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Title: Characterization of Channel Morphology and Sediment Loads for the Yazoo River Basin, Mississippi

item Simon, Andrew
item Klimetz Ii, Paul

Submitted to: Laboratory Publication
Publication Type: Government Publication
Publication Acceptance Date: 8/1/2008
Publication Date: 9/1/2009
Citation: Simon, A., Klimetz, L., Klimetz K Ii, P.D. 2009. Characterization of Channel Morphology and Sediment Loads for the Yazoo River Basin, Mississippi. Laboratory Publication. NSL Research Report No. 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 Yazoo River Basin 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 Yazoo River Basin and the ecoregions in which it lies (the Southeastern Plains, 65; the Mississippi Alluvial Plains, 73; and the Mississippi Valley Loess Plains, 74), 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 biological sampling locations was also determined with the intention of linking this to ecoregion scale ‘reference’ values. With a general lack of stable sites within the Yazoo River Basin, ‘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 3.98 – 17.2 T/y/km2, at the effective discharge (that which occurs on average every 1.5 years, Q1.5) this range is 0.943 – 0.471 T/d/km2. A target ‘reference’ range for suspended-sediment mean annual yield in stable or ‘reference’ channels of the Mississippi Alluvial Plains is 39.7 – 70.5 T/y/km2, at the Q1.5 this range is 0.317 – 1.01 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: The main purpose of this report was to provide the Mississippi Department of Environmental Quality (MDEQ) with geomorphic evaluations of previously visited biological sampling locations and update ‘reference’ suspended-sediment transport conditions within the Yazoo River Basin. This information would then aid MDEQ in calculating maximum allowable suspended-sediment loadings to, or in a stream or water-body that does not impair designated uses; the total maximum daily load (TMDL). Through the use of Rapid Geomorphic Assessments (RGA), 80 % of the approximately 200 sites visited within the Yazoo River Basin, were determined to be in a state of recovery from disturbance (stages V and VI), with evidence of deposition and channel bed aggradation. Suspended-sediment transport rates were calculated at many different scales: Level III Ecoregions, Level IV Ecoregions, The Yazoo River Basin, and for sub-watersheds within the study area. The use of turbidity as a more economical method of gauging suspended-sediment concentration to traditional point samplers was also examined at four locations within the Yazoo River Basin, to further reduce uncertainty in the calculation of suspended-sediment transport rates and as a potential substitute for traditional point samplers as a more economical method of collecting suspended-sediment concentration data. An overall lack of precipitation during the study period meant that few storms were sampled and consequently only a limited range of suspended-sediment concentrations were measured. To increase the size of the dataset available for analysis, data recorded as part of this study was combined with United States Geologic Society (USGS) data and rating relations developed at both the study basin scale and for Ecoregion 73 and 74. Depending upon the level of accuracy required, the rating relations developed allow the estimation of suspended-sediment at remote sites where discharge or concentration measurements are not available. Whilst turbidity has shown potential as a surrogate to traditional suspend-sediment point sampling methods currently in practice, further collection of depth-integrated samples over a greater range of turbidity measurements would increase confidence in the turbidity-concentration relations provided within the body of this report. Suspended-sediment loads for the 1.5-year recurrence interval flow and annual loads were calculated from derived relations between instantaneous discharge and suspended-sediment load and normalized for drainage area. Distributions for stable sites were then considered representative of ‘reference’ sediment-transport rates and inter-quartile ranges identified as ‘reference’ or target yields. Mean annual ‘reference’ yields ranged from 9 T/y/km2 for Ecoregion 65, the Southeastern Plain, to 79 T/y/km2 for the loess-dominated regions bordering the Mississippi River, Ecoregion 74; with Ecoregion 73 lying somewhere in between. Median ‘reference’ yields at the 1.5-year recurrence interval flow show a similar pattern: 0.2, 0.8 and 2 T/d/km2 for Ecoregion 65, 73 and 74 respectively. Due to an insufficient number of USGS gauging stations located on stable channels with adequate data within the seven Level IV Ecoregions of the Yazoo River Basin, Level IV ‘reference’ suspended-sediment transport rates should not be used in preference to Level III Ecoregion values.