Morphology and structure of the valley bottoms of the steppe rivers with beaded channels (on the example of the kardail and Kupava Rivers, Khoper-Buzuluk Lowland)

Beaded channels, which are alternating lake-like extensions (beads) and narrow runs between them, are widespread on small rivers in the steppe zone, but there are different points of view regarding their genesis. To establish the origin of beaded channels, a detailed topographic survey was conducted in two key sites of the valleys, sediments in sections and boreholes were studied, and different versions of the origin of the channel extensions were analyzed. The most ancient rear part of the wide valley bottoms is occupied by the first terrace – a leveled surface with relict cryogenic microrelief and long beaded oxbows, covered by loess-like deposits. The high-level floodplain forms a belt of river meandering at the level of the big meanders, it differs slightly from the terrace in height, but beaded oxbow lakes, ridges and small depressions are clearly visible on it. The middle-level floodplain was formed during the period of decreased discharge. Its fragments are located within the belt of displacement of second-order meanders. The low-level floodplain formed as a result of overgrowing of the former bottom of the wide channel. The floodplain of all levels and the modern channel are underlain by dense silt and loams with a total thickness of more than 6 m, similar in composition to the deposits of the first terrace. The channel lag facies formed by bedload sediments is not distinguished in the deposits. The beaded structure of the channels is maintained due to the absence of bedload sediments in the modern channel. Beaded-shaped oxbow lakes on the high-level floodplain and terrace may have a cryogenic origin, since permafrost existed during the formation of these surfaces, and the fine-grained texture of the underlying sediments is favorable for ground ice aggregation. However, they acquired a bead-shaped form after the channel was abandoned. The possible cryogenic origin of beaded extensions of the modern channel requires a more detailed study of the age and structure of the enclosing sediments. The formation of deep extensions of the modern channel is not associated with silting of the riffles, since the thickness of the agrogenic silt does not exceed 1 m.

Photographs of distinct channels and cross-sections of the valleys of the Kardaila and Kupavna Rivers are available at: https://geomorphology.igras.ru/jour/pages/view/dopmat

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