We examine the diversity of formation conditions for river ecosystems within the boundaries of volcanic territories of Kamchatka in the context of the specific changes in water discharge, turbidity and sediment yield, the temperature and channel regimes as well as in chemical composition of river waters. On the basis of ranking the factors under consideration according to the degree of impact on the state of communities of aquatic organisms, we explore the possibilities for a classification of rivers. An analysis is made of the ecological consequences of the manifestation of volcanic processes for the diversity, structure and spatiotemporal distribution of the fish fauna. An assessment is made of the conditions under which there is taking place a degradation and extinction of the ichthyofauna. We suggest a subdivision of the streams of the volcanic territories into three groups: without specific manifestations (ordinary rivers), with volcanic manifestations, and volcanic rivers.Keywords: rivers, river ecosystems, Kamchatka, salmonid fishes.