When River Ice Breaks Faster Than Expected: One Week of Distributed Acoustic Sensing on the Sävar River in Sweden

In cold climates, rivers are affected by ice cover for several months a year, seasonally transforming hydraulic and hydrological conditions. This, in turn, impacts channel morphology and ecology. During ongoing climate warming, river-ice extent is declining and freeze-up and break-up patterns are changing. River ice break-up in the spring is considered the most dynamic period of the year. It is driven by thermal processes like surface melting in response to rising air temperatures and/or mechanical forces like increased discharge and flow-induced fracturing. However, these processes remain difficult to constrain with observations as field sites are difficult to access and instrument at a sufficient spatial coverage. Here, we present a comprehensive observational dataset combining seismic, Distributed Acoustic Sensing (DAS), and auxiliary measurements that captures the complete river-ice breakup process in a northern river.

We deployed a DAS system along a 400-meter, regulated reach of the Sävar River located at around 64 N^o latitude in northern Sweden. The fiber-optic cable configuration included a longitudinal section mid-channel on the river ice and a sawtooth pattern across the channel. Additionally, we installed eleven three-component geophones at key cable crossing points to collect benchmark seismograms. During our field campaign between 26 March and 3 April 2025, we captured the complete breakup of the ice cover. Ice failure began on 30 March, and the channel was ice-free on 3 April.

Detections with short-term over long-term averages (STA/LTA) and visual inspection revealed over 2000 ice cracking events. Frequency-wavenumber analysis of the DAS data along the longitudinal cable indicates the presence of the fundamental quasi-symmetric mode (QS₀) and the quasi-Scholte (QS) mode. We further discuss event location and waveform modelling to advance the characterization of crack event frequency and orientation (longitudinal vs. cross-channel). Our measurements allow us to asses the roles of environmental factors, particularly river discharge and temperature, in the breakup process. By resolving fine-scale ice fracturing processes, our results provide new constraints on the timing of river-ice breakup and the corresponding ice thickness evolution, with implications for flood hazard assessment, sediment transport, and river management in cold regions under a warming climate.