Jökulhlaup season is here: be aware of glacier conditions

Jökulhlaup season is here, and with it comes increased flooding potential along the Mendenhall River system. While not every season has a Jökulhlaup, and not every Jökulhlaup blowout results in major flooding, it’s important to be aware of your surroundings at this time of year, especially if you live near creeks and rivers in the Mendenhall Valley.

Weather patterns change rapidly from year to year and we still do not know how the 2023 eruption will have affected the river and basin. Regardless of how long you have been in and around the river, take extra care this year and stay alert to weather conditions and advisories.

New to Juneau? Start at the beginning. Otherwise, see How can I get the latest updates and forecasts on glacier bursts?.

What is a jökulhlaup? How is it formed?

A jökulhlaup is also known as a glacial outburst. Jökulhlaups occur when a body of water in a glacier breaks through its dam. In Juneau, this often happens when glacial ice builds up to form a natural dam. The basin created by this dam fills with meltwater and more ice. However, the glacial ice of the dam wall is vulnerable to pressure, heat, and movement. If the wall breaks or collapses, the basin empties into adjacent waterways and the water level rises rapidly. Depending on the amount of water released, in other words, how full the basin is, this can cause flooding downstream.

Where in Juneau do jökulhlaups take place?

Jökulhlaups are most common near the Mendenhall Glacier in the area known as Suicide Basin. The basin drains into Mendenhall Lake, and a jökulhlaup from Suicide Basin can lead to flooding along the Mendenhall River system (including Montana Creek).

What conditions lead to jökulhlaups?

Any weather conditions or events that affect the glacier ice can lead to a jökulhlaup. Examples of these are unusually warm weather, heavy rainfall and seismic activity.

Do jökulhlaups always cause major floods?

The extent of flooding in surrounding waterways after a jökulhlaup depends on how much water is released and on the water levels before the eruption. For example, an eruption during heavy rainfall can cause the basin to overflow into a high river, making it more likely to cause significant flooding. In contrast, a partial breach when the river level is low can release smaller amounts of water from the basin without causing major flooding.

How can I get the latest updates and forecasts on glacier bursts?

The U.S. Geological Survey and the National Weather Service use a number of monitoring tools to track conditions at Suicide Basin, Mendenhall Lake, and Mendenhall River. Many of these tools can be followed online. Visit the Suicide Basin monitoring page at weather.gov/ajk/SuicideBasin and the Mendenhall River gauge at water.noaa.gov/gauges/MNDA2.

Watch the recording of a recent town hall meeting with scientists from CBJ, NWS, USGS, and the University of Alaska Southeast to learn more about how the forecasting and monitoring tools work, including new improvements for 2024..

Follow the NWS Juneau office on Facebook or Twitter for the latest weather information for Juneau.

How do I know if a flood is coming in the jökulhlaup?

CBJ, the National Weather Service and other agencies closely monitor Suicide Basin and can warn the public when a jökulhlaup occurs. It takes several hours for the water released from a glacier burst to reach Mendenhall Lake and several more hours for water levels along the Mendenhall River to peak. In 2023, this allowed residents along the river to evacuate before floodwaters reached their homes.

Glacier ice is unpredictable and there is no easy way to predict how or when a jökulhlaup will occur. The best way to be prepared is to pay attention to weather warnings, be alert to emergency messages, and have a plan in place in case you need to take quick action.

How can I prepare for a flood?

By asking this question, you’ve already taken the most important step toward emergency preparedness. Simple advance planning—arranging where your household will meet, locating power and gas shutoffs, and gathering basic emergency supplies—can make all the difference when you need to respond quickly to a stressful situation. Take a moment to read up on personal and household emergency preparedness, then take the first steps to prepare yourself and your household.

For more information about how to stay safe during a flood, visit CBJ’s Juneau Hazards page (click on the “Floods” tab).