Changing cold-region rivers: Flow characteristics and sediment transport beneath ice cover and shifts in discharge regimes

Rivers in cold regions are integral to the global hydrological cycle, yet their flow and ice regimes are undergoing rapid transformation under anthropogenic climate warming. The Arctic has warmed nearly four times faster than the global average, intensifying hydroclimatic extremes and altering processes that regulate river discharge, ice formation, and seasonal flow dynamics. Earlier snowmelt, increasing winter rainfall, reduced snow storage, and more frequent freeze-thaw cycles are weakening flow seasonality and shifting runoff peaks toward winter and early spring. Concurrently, river ice cover is becoming thinner, shorter in duration, and reduced in extent, modifying freeze-up and breakup dynamics, sediment transport, and ecological conditions. Despite growing recognition of these large-scale changes, field-based understanding of under-ice hydraulics and sediment processes remains limited.

This study investigates how changing hydroclimatic conditions influence river flow regimes and sediment dynamics in cold-region rivers of Finland (60°–70°N). An integrated, multi-scale approach is applied, combining (1) statistical analyses of multi-decadal discharge records at the watershed scale, (2) in-situ winter measurements of under-ice flow and sediment transport, and (3) spatial analyses of flow structure and turbulence beneath ice cover across a meander bend. Together, these complementary methods provide new insights into how climate-driven hydrological shifts are reshaping discharge regimes and governing flow and sediment processes during the ice-covered season. The results contribute to improved understanding of winter river dynamics in rapidly warming cold regions.