This typically occurs where streams carry a high relative quantity of silt and clay which is easy to transport but is difficult to re-erode once deposited on stream banks. Narrow, deep channels favoring a meandering pattern require appreciable bank strength. If stream banks are composed of loose gravel or sand of the same size range that is transported on the channel bed, channels tend to be wide, shallow, and braided. The occurrence of braiding has been related to the natural tendency for sediment transport to produce multiple depositional bars in sufficiently wide channels, which tends to split the flow as the bars grow ( 13, 15). For a stream of a given flow discharge, steep channel gradients and large ratios of channel width to flow depth are associated with braiding, with the converse for meandering. Both empirical studies and theory have helped to define the conditions that control channel pattern ( 11 – 15). In settings where rivers are not laterally confined by resistant valley walls either the planform generally displays sinuous meandering of a single channel or the river splits into multiply interconnected braided channels. These complications have been incorporated into increasingly detailed models of stream meander evolution ( 10). Interaction of the bend-induced flow and bed topography with superimposed alternate bar bedforms complicates the flow and bed topography in wider meandering channels ( 8, 9). Linear theory of flow in bends ( 5) has permitted construction of simulation models that replicate many aspects of meandering behavior, including meander cutoffs, creation of oxbow lakes, and patterns of floodplain sedimentation ( 6, 7). ![]() The secondary current has an intrinsic downstream scale related to flow velocity and depth this results in gradual increase in bend amplitude and propagation of the meandering pattern upstream and downstream. When flow enters a channel bed, a helical secondary current is set up that increases flow velocity and channel depth along the outer bank in proportion to bed curvature, which encourages bank erosion. The mechanics of formation of meanders is reasonably well understood ( 4). Image is a portion of NASA HiRISE image PSP_006683_1740. ![]() Rough terrain at upper right and lower left is due to wind scour. A cutoff may have occurred shortly before flow ceased above location “X,” resulting in abandonment of the loop lying below. The low curvilinear ridges interior of the main sinuous ridge are remnants of the meander loops as they grew through bank erosion along the outside of bends. The channel bed is now a ridge (in inverted relief) because wind erosion has removed finer sediment from the floodplain and surrounding terrain. Red arrows point to representative locations along channel. Fossil highly sinuous meandering channel and floodplain on Mars.
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