<p>Historically snowmelt-dominated rivers in European Russia are undergoing substantial changes in streamflow seasonality, with important implications for flood behavior and water resource management. This study focuses on the Moskva River, the primary tributary to the Mozhaysk Reservoir, to quantify projected changes in streamflow seasonality and flood characteristics under future climate scenarios and evaluate their implications for reservoir operations. Employing the HBV hydrological model driven by a multi-model ensemble of bias-adjusted CMIP6 climate projections (SSP1-2.6, SSP3-7.0, and SSP5-8.5), the research projects hydrological dynamics to 2100. Projections reveal a marked shift in streamflow seasonality. Under the SSP5-8.5 scenario, winter runoff is projected to increase by 73% due to warmer temperatures and a transition from snow to rain, while the historically dominant spring freshet is expected to decrease by 42%, consistent with reduced snow accumulation potential from lower winter snowfall. Despite the substantial increase in winter flow, enhanced warm-season evaporative losses result in an overall annual streamflow deficit relative to the historical baseline. Further analysis shows that while extreme precipitation (R95p) is strongly and positively correlated with flood characteristics in summer and autumn, the strength of this relationship does not systematically increase under higher emission scenarios. This points to a nonlinear basin-scale response, likely modulated by shifts in the spatial organization of heavy rainfall. These findings highlight a critical vulnerability in current reservoir operating rules, which have remained static since 1968. The altered intra-annual flow distribution and the projected annual volume deficit necessitate a fundamental reevaluation of water management frameworks to balance the capture of early-year inflows with the mitigation of evolving flood risks under nonstationary climate conditions.</p>