Apparently, it is a phenomenon that happens ever year, typically once or twice. They call it jokulhlaup, which is Icelandic, because this is also something that is observed in Icelandic glaciers. A very good description of how the seasonal glacier release and consequent flood waters is here for anyone interested:   https://alaskapublic.org/2020/07/28/a-glacial-dam-near-juneau-could-burst-any-time/
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