# Spatio-Temporal Variability of Water Temperature of Arctic Rivers in Russia over the Past 60 Years
## 5 Conclusions  
The highest river water temperatures in the Russian Arctic are reached in the rivers north of the EPR, especially in the Vychegda River and its tributaries, but also in parts of the Ob, Yenisey, and Lena basins south of the Arctic Circle. The size and flow rate of rivers appears to have little influence on the water temperature of rivers at similar altitude and latitude. The maximum differences in water temperature between rivers of different size are 3-4 K.  
The rivers Ob and Lena transport heat from the southern parts of their basins beyond the Arctic Circle and become warmer in their Arctic parts for 1-2 K compared to the rivers in the surrounding areas. However, this is not observed in the Yenisei River. This may in part be due to a lower number of gauging stations, especially in the lower reaches. The coldest rivers are located in the mountainous part of the Kolyma basin and are mainly small and medium-sized rivers. Other cold rivers are the Ob in its lower part and the rivers of the Pur and Taz basins. The water temperature in rivers of the Russian Arctic is largely controlled by air temperature. The water temperature of rivers in the Russian Arctic increased only slightly over the past 60 years, if at all. If present, increases are most noticeable in May, June, and October, reaching a maximum of 2.5 K (in Pure and Tas basins in June), mainly during the last 20-30 years (after a change point year). The largest increase occurred in the north of Western Siberia and in the European part of the Russian Arctic (excluding Kola Peninsula and Karelia) in May and June, i.e., in the areas where the relationship between water and air temperature is the closest. In more eastern hydrographic regions, the increases in water temperature are less pronounced, as an air-water temperature relationship is less pronounced. This is attributed to local factors, such as the presence of permafrost, latitude, and the gauge elevation. Nearly 70% of changes are statistically significant. The magnitude of the changes in relation to the change point years (mostly 1990-2000) and 1991 are almost identical. The largest changes in water temperature occur in the transitional seasons of the year, which, for most rivers, corresponds to floods in spring and receding runoff in autumn following the autumn floods. In summer, the mean monthly water temperature increased the most in the rivers of the Kola Peninsula and Karelia, where the high retention capacity of lakes naturally regulates flow rates. The standard deviation (SD) of water temperature changed from region to region of the Russian Arctic within (module) \(-\)0.5-0.6 \({}^{\circ}\)C after the change point year. In most rivers, the changes in SD are zero or negative. The rising SD of monthly water temperature averages indicates an increase in natural temperature oscillation in the Arctic part of Siberia, especially during May-June and October. Statistically significant changes in water temperature in different sites are generally higher than changes in the water temperature SD during the whole warm period. The intensity of changes in river water temperature is low enough. It is 0.31 K/decade (1961-2020) for the whole warm period in hydrographic regions. This value is lower than the intensity of water temperature changes in rivers south of the Arctic. After 1961, in some fairly long periods (>15 years), trends in various directions and intensities, including negative ones, can be observed. Hence, the intensity of the river water temperature changes in the Russian Arctic is less than in rivers in more southern regions. The application of a comparison of the characteristics of periods relative to critical years showed that, firstly, it is possible to identify more violations of uniformity when compared to the characteristics relative to 1990. Secondly, it is shown that the last 2 decades make the most significant contribution to the transformation of water temperature.