Fluvial thermal erosion following ice breakups of the Lena River (Yakutia, Siberia) is a significant geomorphic process. During the initial stage of ice breakup, ice pushes up onto the river banks and produces large accumulations of ice and sediments that protect the islands' heads against the mechanical and thermal effects of the river's flow. This initial stage is relatively short and terminates only a few days after breakup begins. In the second phase of flooding, after the river ice has melted, the island heads become free of ice. Hence, when water levels are high, the floodwaters are in sustained contact with the frozen banks of the islands, causing efficient thermal and mechanical erosion of their banks. Such erosion may also occur later in summer, if there is a second discharge peak. Between 2009 and 2012, the retreat of the banks of the river islands displayed high interannual variation that is attributed to the variability of the duration and timing of the flood season. For a given island, the annual rate varied from 2m to 40m and the duration of active thermal erosion of the frozen islands varied from 6 days to 39 days. Copyright (C) 2014 John Wiley & Sons, Ltd.