Integrity (IBI) sites was also determined with the intention of linking this to ecoregion scale 'reference' values, however only 6 and 12 % of sites in the Big Black and Tombigbee River Basins respectively, were found to be stable. With a lack of data, 'reference'; values at the IBI sites could not be compared to those at the basin or ecoregion scale. At the Level III Ecoregion scale, a suggested range for suspended-sediment mean annual yield in stable or 'reference' channels of the Southeastern Plains is 3.84 – 16.4 T/y/km2, at the effective discharge (that which occurs on average every 1.5 years, Q1.5) this range is 0.0929 – 0.415 T/d/km2. Data for the Mississippi Valley Loess Plains is slightly less reliable as there are less stable sites in this inherently unstable region. However, a suggested range for suspended-sediment mean annual yield in stable or "reference" Ecoregion 74 is 36.2 – 143 T/y/km2, at Q1.5 this range is 0.746 – 0.314 T/d/km2.
Technical Abstract: Three segments within the Big Black River Basin, and nine within the Tombigbee River Basin are on the Mississippi 303d list of water bodies as having impaired conditions for aquatic life due to sediment. An additional 56 reaches of channel are listed for biologic impairment between the two basins. At the request and support of the Mississippi Department of Environmental Quality (MDEQ), this study and report examines sediment loadings in the Big Black River and Tombigbee River Basins, Mississippi. The principal objective of the study was to determine reference conditions for suspended-sediment loads and yields, and bed-material characteristics for channels in the Big Black and Tombigbee River Basins. The Big Black and Tombigbee drainage basins lie within the Southeastern Plains (Ecoregion 65) and the Mississippi Valley Loess Plains (Ecoregion 74). "Reference" suspended-sediment transport rates were obtained from stable streams with historical flow and sediment-transport data in these two ecoregions. A combination of methods was used, at a number of different scales. Channel stability was determined at the basin scale, at both MDEQ Index of Biologic Integrity (IBI) sites and historical United States Geologic Society (USGS) gage sites. The majority of sites in the Big Black and Tombigbee River Basins were classified unstable or 'impacted', using a Rapid Geomorphic Assessments (RGA); 94 % in the Big Black River Basin, 88 % in the Tombigbee River Basin. Due to a lack of stable, 'reference' streams in the study basins, relations between bankfull height, width and cross-sectional area, width-depth ratio, slope and drainage basin area could not be made based on stability. The characterization of 'reference' streams in terms of embeddedness of bed material was not possible as the majority of the streams were dominated by fine material, < 2 mm. At the Ecoregion Level III scale, there were sufficient data in the Southeastern Plains, Ecoregion 65, to calculate 'reference' or target value rates of suspended-sediment transport. It is more appropriate to consider transport rates in terms of attainable goals rather than absolute values. Therefore, 25th and 75th percentile ranges for suspended-sediment mean annual yield are 3.84 – 16.4 T/y/km2 and 0.0929 – 0.415 T/d/km2 for the effective discharge, that which occurs on average every 1.5 years (Q1.5). Ecoregion 74, the Mississippi Valley Loess Plains is characterized by a general lack of stable, "reference" streams with supporting data. Therefore for this Ecoregion the data required to produce a suspended-sediment rating curve was reduced from 30 to 9 suspended-sediment samples. This must be considered when using the "reference" values calculated for Ecoregion 74. The 25th and 75th percentile ranges of "reference" mean annual suspended-sediment yield values for Ecoregion 74 is 36.2 – 143 T/y/km2 and 0.746 – 3.14 T/d/km2 at the Q1.5 discharge. Data were further parameterized in terms of magnitude, frequency and duration of suspended-sediment concentrations to establish alternative ways of providing "reference" values that may be more appropriate to linking ecologic function of the river channels.