Debris Flow Activity and Specific Features of Debris Flow Formation in the Geysernaya River Valley (Kamchatka)

Debris flow activity in the Geysernaya River basin was mapped. Large-scale debris flow processes due to the collapse of fragments of the valley's left wall were observed in this area three times over the past 40 years. Three independent debris flow areas of the river basin were identified and described: the upper reaches of the river, the Levaya Geysernaya basin; the middle reaches of the river, below the dammed lake that formed in 2014; and the lower downstream part, below the dammed lake that existed in 2007-2014. Debris flow processes are typical of most tributaries of the Geysernaya River. Moreover, debris flows can be aseasonal in the left tributaries, because they are formed under constant heating and active gas-hydrothermal influence within the thermal fields. The landslide dam formed in 2014 is resistant to erosion, and an independent outburst of the dammed lake is unlikely. With further erosion of the 2007 dam and expansion of the breach in the dam body, collapse of its sides and debris flow in the lower part of the Geysernaya River valley should not be ruled out. Risks of rockfall, landslide, and debris flow processes, most likely on the left wall of the valley, should be taken into consideration in planning the development of recreation facilities.