Detecting early winter open-water zones on Alaska rivers using dual-polarized C-band Sentinel-1 synthetic aperture radar (SAR)

Northern high-latitude river ice provides critical natural infrastructure for winter travel, commerce, hunting, fishing, and recreation in rural areas with little or no road access. Open water zones (OWZs) in river ice are dangerous for such travel and are most common during early winter. Changes in the occurrence and duration of OWZs may also indicate more widespread shifts in ice regimes across Alaska and other northern regions. To aid in detecting open water hazards and broader changes in winter conditions, we developed a supervised classification with a principal component analysis (PCA) using both polarizations of Sentinel-1 C-band synthetic aperture radar (SAR) dual-polarized data for rivers in early winter to discriminate between ice cover and open water. Previous SAR river ice classifications have focused on one or two river reaches often with an emphasis on moving ice during spring break-up, hampering generalization of these results to other rivers and seasons. To address this limitation, we used 12 reaches from eight rivers for training and validation with the aim to combine data from different river types to create an ice classification that could be applied to northern high-latitude rivers from October through January. The classification was trained using shore-based time-lapse photos, aerial photos, and on-ice observations, and validated with shore-based time-lapse photos and independent citizen scientists' photo observations. Overall accuracy for the classification ranged from 65 to 93% with a corresponding range of 0.31–0.84 Cohen's Kappa statistic (K̂). We report some ambiguity between open water and smooth ice, especially in slower-flowing parts of rivers. We conclude that VV and VH thresholds can therefore be customized to increase accuracy, depending on specific river attributes such as river morphology, silt/sediment load, and channel flow velocity. This classification, which allows for mapping long river reaches in low-light winter conditions, can be performed on historical Sentinel-1 imagery to determine areas that display open water year after year. Once customized to a particular river, it can be automated to provide current open water zone maps to Alaskan and other rural northern communities worldwide to aid safer travel on ice.