The transgressive and regressive movements of the islands and island clusters in the braided riverbeds at the wide flood-plains

Abstract

The mechanism of the islands and island clusters movements is suggested. The velocity field and circulating flows usually lead to washing of their upper part and to formation of spits in their tails which result in the island's displacement. If the sediment run-off exceeds carrying capacity of the river, then backwater due to the island existence causes accumulation at the upper part of the island and its regressive displacement. Symmetrically braided river bed expresses directional development of the bed deformation (the sediment accumulation and the bottom level rising). Regressive displacement often is manifested in the formation of midstream sandbank upstream the island due to water surface inclination. The backwater appears at some distance from the head of the island and the flow is directed to the one or another braid during different phases of a regime. These flows are carrying sediments out from under the ridge, determine the midstream sandbank stabilization, and prevent its coming onto the island. Localities of the island's regressive movement were determined: in the periphery of the bed, in the zones of backwater upstream the confluence, before the valley bottom narrowing, downstream the local sources of sediments influx. Conditions of island stabilization depend on the meanders formation in the braids circumflexing the island, geomorphologic limitations (transition from the wide flood-plain to incised river-bed, constraints by bedrock coast promontories), and transition from the braided bed to the wandering or relatively rectilinear not braided one. The meander bars and midstream sandbanks consecutive coming on to the point of braiding from the upstream reach is also significant.