Changes

==Braided-channel deposits==

==Braided-channel deposits==

Braided channels are marked by successive divisions and rejoinings of the flow around [[alluvial]] islands. Most braided river channels are characterized by a dominant bedload transport of sediment, high variations in water discharge, high downstream gradients, and large width-depth ratio of channels. Most braided rivers display rapid and continuous shifting of sediment and position of channels. These channels are found in all climate zones, but, because of their dependence on erratic discharge and high bedload, they are most common in arid and arctic settings. Braiding characteristics of the channel often extend all the way into the delta plain. However, one of the largest braided channels in the world, the Brahmaputra River, has formed in a humid climatic setting. Lateral migration of c annels can be dramatic, as in the Brahmaputra River, where lateral migration rates of several thousand meters during a single flood are not uncommon.<ref name=Coleman_1969 /> In the Rosi River (a tributary of the Ganges River), lateral migration of the channel over the past two centuries has been about 170 km; in a single year the channel may shift over 30 km laterally.<ref name=Reineckandsingh_1973>Reineck, H. E., and I. B. Singh, 1967, Primary sedimentary structures in the Recent sediments of the Jade, North Sea: Marine Geol., v. 5, p. 227-235.</ref> Because of high lateral migration rates and shallow depths of scour, most braided-channel deposits display high lateral continuity but are rather thin (rarely over 30 m thick).

Braided channels are marked by successive divisions and rejoinings of the flow around [[alluvial]] islands. Most braided river channels are characterized by a dominant bedload transport of sediment, high variations in water discharge, high downstream gradients, and large width-depth ratio of channels. Most braided rivers display rapid and continuous shifting of sediment and position of channels. These channels are found in all climate zones, but, because of their dependence on erratic discharge and high bedload, they are most common in arid and arctic settings. Braiding characteristics of the channel often extend all the way into the delta plain. However, one of the largest braided channels in the world, the Brahmaputra River, has formed in a humid climatic setting. Lateral migration of c annels can be dramatic, as in the Brahmaputra River, where lateral migration rates of several thousand meters during a single flood are not uncommon.<ref name=Coleman_1969>Coleman, J. M., 1969, Brahmaputra River: channel processes and sedimentation: Sedimentary Geology, v. 3, p. 129-239.</ref> In the Rosi River (a tributary of the Ganges River), lateral migration of the channel over the past two centuries has been about 170 km; in a single year the channel may shift over 30 km laterally.<ref name=Reineckandsingh_1973>Reineck, H. E., and I. B. Singh, 1967, Primary sedimentary structures in the Recent sediments of the Jade, North Sea: Marine Geol., v. 5, p. 227-235.</ref> Because of high lateral migration rates and shallow depths of scour, most braided-channel deposits display high lateral continuity but are rather thin (rarely over 30 m thick).

[[file:M31F2.jpg|thumb|200px|{{figure number|1}}Photographs of bedding in a braided channel deposit. A. Large-scale cross-bedding in the lower part of a fining-upward cycle on a braided channel. B. Trough-shaped cross-bedding in lenticular sets that form the overlying zone in a fining-upward cycle of a braided channel. C. Ripple drift bedding separated by parallel sand laminations.<ref name=Colemanandprior_1981>Coleman, J. M., and D. B. Prior, 1981, Deltaic environments of deposition in P. A. Scholle and D. Spearing, eds., Sandstone depositional environments: AAPG Memoir 31, p. 139-178.</ref>]]

[[file:M31F2.jpg|thumb|200px|{{figure number|1}}Photographs of bedding in a braided channel deposit. A. Large-scale cross-bedding in the lower part of a fining-upward cycle on a braided channel. B. Trough-shaped cross-bedding in lenticular sets that form the overlying zone in a fining-upward cycle of a braided channel. C. Ripple drift bedding separated by parallel sand laminations.<ref name=Colemanandprior_1981>Coleman, J. M., and D. B. Prior, 1981, Deltaic environments of deposition in P. A. Scholle and D. Spearing, eds., Sandstone depositional environments: AAPG Memoir 31, p. 139-178.</ref>]]