Eighteen-year Changes in Ice Breakup and Freeze-up on Canadian and Alaskan Rivers Wider Than 150 m Using MODIS Imagery

Annual river ice breakup and freeze-up in North America have major implications for northern ecosystems and infrastructure. Additionally, both processes are impacted by air temperature and are therefore important to study as the Arctic climate changes. Quantification of river ice breakup and freeze-up in the Arctic has been spatially limited due to the lack of ground-based observations. Previous studies have used imagery from the Moderate Resolution Imaging Spectroradiometer (MODIS) to study ice breakup and, less commonly, freeze-up on individual Arctic rivers wider than 500 m. However, rivers narrower than 500 m are much more common than wide rivers and are therefore important to study. Freeze-up quantification on a large scale has previously not been attempted due to the complexity of freeze-up processes and poor optical image quality during the fall. Here we present an algorithm that uses MODIS imagery to detect ice breakup and freeze-up on all Alaskan and Canadian river reaches wider than 150 m from 2000 through 2017. The algorithm uses reach-specific NIR-band thresholds to account for the inclusion of vegetation in observations of rivers that are sub-pixel in width. Interannual variability is clearly visible in breakup date results in Alaska. Validation of breakup results for ten randomly selected Alaskan river reaches against low-cloud Landsat images have an overall accuracy of 96.1% and a kappa statistic of 0.92. Freeze-up is detectable in fewer reaches due to poor optical image quality. However, findings from this study provide the first large-scale historical record of how river ice has changed on boreal North American rivers wider than 150 m over the last eighteen years. The methods presented here can be expanded to quantify breakup and freeze-up on pan-Arctic rivers.