Glacial lake outburst floods (GLOFs) are floods that occur from an unstable natural dam formed from a glacial retreat. Glaciers are dynamic bodies of ice that change frequently. When a glacier retreats, it can leave behind a large impression in the ground that fills with water, turning it into a lake. This is typically known as a moraine. These lakes can be impounded by an unstable pile of debris and buried ice. As the climate warms, glaciers generally shrink. In many mountains such as the Andes, Himalaya, Alps, Rockies, and elsewhere, the retreat of glacier tongues sometimes allows unstable moraine dammed lakes to form. Terminal moraines act as dams for these lakes, but as the lakes swell from rising water levels and the retreating glacial ice tongue, the moraine dam can weaken. Moraine dams that become too weak may crumble under too much pressure from the swelling lake, creating a GLOF. If the moraine dams of a glacial lake fail, the water can burst out, leading to massive floods and debris flows with potentially extensive damage downstream, including loss of life and infrastructure. Alternatively, and more commonly, glacial ice from the retreating glacier can crash into lakes, generating giant waves that erode weak moraine dams in a matter of minutes, thereby also triggering GLOFs.
In the summer of 2002 Alaska's Hubbard glacier pinched off the connection between Russel Fjord and Disenchantment bay. This made the Russel Fjord into a glacial lake, for a short period of time. The lake rose 15 m above sea level. When the lake burst, it sent almost 2 million cubic feet of water per second back into the bay.
In the past century, GLOFs have caused disasters in high mountain regions of the world which include the Andes, areas in Caucasus and Central Asia, the Himalayas, North America and the European Alps. There are many examples in Nepal in which loss of lives and property have occurred as a result of glacial lake outburst floods. Glaciers, being ice sheets, can indicate increased air temperature due to being sensitive to climate change. Glacial lakes are not only a risk of potential danger, but they are also an important natural resource, providing a fresh water source for people. Over the past 20-30 years, human measures to mitigate unstable glacier lakes in the Himalaya and European Alps may have prevented some potential GLOF disasters. Research has shown that due to global climate change and retreating glaciers, there is an escalated risk in GLOFs worldwide. This is leading glacial lakes to swell and increase the risk of flooding. For example, in Bhutan, a region with 677 glaciers and 2,794 glacial lakes has experience more than 21 glacial lake outburst floods in the last two centuries. From the 21 mentioned, 4 of the outbursts have occurred in the last 40 years. However, because GLOFs are quite rare, it is uncertain whether their frequency of occurrence is changing at either the regional or global scale. Researchers argue that outburst floods from moraine dammed lakes in North America for example may have increased due to a reduction in the number of the lakes since the end of the Little Ice Age.
Below is a video explaining climate change impacts and the risks associated with glacial lake outburst floods in regions of the Himalayas. For more information: