Mendenhall Glacier and other glaciers above Juneau, Alaska, while rapidly receding are trapping meltwaters that are more frequently being released as devastating ‘glacial outburst floods’ in mini-examples of the processes that produced the Ice Age Floods. The Washington Post recently produced a detailed article about the Aug. 5th flood that roared through Juneau on the order of a 500-year flood. Excerpted from Washington Post, Sept. 4th article by Joshua Partlow
On the morning of Aug. 4 … “Looks like the basin is going.”
The U.S. Geological Survey (USGS) gauge at Mendenhall Lake — at the foot of the glacier — was showing water levels rising sharply. … All summer, a jumble of icebergs and meltwater had been filling Suicide Basin. And when it eventually flushed out, as it had done more than 30 times since 2011, the water would pour into Mendenhall Lake and down the river to Juneau.
… The glacier normally served as a dam for that reservoir of ice melt and rainwater, but when enough of it accumulated, the tremendous pressure could lift the glacier and let water escape underneath. This was known as going “subglacial.” When that happened … the passageway within the glacier can rapidly expand, emptying billions of gallons of water downstream in a matter of hours … About a week later,water began overtopping the ice dam and flowing down along the glacier’s flank.
When this overtopping had happened in two previous years, the water found its subglacial escape hatch about a week later. But each year the glacier is changing, and the holes made the summer before may be gone. No one knew exactly when it might burst. On Aug. 4, with lake levels rising, the National Weather Service issued a warning predicting that Mendenhall Lake would peak the following evening around 10.7 feet — about five feet above its typical level … the next day … observing the raging river as water levels surpassed that initial projection and then kept going beyond the 12-foot record set in July 2016. Before the night was over, it would rise three feet higher.