Natural drivers of high floods in the middle reach of the Amga River, Central Yakutia

Abstract

Major flooding in the middle Amga River occurs in late spring, early to mid-May. Spring flood is the main hydrological season in the middle and lower reaches of the Amga River, marked with the annual maximum of the level and discharge of water at the Amga hudropost. In the last decade, spring freshet peak was observed in first 10 days of May, which was 6-8 days earlier than average. The Amga River channel in its middle reaches hosts multiple fluvial forms – potential ice jam concentrators, i.e. shallows and sharp channel bends, but ice jam floods are not observed annually. The weather conditions of the high water period can play a decisive role in the formation of floods, simultaneously favoring rapid snowmelt in the upper reaches of the river and the formation of jams in the middle reaches. A criterial analysis showed higher snow accumulation in the upstream catchment section, lower air temperature in the last 10 days of April and higher air pressure in mid-May in central Yakutia in the years with high floods in the middle Amga River reaches. Most floods were observed in the years with total winter precipitation over 120 mm in the upper part of the catchment. Ice thickness appear to have no influence on the ice jam probability of occurrence. Maximum water stage with return period of 100 years, calculated using ‘peak-over-threshold’ approach, is 145.91 m, and with return period of 1000 years – 146.88 m.