Influence of basin connectivity on sediment source, transport, and storage within the Mkabela Basin, South Africa

Abstract

Abstract. The management of sediment and other non-point source (NPS) pollution has proven difficult, and requires a sound understanding of particle movement through the drainage system. The primary objective of this investigation was to obtain an understanding of NPS sediment source(s), transport, and storage within the Mkabela Basin, a representative agricultural catchment within the KwaZulu-Natal Midlands of eastern South Africa, by combining geomorphic, hydrologic and geochemical fingerprinting analyses.The Mkabela Basin can be subdivided into three distinct subcatchments that differ in their ability to transport and store sediment along the axial valley. Headwater (upper catchment) areas are characterized by extensive wetlands that act as significant sediment sinks. Mid-catchment areas, characterized by higher relief and valley gradients, exhibit few wetlands, but rather are dominated by a combination of alluvial and bedrock channels that are conducive to sediment transport. The lower catchment exhibits a low-gradient alluvial channel that is boarded by extensive riparian wetlands that accumulate large quantities of sediment (and NPS pollutants).Fingerprinting studies suggest that silt-and clay-rich layers found within wetland and reservoir deposits of the upper and upper-mid subcatchments are derived from the erosion of fine-grained, valley bottom soils frequently utilized as vegetable fields. Coarser-grained deposits within these wetlands and reservoirs result from the erosion of sandier hillslope soils extensively utilized for sugar cane, during relatively high magnitude runoff events that are capable of transporting sand-sized sediment off the slopes. Thus, the source of sediment to the axial valley varies as a function of sediment size and runoff magnitude. Sediment export from upper to lower catchment areas was limited until the early 1990s, in part because the upper catchment wetlands were hydrologically disconnected from lower parts of the watershed during low to moderate flood events. The construction of a drainage ditch through a previously unchanneled wetland altered the hydrologic connectivity of the catchment, allowing sediment to be transported from the headwaters to the lower basin where much of it was deposited within riparian wetlands. The axial drainage system is now geomorphically and hydrologically connected during events capable of overflowing dams located throughout the study basin. The study indicates that increased valley connectivity partly negated the positive benefits of controlling sediment/nutrient exports from the catchment by means of upland based, best management practices.

Anthropogenic Land Use and Land Cover Change as Potential Drivers of Sediment Sources in the Upper Crocodile River, North West Province, South Africa

“…The multivariate mixing model was used to estimate the proportions of each potential sediment source, as prescribed by Manjoro, Rowntree, Kakembo, Foster and Collins [ 25 ] and Miller et al [ 64 ], in order to estimate the sources sediment flowing from the surface and subsurface to the river, using Equation (3): where n is the number of tracer properties of the optimum composite tracer, C i is the concentration of the i th tracer property in the sediment, which is the optimised percentage contribution of source s , Ss i , is the mean concentration of the i th tracer property in source S and m is the number of sediment sources. Constraints on the mixing model ensured that (1) the contributions of the individual source areas were non-negative and (2) the sum of the relative contributions from all of the source areas was equal to unity (Equation (4)) [ 64 ]. …”

Upstream to downstream: a multiple-assessment-point approach for targeting non-point-source priority management areas at large watershed scale

“…Today, the targeting of watershed PMAs has been integrated as an inherent part of largescale watershed management programs, such as the Total Maximum Daily Load (TMDL) (Savage and Ribaudo, 2013;Sahoo et al, 2013;White et al, 2009). As a geographically connected unit, a watershed can be broken into a distinct stream network and corresponding subwatersheds (Gérard-Marchanti et al, 2006;Liu and Weller, 2008;Miller et al, 2013). A river assessment point, where water quality is sampled and evaluated, is usually designed as the key variable in assessing and protecting water quality within a river network (Lee et al, 2012).…”

“…2 The targeting results of the MAP-PMA, the AP-1, the AP-2, and traditional approach Traditionally, the upstream pollutant inputs are assumed to transport semi-systematically downstream. In the MAP-PMA framework, this classical continuum idea was replaced by a hierarchical idea, in which the river network is divided into smaller river sections between multiple assessment points (Brierley and Fryirs, 2011;Miller et al, 2012Miller et al, , 2013. Each river section represents a homogeneous spatial unit, which associated with a specific assessment point within the river network.…”

Upstream to downstream: a multiple-assessment-point approach for targeting non-point-source priority management areas at large watershed scale

“…Traditionally, the upstream pollutant inputs are assumed to transport semi-systematically downstream. In the MAP-PMA framework, this classical continuum idea was replaced by a hierarchical idea, in which the river network is divided into smaller river sections between multiple assessment points (Brierley and Fryirs, 2011;Miller et al, 2012Miller et al, , 2013. Each river section represents a homogeneous spatial unit, which associated with a specific assessment point within the river network.…”

“…As a geographically connected unit, a watershed can be broken into a distinct stream network and corresponding subwatersheds (Gérard-Marchanti et al, 2006;Liu and Weller, 2008;Miller et al, 2013). A river assessment point, where water quality is sampled and evaluated, is usually designed as the key variable in assessing and protecting water quality within a river network (Lee et al, 2012).…”