Secular Sediment Waves, Channel Bed Waves, and Legacy Sediment

Abstract

The concept of sediment waves is reviewed and clarifications are proposed for nomenclature concerning vertical channel responses to large fluvial sediment fluxes over a period of a decade or longer. Gilbert’s (1917) original sediment waves are re‐evaluated at their type locale and used to develop a consistent set of definitions. A ‘sediment wave’ represents a transient sediment flux that includes but is not necessarily identical to a ‘channel bed wave’ that represents the rise and fall of the bed in response to sedimentation. A large‐scale sediment wave results when a major sediment‐delivery event generates an aggradation–degradation episode (ADE). It may leave a legacy of sediment deposited on valley bottoms. Gilbert’s classic sediment‐wave model was empirically based on changes in bed elevations but was described as a sediment flux. He described downstream translation of an attenuating symmetrical wave although it was too large and disjointed to have a coherent waveform. Rapid return of bed elevations to pre‐sedimentation levels in Gilbert’s wave should not be mistaken for exhaustion of sediment. The common concept of a symmetrical sediment wave representing the time series of sediment loads is not accurate for large‐scale sedimentation events that store and slowly release sediment on floodplains. Large sediment waves composed of relatively fine material and limited bed armoring tend to be right‐skewed owing to rapid vertical readjustments relative to slow lateral recruitment of stored sediment. A revised right‐skewed conceptual model of large sediment and bed waves is presented that fits modern observations and incorporates stochastic elements of flood events. Large sediment waves may be linked to legacy sediment; that is, large repositories of anthropogenic alluvium stored on valley bottom floodplains, wetlands, mill ponds, and reservoirs. This illustrates the ubiquity and importance of secular sediment waves to river management.

Placer mining along the Fraser River, British Columbia: The geomorphic impact

“…Church, 1980;Baker, 1988;James, 1999James, , 2010. In particular, we observe that many relatively small excavations along the Fraser River have had a substantial cumulative impact on the river.…”

“…Mining-waste sediment moved rapidly downstream, causing several meters of aggradation and destruction of farmland along the Sacramento River. Channel degradation followed aggradation (Gilbert, 1917), though large amounts of sediment remain stored in overbank deposits and continue to affect the river system (James, 1997(James, , 2006(James, , 2010.…”

“…This sediment was processed through sluice boxes and commonly disposed of in streams or rivers, resulting in downstream aggradation and instability. The geomorphic impact of concentrated, large-scale placer mining has been well documented in several regions, including California (e.g., Gilbert, 1917;Rohe, 1986;James, 1997James, , 2006James, , 2010, New Zealand (e.g., Barker, 2001;Otago Regional Council, 2008), and Australia (e.g., Knighton, 1989 The Middle Fraser River reach is the area between Hope and Quesnel. Here, the Fraser River fl ows through a series of deep canyons.…”

“…Without historical context, modern process studies might reach erroneous conclusions regarding natural processes, because the observed event sequences on which the conclusions are based are frequently much shorter than the time scale of trends in geomorphic systems (Church, 1980). Historical study and observation of legacy effects in geomorphic systems have the potential to provide context for the geophysical event sequences on which modern process studies are based (James, 2010).…”

Legacy sediment stored in sinkholes: a case study of three urban watersheds in Tennessee, United States of America

“…Intensive episodes of upland erosion related to natural or anthropogenic forcing may produce sediment waves that are gradually translated downstream affecting aquatic habitat and changing channel bed elevations (Gilbert, 1917;James, 2010). Some of the material left behind by sediment waves may become trapped on floodplains or in reservoirs and is referred to as legacy sediment.…”