Temporal transformations of large braided river channel (near Aldan reach of the Lena River as an example)

Abstract

River channels characterized by the multi-thread patterns are most complex form points of view of morphology, structure and deformation regime. The paper presents an analysis of the largest river channel deformations at the adjoint braids with an example of the Lena River reach near its confluence with the Aldan River including the three consecutive braids – Haityallah, Omulagan and Arbyn. During the 20 th century and the first decade of the 21 st century the studied Lena River reach has experienced periodic shifts of the main arm, associated changes of discharge ratios between arms within braids and the resulting relocation of the navigable fairway track. Temporal patterns of discharge fluctuations in the Lena and Aldan Rivers do not coincide, causing alternations of the dominant influence of the main river flow onto its tributary or the opposite during the peak discharge periods. It is shown that the intermittent phase of temporal transformation of the adjoint braids channel pattern into the parallel arms channel pattern occurs between the longer stages of dominant development of left or right arms. Duration of that phase depends mainly upon the average water discharge relatively to its long-term fluctuations (increases during higher runoff periods and decreases during lower runoff periods), flow backing conditions caused by the inflowing tributary (the Aldan River) affecting the entire length of the studied Lena River reach and local channel deformation conditions. In general, such a transitional phase of the channel development (between the alternating stages of dominance of left or right arms within the adjoint braids chain) can last for significant period of 10-15 years or even more. Duration of the transitional phase is comparable to that of the alternating stages of dominance of left or right arms on large- or medium-sized rivers where parallel arms pattern of the multi-thread channel does not develop. In turn, channel deformations at the adjoint braids exert significant influence on development of main arms of the internal delta situated at the confluence of the Lena and Aldan Rivers, where the main control is associated with alternation of flow backing or no backing conditions related to different discharge ratios between the main river and its main tributary.