ИСТИНА

Volcanism is the unique factor of river runoff components formation and variability. Analysis of water runoff in major World rivers between strong volcanic eruptions in XIX and XX centuries demonstrates statistically significant dimension of water runoff in some river systems. Volcanic eruptions are the largest planetary source of sediments. A huge amount of friable volcanic deposits come onto the catchment surface and could be transported by water easily. As the result, rivers draining volcanic areas are characterised by the maximum specific suspended sediment yield (SSSY) among mountain rivers, and SSSY can exceed 106 t km-2 yr-1. Potential sediment washout in the Eastern volcanic region of Kamchatka is the maximum one in the Kamchatsky Krai (Russian Far East) and varies between 1,000 and 5,000 t km-2 yr-1. The average maximum suspended sediment concentration (SSC) is 1–10 kg m-3 in rivers draining volcanic flanks, while in some water courses SSC can approach 200 kg m-3. The maximum SSY is usually measured after the major volcanic eruptions in the Kamchatka River. At the same time water runoff is usually decreased. Reduction or destruction of the river feeding source could also be the reason of water runoff decrease in small rivers basins. Volcanic eruptions influence on hydrological regime of rivers through changes of transpiration and infiltration conditions. In addition, rivers with specific features of hydrological regime are formed on volcanic flanks. They are characterised by episodic surface runoff and significant underflow. Maximum water discharge and SSC are usually observed in 12:00–18:00 in these rivers due to air temperature reaches its maximum, and intensity of snow melting is also the maximum correspondingly. In some cases such as snow and ice melting during eruptions in winter and spring time or heavy rainfall friable volcanic deposits start to move and form volcanic debris flows (lahars), which become new source of sediments in river channel then. Emission of acid gases to the atmosphere during eruption impacts on chemical composition of rain and melt water, and surface runoff. The acidity, F, Al, Fe, Mn, Cd, Cu and Pb content could exceed background level significantly after tephra fallout.