Channel Morphology and Flow Distribution between Islands in Complex Parallel-Branch Bifurcation in the Lower Lena and Their Long-Term Variations

The first full morphological–hydrological characteristic is given to complex parallel-branch bifurcations of the lower Lena, which have formed at the total (taken with islands) channel width in individual links from 12.6 to 28 km and their length from 36 to 45 km. The number of islands in the branching links is 29 in the Upper Monastyrskii and more than 40 in the Cherpalskii bifurcations. It is shown that, all over the length, two branches with largest water abundance can be identified—the left and the right—of which the latter, navigable in all links and more abundant in water, runs along or near the right, bedrock bank. The left branch concentrates the runoff of water and sediments from the Vilyui, the left tributary of the Lena; this, along with the sediment-removing role, determines its shallowness and higher secondary branching. The links of the branches are separated by short segments of channel narrowing down to 5–8 km, in which two-midstream flow persists. It is shown that groups of islands (archipelagoes) in the middle of the river between the main branches are separated by relatively low-flow central branches subparallel to them (overall, within 10–15% of river flow), numerous transverse channels between islands and divided into into two parts (upstream and downstream) by diagonal branches, through which water partly redistributes between the main branches. It was shown that, at many-year scale and at seasonal runoff variations, the water abundance in the main and subparallel branches is practically constant, while the discharge in diagonal branches varies periodically from 10–15 to 30–35%, depending on channel transformations in the right main branch. This coincides with the periodic redistribution of water discharges, the development and shallowing of channels in secondary branching of the main channel, which lie near the sites of separation of diagonal branches from them. The revealed regularities for the first time give data on the channel regime of parallel-branch bifurcation of the channel of a large river, which is of great importance for the improvement of navigation in the Lower Lena