Fifty-year dynamics of the Lena River islands (Russia): Spatio-temporal pattern of large periglacial anabranching river and influence of climate change.

The Lena, a large river that drains the northern coldest region of the Northern Hemisphere, is deeply influenced by the continuous permafrost and degradation of the frozen ground has been shown to be the main cause of the marked increase in water discharge. The first objective of this study conducted on the middle Lena was to analyze the island dynamics for the last 50 years (1967 to 2017). Several morphological parameters were surveyed using a GIS on seven series of aerial photographs and satellite images of a 100 km-long reach: island size, eroded and deposited areas, position and morphology of the islands. This approach enabled the identification of evolutionary models. Our second objective was to evaluate the potential impact of ongoing climate change. We analyzed morphological parameters with respect to two main factors: efficient discharge (bar-full, bankfull and flood discharges) and water temperature. A potential erosion index (PEI) was calculated by coupling the duration of discharge exceeding the bar-full level and water temperature. The results identified several morphological changes that occurred at the end of the 20th century: an increase in the number of islands, greater eroded surfaces and accelerated migration of islands. Comparing the dynamics of islands with and without permafrost is a good indicator of their sensitivity to climate change. A major change was observed concerning the erosion and migration of islands with and without permafrost. This evolution seems to be linked both with the duration of the discharge that exceeds the bar-full level and with the number of flood peaks. The water temperature in May and August have a major influence on permafrost islands that become increasingly destabilized. Thus, as large rivers are assumed to slowly react to climate change, the recent changes in the Lena River prove that the global change deeply impacts periglacial rivers.