Sediment transport and channel form in gravel-bed river meanders

Location

Continent: North America
Country: USA
State/Province: Colorado
City/Town: Rocky Mountain National Park
UTM coordinates and datum: none

Setting

Climate Setting: Semi-Arid
Tectonic setting: Craton
Type: Process

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Transport of individual grain size fractions per region of the channel for less than bankfull flow, roughly equal to bankfull flow, and greater than bankfull flow. Jordan Clayton, Georgia State University.

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Increases in bed load grain size towards the outer region of bend are balanced by differences in transport rates and changes in transport patterns with discharge, resulting in a volumetric balance between the inner and outer regions of the bend. Jordan Clayton, Geogia State University.

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Sediment laden water in the Colorado River in 2003. Jordan Clayton, Georgia State University.

Description

Fluvial geomorphologists tend to see the river for the rocks. This perspective is justified because the shape of the stream channel is dictated, in most cases, by the movement of sediment. A key objective of much fluvial geomorphic research therefore is focused on predicting how the streamflow, channel shape, and sediment distribution interact. In simple, experimental channels, feedbacks between these parameters determine the channel's equilibrium form. In most natural environments, however, these parameters are subject to large temporal and spatial variability in driving hydrologic processes, channel morphology, and land-use patterns, and are also affected by flow obstructions. For example, many channels have flow complications such as woody debris jams, tight bends, banks that have blown out, sediment that has been introduced, and perhaps even old cars that have become bed load. These complications make quantitative predictions of flow strength, bed load distribution, and geomorphic adjustment difficult. My research in the Colorado River in Rocky Mountain National Park examined these interactions for several bends of different curvature. I obtained a high-resolution dataset of depth, bed load, and surface versus subsurface grain size information, and documented how variations in sediment transport intensity allow stream reaches of differing morphology to transport equivalent amounts of sediment and thereby promote geomorphic stability. Bed load data taken throughout the reach indicate that the fine and coarse fractions of the load are differentially routed, such that fine grains are swept inward over the point bar, while coarse grains are routed outwards towards the pool. At bankfull flow, all size fractions of the available sediment are transported throughout the bend, but the median grain size of the bed load increases with distance towards the outer bank. This differential routing decreases slightly with increasing discharge beyond bankfull, as indicated by a shift in the locus of coarse particle transport from the outside of the bend inward to the channel center. Furthermore, while fine sediments are fully mobile throughout the bend, partial mobility of the coarse fractions of the load transitions to full mobility towards the bend's outer region. These variations in transport intensity lead to a volumetric balance of transported sediment between the inner and outer regions of the channel, which thereby promotes the geomorphic stability of the bend. I plan to continue research on this topic with the aim of connecting the short-term sediment fluxes with the longer-timescale morphological evolution of meandering systems.

Associated References:

• Clayton, J.A. and Pitlick, J. (2007), Spatial and temporal variations in bed load transport intensity in a gravel-bed river bend. Water Resources Research, v. 43, W024626, doi: 10.1029/2006WR005253.
• Clayton, J.A. and Pitlick, J. (2007), Persistence of the surface texture of a gravel bed river during a large flood. Earth Surface Processes and Landforms, doi: 10.1002/esp. 1567, Accepted, in press.

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