From Intermittent Observations to Continuous Monitoring: Advancing Under-Ice River Flow Measurements

Ice cover significantly alters river flow dynamics by introducing friction at the ice-water interface, shifting the high-velocity core closer to the riverbed, and modifying shear forces that influence sediment transport. Climate change further complicates these processes as winter base flow increases and ice conditions change. Rising temperatures reduce the extent and stability of river ice, making discharge patterns more variable and impacting under-ice sediment transport processes. Therefore, real-time, continuous monitoring of under-ice flow and sediment dynamics is essential. As part of an experimental study, a new measurement approach was tested against the traditional, labour-intensive method in a Finnish river under mid-winter conditions. Instead of conventional cross-sectional stationary measurements taken from ice-auger holes at one-metre intervals once or twice per winter, the new approach used side-looking acoustic Doppler sensors mounted on aluminium frames, with three sensors per frame evenly spaced. These frames were placed vertically into the river along the outer bank through large holes in the ice. This enabled the sensors to measure flow across the entire horizontal cross-section and at three depths: near the ice surface, mid-water column, and near the riverbed. The sensors remained in the river for several weeks, continuously recording under-ice flow and sediment transport conditions. This pilot deployment aimed to assess whether these sensors could replace the conventional method for long-term,  enable continuous monitoring of under-ice flow conditions and possibly reveal previously unresolved temporal variability in under-ice hydraulics and sediment transport, including short-lived flow pulses, vertical velocity redistribution, and event-scale sediment mobilisation that are not captured by episodic winter measurements. Traditional cross-sectional measurements were conducted at sensor locations to compare flow dynamics, data accuracy and spatiotemporal resolution. The results will help evaluate the feasibility of continuous hydrological monitoring in ice-covered conditions, which remains challenging today.