Tracking Hydrological Regime Shifts Through the Anthropocene: The Samara River, Southern Cis-Urals

The Samara River is a left-bank tributary of the Volga with a total basin area of 46,500 km2, of which 22,800 km2 is the gauged area analyzed in this study. This river system serves as a case study for examining hydrological transformations in the extreme southeastern region of European Russia (the Southern Cis-Urals) driven by progressive climate change and historical land-use shifts. To achieve this, the study analyzes long-term, station-based gauge observations of monthly river runoff for the period 1933–2022. The methodology employs a baseline set of standard statistical criteria combined with specialized hydromathematical procedures to evaluate runoff structure. The results demonstrate that while the river’s annual runoff lacked a statistically significant trend, its intra-annual structure underwent profound transformations. While April and May exhibited non-significant declines, the vast majority of months demonstrated prominent upward runoff trends. This systematic increase compressed the amplitude between hydrologically contrasting months by a factor of 2.5 to 3 from the historical climatological baseline (1933–1960) period to the modern baseline (1991–2020) period. Since 1989, the intra-annual variability coefficient has decreased by approximately one-third, especially during low-flow years. Concurrently, the spring flood contribution decreased by approximately 25%, and the frequency of prolonged, three- to four-month snowmelt-driven freshets during the 1987–2022 period doubled to ~22% compared to the preceding period, whereas the summer–autumn and winter low-flow shares increased substantially. Spectral analysis demonstrates a structural asymmetry in runoff generation mechanisms: low-flow dynamics are predominantly governed by long-term macrocycles (spanning 11 years or more), whereas maximum discharge exhibits a complex spectral configuration rather than a purely stochastic white noise process, reflecting an underlying statistical ambiguity between the applied criteria. Finally, five alternating multiyear flow phases occurred since 1933, characterized by low-flow conditions during 1933–1945, 1966–1984, and 2014–2022, and high-flow conditions during 1946–1965 and 1985–2013, with the respective contributions of the spring snowmelt flood and low-flow runoff differing substantially across these phases. This research advances the understanding of regional river runoff transformations in the Anthropocene, providing an objective basis for long-term water balance forecasting in the Southern Cis-Urals, strategic environmental management, climate change adaptation, and environmental risk assessment.