Residents of Juneau, Alaska, are currently facing a critical situation as authorities have issued an urgent evacuation order. This comes in the wake of an overflowing glacial lake that has sent significant volumes of water surging down the Mendenhall River. Meteorological forecasts have warned that river levels may reach record-breaking heights between 8 a.m. and noon local time on Wednesday, posing a serious threat to homes and roads in the state capital, which is home to over 30,000 residents.
Flooding events have become a troubling pattern in Juneau since 2011. However, the past few years have witnessed unprecedented surges, primarily driven by rising temperatures that are causing glaciers in the region to melt at an alarming rate. According to the Environmental Protection Agency, Alaska has experienced warming at a pace faster than the global average, making it the state most affected by climate change.
Last August, Juneau suffered from severe flooding caused by the glacial lake, which inundated several hundred homes with four to six feet of water. Fortunately, there were no reported fatalities or injuries, but the city responded by constructing a temporary levee along the river to mitigate future risks. Below, we delve into the details concerning these floods and their implications.
So, what exactly is a glacial lake outburst flood, commonly known as a GLOF? As glaciers melt, they retreat uphill, leaving behind a bowl-shaped depression in the valley where the ice once resided. Over time, meltwater accumulates in this depression, forming a lake. However, the lake's sides are often unstable, composed of loose dirt, rock, or ice. If an avalanche, landslide, or a falling cliff disrupts the lake's structure, the resulting disturbance can lead to a catastrophic collapse of the lake’s banks. This can cause a sudden and massive release of water, cascading down the valley and threatening downstream towns and cities.
GLOFs can be extremely destructive; as the water rushes downstream, it picks up significant amounts of sediment and boulders, transforming into a thick slurry capable of demolishing structures in its path. A recent GLOF in northern India in 2023 tragically resulted in the deaths of at least 55 individuals and the destruction of a hydropower dam. According to a 2023 study, approximately 15 million people globally reside within 50 kilometers (31 miles) of a glacial lake and less than a kilometer from the potential path of a GLOF.
The glacial lake currently overflowing in Alaska is located at the base of the Suicide Glacier, situated north of Juneau. Historically, this glacier flowed into the much larger Mendenhall Glacier. However, as the Suicide Glacier melts and recedes, a steep gap has formed between the two ice masses, known as Suicide Basin. Experts have suggested renaming Suicide Basin to Kʼóox Ḵaadí Basin, which translates to “Marten’s Slide Basin” in the Tlingit language, referencing a local weasel-like animal.
Snowmelt and rainfall accumulate in Suicide Basin, and when the water level rises sufficiently, it begins to drain through cracks in the Mendenhall Glacier, ultimately flooding the Mendenhall River. The first significant outburst occurred in July 2011, catching nearby communities off guard. Since then, the basin has filled and drained at least 39 times, as reported by the National Weather Service. Early Wednesday, the Mendenhall River entered a major flood stage, with water levels surpassing 14 feet, and it was projected to crest at 16.75 feet.
Glaciers in this area are part of the Juneau Ice Field, which is rapidly melting—twice as fast as it did prior to 2010, according to scientific reports. Researchers have also noted that more glaciers are detaching from one another, leading to the formation of lakes such as Suicide Basin. The accelerated melting is contributing to increased water levels in these lakes, which subsequently exacerbate the flooding risk for neighborhoods downstream.
Climate change is significantly influencing the occurrence of GLOFs. The world’s high mountain regions are warming at a rate faster than the global average, resulting in the shrinkage of thousands of glaciers and the formation of new lakes beneath them. Since 1990, the number, area, and volume of glacial lakes worldwide have surged by approximately 50%, as estimated in a 2020 study. However, the presence of larger lakes does not necessarily correlate with increased GLOF hazards, as each lake and valley has its own characteristics that determine the likelihood and consequences of an outburst.
In the context of the Mendenhall Valley, rising temperatures are responsible for the retreat of the Suicide Glacier and the creation of Suicide Basin. Interestingly, scientists have indicated that as global temperatures continue to rise, the Mendenhall Glacier may eventually melt to a point where the flood risk diminishes. This scenario could emerge within the next decade or two, as there may no longer be enough ice surrounding Suicide Basin to trap substantial amounts of meltwater. Instead, the water may gradually flow into the valley.
For now, the residents of Juneau must remain vigilant as they navigate the ongoing threats posed by their warming environment, just a few miles away from the rapidly changing landscape.
