Аннотация:The main ambiguities related to the Usoi natural dam and the Sarez Lake evolution are
described. They are predetermined by insufficient knowledge of the geological structure
of the dam and valley slopes and by uncertain seismological conditions of the region. All
these uncertainties do not allow to guarantee long-term (decades – centuries) safety of
the Sarez Lake without special technical measures aimed to reduce risk of the Usoi dam
breach. These laborious and time-consuming measures could be implemented while the
situation remains stable.