Terrain Elevation as a Driver of Anthropocene Trends in the Runoff of Rivers: Insights from the East European Plain

Relief is an important driver in the spatial differentiation of river runoff and its regime both in the mountains and on the plains, which is most evident in arid and semi-arid regions of the Earth’s land. Based on 22 small and medium-sized rivers of the zones of forest–steppe and steppe in the temperate climate zone of the eastern part of the East European (Russian) Plain, within the Middle Volga region and the eastern part of the Don River basin, the role of this factor in the spatiotemporal changes in various key runoff parameters (annual average runoff (Q), annual maximum runoff (Qmax), and annual minimum runoff for both cold (Qmin-CP) and warm (Qmin-WP) seasons) between two baseline climatic periods of the Anthropocene (1961–1990 and 1991–2020) is considered using the average elevation of river basin (H) as its quantitative indicator and statistical procedures of regression and correlation analysis. It is found that in the interperiod trends of the Anthropocene, the H factor was the leading (and statistically significant) cause of spatial variability in the changes in Qmax in the forest–steppe zone, through its inverse relationship with H (with a 70% contribution of influence), Qmin-CP in the forest–steppe and steppe zones (with a 55–75% contribution of influence), and Qmin-WP in the steppe zone (with a 64% contribution of influence), through their direct relationships with H. It is also shown that H acted as an important factor (with a 47% contribution of influence) of statistically significant strengthening of the spatiotemporal coherence of Q and Qmax values between the studied periods, but only in the river basins of the Middle Volga region: during the period of the most active climate warming (1991–2020), the region’s upland rivers turned out to be more coherent in these two runoffs than the lowland ones. An ambiguous influence of H on the mutual correlation of all the examined runoff parameters over the baseline periods was revealed. The identified patterns are a consequence of the reaction of the complex altitudinal zoning of the plain’s landscapes mainly to climate changes, especially during the cold season (frequent thaws, decreasing soil freezing depth, etc.). The achieved results are intended to contribute to the understanding of the role of so-called “passive” driving forces in contemporary regional changes in river runoff.