The distribution and residence time of suspended sediment stored within the channel margins of a gravel‐bed bedrock river

Abstract

Previously undocumented deposits are described that store suspended sediment in gravel-bedded rivers, termed 'fi ne-grained channel margin' (FGCM) deposits. FGCM deposits consist of sand, silt, clay, and organic matter that accumulate behind large woody debris (LWD) along the margins of the wetted perimeter of the single-thread, gravel-bed South River in Virginia. These deposits store a total mass equivalent to 17% to 43% of the annual suspended sediment load. Radiocarbon, 210 Pb and 137 C dating indicate that sediment in FGCM deposits ranges in age from 1 to more than 60 years. Reservoir theory suggests an average turnover time of 1·75 years and an annual exchange with the water column of a mass of sediment equivalent to 10% to 25% of the annual sediment load. The distribution of ages in the deposits can be fi tted by a power function, suggesting that sediment stored in the deposits has a wide variety of transit times. Most sediment in storage is reworked quickly, but a small portion may remain in place for many decades. The presence of FGCM deposits indicates that suspended sediment is not simply transported downstream in gravel-bed rivers in agricultural watersheds: signifi cant storage can occur over decadal timescales. South River has a history of mercury contamination and identifying sediment sources and sinks is critical for documenting the extent of contamination and for developing remediation plans. FGCM deposits should be considered in future sediment budget and sediment transport modeling studies of gravel-bed rivers in agricultural watersheds.

Sediment contributions from floodplains and legacy sediments to Piedmont streams of Baltimore County, Maryland

“…Thus, our findings are consistent with previous results (Walling et al, 1998;Trimble, 1999;Hupp et al, 2010Hupp et al, , 2013Skalak and Pizzuto, 2010;Smith et al, 2011). Extrapolations suggested that within the drainage network, sediment contributions per meter of channel length were greater for larger streams, but the majority (41%-62%) of cumulative streambank sediment loads were from first-and second-order tributaries.…”

Surficial redistribution of fallout131iodine in a small temperate catchment

“…We demonstrate that these zones, even in low-order streams, may sequester significant quantities of fine particulate material, contaminants, and nutrients (33). The continuous processes of fluvial redistribution in small catchments are, in turn, important in supplying organic material (34,35,36) and soil carbon (37,38) to larger river systems, lakes, wetlands, and estuaries (39,40).…”

Effects of reservoir stratification and watershed hydrology on manganese and iron in a dam‐regulated river

“…A number of factors contribute to the spiraling of particulates, notably the particle size, flow velocity, channel depth, and Fe or Mn fingerprint of channel sediments (Whiting, Matisoff, Fornes, & Soster, ). The high‐energy flows created by dam releases (Csiki & Rhoads, ) and pool‐riffle sequences reach provide an environment for erosion, entrainment, and reworking of the suspended particulates (Hupp et al, ; Sear, ; Skalak & Pizzuto, ). Whiting et al, () examined the transport of suspended sediment in an upland river with flow characteristics similar to the Roanoke River and found that the transport distance of suspended particulates could be as short as 3 km due to the presence of pools along the flowpath.…”