Arctic river icings are surface ice accumulations that can be >10km(2) in area and >10m thick. They commonly impact the hydrology, geomorphology, and ecology of Arctic river environments. Previous examination of icing dynamics in Arctic Alaska found no substantial changes in extent through 2005. However, here we use daily time series of satellite imagery for 2000-2015 to demonstrate that the temporal persistence and minimum summertime extent of large icings in part of Arctic Alaska and Canada have declined rapidly. We identified 122 large ephemeral icings, and 70 are disappearing significantly earlier in the summer, with a mean trend of -1.60.9day(-1) for fully ephemeral features. Additionally, 14 of 25 icings that usually persist through the summer have significantly smaller minimum extents (-2.61.6%yr(-1)). These declines are remarkably rapid and suggest that Arctic hydroclimatic systems generating icings, and their associated ecosystems, are changing rapidly. Plain Language Summary Liquid water emerging from groundwater and flowing through Arctic rivers during the winter often freezes into large ice features, which are called river icings. These icings, which are found in nearly all parts of the Arctic, create wide, gravelly river channels that can be important habitat for animals. When icings melt during the summer, they help keep rivers flowing when other water sources are limited. Up until now, no study has systematically looked at whether these features are changing in response to warming temperatures. We use daily satellite imagery available over northern Alaska from 2000 to 2015 in order to test whether icings are becoming smaller or disappearing earlier in the summer. Of 147 features examined, we found that 84 are either becoming smaller (for those that persist throughout the summer) or are disappearing earlier (for those that fully melt each summer). None are becoming larger or disappearing later. These changes may be directly related to warming temperatures, but they may also be happening because climate change is altering how rivers and groundwater interact. If these trends continue, we may see changes in the form of many Arctic rivers and impacts on the habitat of animals like fish and caribou.