Cut-Off Meanders Formation and Braided Reaches Development on Meandering Rivers

The conditions and reasons for the cut-off meanders formation, that create single braided reaches on large rivers with meandering channel, are considered. They appear when developed and steep bends, which have reached critical ratios of parameters in the process of their evolution (l/L > 1.4–1.7; l is the length, L is the step of the bend), are straightened. Cut-off bends are characteristic of rivers, where floodplains are flooded deeply and for a long time during high-water period. In this case, most often, the runoff is redistributed into the new straightening channel branch, although the old channel continues functioning. However, in many cases, in cut-off bends a larger part of water runoff remains in the old channel. This happens if the new straightening channel branch is located behind the reach of the bedrock bank, which exerts a directing (toward the old channel) effect on the flow; the new channel meanders and remains shallow. If the effective water discharge passes within floodplain edges, cut-off bends are formed during extremely high floods. At the same time, they exist for 10–20 years, while the old channel (the old bend) is filled with sediments and dies off, turning into a floodplain oxbow lake. The short-term existence of braided reaches (for a number of years) before the death of the old channel is possible when the loop-shaped bends are straightened due to the oncoming erosion of the banks. In any case, the development of cut off bends is favored by the discharge of clarified waters from the floodplain into the old channel. The presence of cut-off bends, i.e., forms of braided channel, is a factor in the dispersal of runoff along channel branches, which has a significant impact on their channel changes, development of bends and conditions for channel processes management while water management of rivers as a whole.

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