Abstract <p>This article is of methodological and theoretical nature; it contains the results of the development of terminology and typification of hazardous hydrological processes and events in the mouth areas of rivers. A comprehensive classification of hazardous hydrological processes and events, which takes into account their type and origin and the pattern of their impact on the natural and socio-economic complex of river mouths, is given; other approaches to dividing and grouping such processes with clarifications and new formulations of the concepts related to them are also considered. The list of the described hazardous processes includes floods and hazardous shallowing events of various genesis, as well as hazardous ice events and morphodynamic processes, negative changes in the moisture regime, hydrochemical regime, and water quality, and hazardous hydrodynamic events of marine origin. The main factors, mechanisms of development, and the pattern of negative consequences of these processes are analyzed. The theoretical assumptions are supported by extensive factual material on specific river mouths or by the results of regional generalizations. The results can be used in works on studying, monitoring, identifying, quantifying, and ranking hazardous hydrological processes and zoning territories and water areas subject to their impact.</p>