It’s an “Outburst Flood” or GLOF (“glacial lake outburst flood”). The weather.gov page explains in one graphic:So there is the famous Mendenhall glacier. But at some point, the basin gets so full, the water from it starts flowing underneath/through the glacier into Mendenhall lake. There is a river from Mendenhall lake out to the ocean. Half the people in Juneau live in the Mendenhall Valley where that river runs directly through.