Depositional processes and sediment supply in resistant-boundary channels: Examples from two case studies

Abstract

Coarse-grained deposits produced by a rare, extreme flood in the Mt. Everest region of Nepal and fine-grained deposits produced by frequent, moderate floods along the Colorado River in and near the Grand Canyon, U.S.A. illustrate a wide range of depositional patterns, processes, and mechanics along resistant-boundary channels. Deposition processes in each of these case studies were strongly influenced by sediment supply. The availability of sediment was an important factor in both case studies in determining the occurrence, location, stability, and sedimentologic characteristics of deposits produced by their respective floods. A glacial-lake outburst flood in the Mt. Everest region of Nepal eroded, transported, and deposited large quantities of boulders, cobbles, pebbles, and sand along the outburst-flood route. Depositional macroforms produced by this flood include: (1) expansion bars at abrupt expansions immediately downstream of constricted reaches, (2) longitudinal bars at local flow expansions along valley margins, (3) point bars along the inner margins of valley bends, (4) pendant bars immediately downstream of obstructions, and (5) imbricate clusters of cobbles and boulders immediately upstream of obstructions. Along the upper 16 km of the outburst-flood route, deposition is more extensive and deposits are thicker, are more poorly sorted, have more finely-skewed distributions, and have a higher percentage of fine-grained sediment than deposits below 16 km. Additionally, deposits along the upper 16 km of the flood route are reverse graded and coarsen in the downstream direction, whereas deposits below 16 km lack grading or are normally graded. These deposit characteristics suggest that the flood waters along the upper 16 km had higher sediment concentrations and were closer to their transport capacity than the flood waters below 16 km. Most resistant-boundary channels are supply limited with respect to entrainment and transport of cobble- and boulder-sized particles during frequent, low to moderate flows. However, superimposed on this supply limited coarse-grained system is a dynamic fine-grained system in which eddy bars along the channel margins are eroded and redeposited during the rising limb of a given flood. Eddy sand bar deposits along the Colorado River in and near the Grand Canyon are an excellent example of this dynamic erosional and depositional cycle that occurs during frequent, moderate floods. The erosional and depositional cycle of eddy bars during a given flood is strongly controlled by sediment delivered to the plunge pool adjacent to the eddy bar. Sediment delivered to the plunge pool on the rising limb of the flood alters the flow patterns and hydraulics in the pool and causes the eddy bar to scour. As discharge increases, plunge pools are scoured and deposition resumes in the recirculating eddies. If fine-grained sediment supply is reduced along a river because of flow regulation structures, this dynamic cycle of erosion and deposition associated with pools and eddy sand bars no longer occurs and the eddy bars become stable.