Spatial variation of sediment yield in the landscape

 
Regional scale relationships between specific sediment yield and drainage area in the Missouri basin reveal varying landscape sedimentary dynamics. Declining specific sediment yield with area is typical of disturbed agricultural areas, while increasing yields indicate the remobilization of stored sediment by bank and floodplain erosion along major tributaries [Hassan A. H., 2008]. Using the developed relationships, areally-adjusted sediment yield maps are created over the entire Missouri basin.

Methodology

Sediment yield from a river basin only partially reflects the rate of sediment mobilization from the land surface. Sediment moves from uplands and hillslopes into stream channels, and eventually streams transfer the sediment to the sea. A proportion of the material eroded from upland areas remains stored in the fluvial system for a long period [Trimble, 1981, 1999; Church and Slaymaker, 1989]. Sediment yield provides a simple lumped representation of the linkages between the erosion processes which operate within the drainage basin and downstream sediment delivery [Walling and Webb, 1992]. It does not provide information on the spatial patterns of sediment mobilization from within the basin. Diversity of climate, geology, land use, and geomorphic connectivity of a basin’s landscape control those patterns of sediment yield. Maps of sediment yield have been generated in order to depict the regional variability of sediment mobilization within a drainage basin [e.g., Fournier, 1960; Walling and Webb, 1983]. In generating these maps, problems have arisen, including lack of data, poor data quality (restricted record length and poor spatial distribution of stations), lack of accepted extrapolation procedures and ignorance of scale dependency [Ashmore and Day, 1988; Church et al., 1989, 1999; Lu et al., 2003; Walling, 2005; Slaymaker, 2006].

Analysis and Result

Based on hydrological unit, the Missouri basin is divided to 7 sub-basins to develop regional sediment yield relationships (Figure 1). Then specific sediment yield is used to compare sediment yield data. L/A = KsA^b is specific sediment yield, in which L is the average annual load, A is the contributing drainage basin area, Ks is the regional unit area yield [Church et al., 1999], and b is an exponent which expresses the areal scale related change of the specific yield (Figures 2-8).

For each sub-basin, a relationship between specific sediment yield and drainage area is developed. For all sub-basins, sediment yield declining with area (b < 0) implies systematically declining sediment mobilization with increasing area or regional fluvial aggradations (Figures 2-8). This normally occurs in heavy agricultural disturbance areas when sediment generated in the upland is deposited near slope base and on floodplains [Church et al., 1999].
Assuming that the observed scaling exponent holds true across its region, the sediment yield per unit area (Ks) was back calculated from the relationships developed between sediment yield and drainage area for each sediment survey as Ksi = Li/Ai^b, where Li is the sediment yield for an individual station [Hassan A. H., 2008]. A map was created for the Missouri basin based on 308 surveyed data. Kriging in ArcGIS is used to interpolate and find the regional unit area yield for the entire basin (Figure 9).

The most remarkable results as shown in Figure 9 include the low sediment yield in the northern and middle parts of the basin, mainly in sub-basins 3 and 6, and very high sediment yields in the south of the basin, mainly sub-basins 1 and 4. These variations may be attributed to: a) agricultural pattern (e.g., high concentration of corn fields in Figure 10), b) different geological conditions (Figure 11), and c) existence of coal mines (Figure 12).

Figure 10- Corn for Grain, Harvested 2007 (Source: National Agricultural Statistic Service)

Figure 11- Geological (Source: National Agricultural Statistic Service)

Figure 12- Coal Feild (Source: National Agricultural Statistic Service)