River ice break-up pattern in Arctic river, Case study: Tornio River and its main tributaries

Extreme hydrologic events are influenced mainly by the river ice processes in cold climates. River ice break-up is particularly notable in Arctic regions since it commonly occurs around the time of the spring freshet. The most significant hydrologic events in the Nordic and Arctic rivers occur following the spring ice jam break-ups. The break-up patterns dominate the hydrology and ecology of downstream parts of a river. Rivers that are already receiving a lot of snowmelt runoff discharge may experience backwater carried on by jammed ice. Neighborhoods along rivers could flood due to the high water, which commonly exceeds open-water peak values in many locations. When ice jams are broken up, the physical action of the ice also results in significant infrastructure and property damage, and disruptions to transportation systems and hydropower production have additional financial costs. Consequently, understanding the ice jam break-up patterns in a riverine system is essential for mitigating such damages. In this study, Tornio River's ice break-up patterns are analyzed since 2002. Tornio River is a transboundary, and Arctic river on the border of Sweden and Finland which discharges into the Gulf of  Bothnia. Tornio river and its main tributaries show different behavior regarding the melting season due to their geographical location, morphology, and delay in reaching the positive temperature phase.

The nine gauges over Tornio River’s tributaries, including Abiskojokk, Abisko, Karesuvanto, Lannavaara, Junosuando, Kallio, Pajala, Naamijoki, and Kukkolankoski have been monitored from 1^st Oct 2002 to  30^th Sep 2020. For this purpose,  daily temperature and flow data have been collected from MODIS (assimilated and gap-filled), Swedish Meteorological and Hydrological Institute (SMHI), and Finnish open hydrology data (provided by SYKE). Furthermore, to observe the events visually and to verify the break-up patterns, Sentinel -1 radar data were used in Google Earth Engine within its period of availability (2016-2020). A tool was developed to estimate the freezing period based on the slope of the annual flow pattern and the consecutive dates following the same slope to find the break-up. The results indicate that, on average, ice in Kukkolankoski, Pajala, Kallio, Naamijoki, and Junosuando stations which are located in lower latitudes, breaks up earlier than Abiskojokk, Abisko, Karesuvanto, and Lannavaara which are relatively situated in higher latitudes. The higher the latitude, the later the ice tends to melt, as it was hypothesized. Additionally, the breakup dates show more dispersed results in Karesuvanto, Kallio, and Abiskojokk than in other stations. It indicates that the breakup pattern in the three mentioned stations is not as stable as in other locations and their pattern changes over time. The results of temperature data show almost the same pattern but with a delay prior to discharge results. On average, 10 days after reaching the positive temperature phase in each station the ice melts.