Floodplain-channel braided reaches: forming conditions and hydrological-morphological characteristics

For the first time, the conditions for floodplain-channel braided reaches formation are analyzed, the criteria for their identification as a structural level of channel processes are justified, and their connection with river water content, channel regime, flooding of floodplains during high water periods and morphology of valleys is discussed. This type of braided reaches is characteristic of wide-floodplain channels, being a specific form of meandering processes of medium and large rivers, such as the Upper and Middle Ob’, Vychegda, and Oka rivers. They are less common for rivers with braided channel and could be found only on large and largest ones (the Severnaya Dvina, the Lower Lena rivers) and in meandering channel branches of split channels (Yuganskaya, Malaya, and Gornaya Ob’ rivers). Such floodplain-channel braided reaches are the result of evolution of meanders or island-braided channels; they are formed as the river passes from one side of the valley to another due to the mismatch between channel and floodplain water flows during seasonal and flash flood periods. They could be formed if the effective water discharge occurs when the floodplain is inundated. They also arise: downstream large headlands of bedrock banks or bends of the valley, which have a directing effect on the flow, deflecting it towards the floodplain and contributing to higher water content in the floodplain channel branches and turning them into river channel branches; as a result of intra-floodplain interceptions of channel branches of split channel and floodplain channel branches; during the formation and evolution of cut-off bends; at confluences of large rivers due to the development of “delta” branches (Ob’ and Tom’ rivers, Amur and Bureya rivers). In floodplain-channel braided reaches with three or four branches they have a polygenetic character. They are rare in incised channels, being associated with large local expansions of channels (the Yenisey River, the Upper Lena River) as a result of water stream spreading and its rising because of the downstream narro wing by bedrock banks.

Between channel branches of the floodplain-channel braided reaches there are large island massifs of complicated shape, with much less ratio of their length (branching step) and width than the optimal one, i.e. 3–4, characteristic of island-braided channels of a drop-shaped or spindle-shaped form. The branches of floodplainchannel braided reaches have their own series of morphodynamic channel types (up to 3–5 bends or islandbraided reaches), the parameters of which depend on the water content of channel branches.

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