Research of the dune formation process on the branched reach of the Tom River according to peculiarities of its water and ice regime

Usually the bed loads moving in lowland rivers form clusters different in size and shape - bed forms asdunes, riffles, antidunes, etc. Dunes, consisting of coarse and fine sand particles, situated at the bottom of large lowland rivers are foundalmost everywhere. These forms are found less frequently in lowland river beds accumulated from fragmentary material as gravel andpebble. Such places are very few and we make an assumption that formation of gravel dune fields, rather small in area, is connectedwith specific hydraulic conditions that are different from the commonly known ones. The dune relief is not developed in general on thebranched reach of the Tom River between Vershinino and Kolarovo settlements, but there are three small areas with dunes consisting ofgravel material. These areas represent great interest because they are typical bed forms neither for the examined site, nor for the riverTom. Sites of three dune fields in low water season are free of water. It is unclear how dunes consisting of coarse gravel material appearand exist in places where water flows during spring flood usually are not sufficient for gravel dunes formation. In the last years, cases ofice jams in the main channel and some branches of the studied area have become more frequent. As a result of ice jams, flows inbranches can be significantly higher than in the absence of ice dams. Thus, the picture of flow distribution between the main channeland its branches becomes different and, depending on the location of the ice jam, some branch can have a function of the main channeland contain the prevailing proportion of the flow. In this context it is possible to assume that dune fields in the examined areas havebeen formed as a result of such flow redistribution from one branch to another. To test this assumption we have made an effort to reconstructthe real possible water discharges, which could form such dune fields in the considered locations of the river Tom, using existingdependences of the morphometric parameters of dunes on the hydraulic characteristics of the stream. We used the parameters of dunes,obtained by direct field measurements: the length and height of dunes, granulometric size composition of sediments from three considereddune areas, and cross profiles of the channel for each area, as the basic data for calculations. Our results give grounds to confirmthe assumption that the considered dune fields have been formed and, in particular, washed off as a consequence of ice jams formationin the studied locations.

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