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Rivers crossing coastal plains are often inefficient conveyors of sediment, so that changes in upstream sediment dynamics are not evident at the river mouth. Extensive accommodation space and low stream power often result in extensive alluvial storage upstream of estuaries and correspondingly low sediment loads at the river mouth. However, gaging stations with sediment records are typically well upstream of the coast, and thus tend to overestimate sediment yields by under-representing the lower coastal plain and because there is often a net loss of sediment in lower coastal plain reaches. Studies of alluvial sediment storage have generally focused on accommodation space, but, using examples from Texas, we show that low transport capacity controlled largely by slope is a crucial factor. [Copyright &y& Elsevier]

Coastal and marine sedimentary archives are sometimes used as indicators of changes in continental sediment production and fluvial sediment transport, but rivers crossing coastal plains may not be efficient conveyors of sediment to the coast. Where this is the case, changes in continental sediment dynamics are not evident at the river mouth. Stream power is typically low and accommodation space high in coastal plain river reaches, resulting in extensive alluvial storage upstream of estuaries and correspondingly low sediment loads at the river mouth. In some cases there is a net loss of sediment in lower coastal plain reaches, so that sediment input from upstream exceeds yield at the river mouth. The lowermost sediment sampling stations on many rivers are too far upstream of the coast to represent lower coastal plain sediment fluxes, and thus tend to overestimate sediment yields. Sediment which does reach the river mouth is often trapped in estuaries and deltas. Assessment of sediment flux from coastal plain rivers is also confounded by the deceptively simple question of the location of the mouth of the river. On low-gradient coastal plains and shelves, the location of the river mouth may have varied by hundreds of kilometers due to sea-level change. The mouth may also differ substantially according to whether it is defined based on channel morphology, network morphology, hydrographic or hydrochemical criteria, elevation of the channel relative to sea level, or the locus of deposition. Further, while direct continent-to-ocean flux may be very low at current sea-level stands, sediment stored in estuaries and lower coastal plain alluvium (including deltas) may eventually become part of the marine sedimentary package. The role of accommodation space in coastal plain alluvial sediment storage has been emphasized in previous work, but low transport capacity controlled largely by slope is also a crucial factor, as we illustrate with examples from Texas. [ABSTRACT FROM AUTHOR]

Presents the results of field measurements conducted in a small agricultural basin on the North Carolina coastal plain. Objective of the study; Role of storm runoff in sediment delivery; Results suggesting soil erosion in the region is more rapid and extensive than previously thought; Soil loss at the fields; Doubt expressed on assessments of the coastal plain as being a stable, non-eroding landscape; Discussion of the mobilized soil, which is redistributed and stored within the watershed with only a small portion of the soil eroded from slopes leaving the basin on an event time frame.

This study examines the particle size characteristics of hillslope soils and fluvial suspended sediments in an agricultural catchment. Samples of surface runoff and stream flow were collected periodically and analysed for the size distributions of the effective (undispersed)sediment. This sediment was subsequently dispersed and the ultimate size distributions determined. The median effective particle size of stream suspended sediment was considerably coarser than the median ultimate particle size, indicating that most of the load included a substantial proportion of aggregates. Moreover, the proportion of fine material (i.e. silt and clay) increased, and the proportion of sand-sized material decreased, with increasing discharge. This decrease in sediment size with increased flow, which is contrary to the traditional assumption of a positive discharge/particle size relationship, is thought to reflect: (i) the influx of silt and clay, predominantly theformer, originating on the catchment slopes and brought to the stream by overland flow along vehicle wheelings, roads and tracks; and (ii) erosion of fine material from the channel bed and banks. During large storms, however, the proportion of sand-sized sediment increased during the rising limb of the hydrograph, as a result of the entrainment of coarser source material from the valley floor during overbank flooding. The stream suspended sediment was finer than the catchment soils and considerably finer than material eroding from the catchment slopes during storms. The degree of clay and silt enrichment in the suspended sediments was largely the result of preferential deposition of the coarser fraction during the transport and delivery of sedimentfrom its source to basin outlet. The data from this study confirm that a significant mode of sediment transport in fluvial systems is in the form of aggregates, and that the dispersed sediment size distribution is inappropriate for determining the transportability of sediment b [ABSTRACT FROM AUTHOR]