Changing Ice and Rising Waters: Glacial Floods in Juneau’s Mendenhall Valley

It was one of those rare cloudless days in Juneau when the Mendenhall Valley felt like a playground. Kids pedaled along trails, families hiked beside the lake, and lawns buzzed with summer chores. But beneath the glacier, a hidden force was building, and it would soon surge into the city.

On July 19, 2011 floodwaters engulfed roads, swallowed backyards, and swept away campgrounds.

Initially, nobody had any idea where the water was coming from. Following a hunch, scientists took to the air in helicopters and discovered that the flood was coming from a water- and ice-filled basin left in the wake of the retreating Suicide Glacier. It was, they discovered, Juneau’s first glacial lake outburst flood.

Glacial lake outburst floods (GLOFs) or Jokulhlaup, as they are known in Iceland, occur when water held behind a glacier, often in a natural reservoir, suddenly releases due to the failure of the dam holding it in place.

These floods are not unique to Juneau; they can happen in any region with glaciers. Glacial lake outburst floods are common in Iceland and also occur in places such as the Swiss Alps, the Himalayas and other high-latitude or high-elevation areas.

Glacial lake outburst floods in other parts of the world often occur without warning. However, Juneau’s outburst flood presents a unique case because the geology of the basin, now called Suicide Basin, is causing the floods to strain the community every year.

As Suicide Glacier retreated, it left behind a bowl-shaped basin cradled by bedrock on three sides. The only thing holding the large lake in is the Mendenhall Glacier, which acts like a massive, icy dam.

Each summer, rain and melting ice fill this lake, slowly building pressure. Eventually, that pressure becomes too much, and the lake finds a way out. The water forces itself beneath the glacier, carving a hidden tunnel through the ice. At first the flow is gentle. Then it surges. The tunnel widens, and the flood roars downstream. But just as suddenly as it starts, it stops. The glacier’s weight crushes the tunnel closed, sealing off the flow until the basin fills again.

The Mendenhall River, which flows from the glacier and through Juneau, is relatively shallow and narrow. This makes it particularly vulnerable during these sudden flood events, as the river cannot accommodate the rapid increase in water. The river winds through residential areas, placing homes and infrastructure in close proximity to the floodwaters, creating a significant risk to both the environment and the local community.

As the climate continues to change, atmospheric temperatures and rainfall will likely increase in Southeast Alaska, meaning the potential for even greater floods exists.

Additionally, the Mendenhall Glacier, which serves as a natural dam for the Suicide Basin lake, is melting and thinning by as much as 10 to 12 feet per year. This thinning reduces the glacier’s ability to hold back water, which can contribute to larger and more sudden floods.

The basin itself is widening over time due to ice calving from the Mendenhall Glacier into the Suicide Basin. As chunks of ice break off and float in the basin, they create additional storage space for water, allowing the basin to fill even higher than before. In 2024, approximately 7.5 million cubic meters of ice were still floating in the basin during the flood, some of which melted and contributed to runoff volumes.

Fourteen years after the first flood, on Aug. 5, 2024 at 5 p.m., I received the warning while I was at work on a whale-watching boat in the middle of Lynn Canal.

The City and Borough of Juneau had issued an evacuation. Suicide Basin was draining once again. We had been anticipating the flood all summer long. Now it was finally happening, and I was stuck on a boat while my car and all my belongings were in the flood zone. I tried to maintain a positive attitude and a straight face for the rest of the workday, but in the back of my mind, I worried that my house would be underwater when I returned.

There was nothing I could do but hope, all while the rest of the town evacuated. People packed their cars with their valuables and secured their belongings. There was an estimate of where the flood might reach, but the extent of the damage to come was unknown. Power was cut along the river. Hard-earned salmon, berries, family memories, and vehicles were left to ruin. Nothing was left to do but watch the river rise and the water encroach on the valley.

Thankfully, I got off work before the river reached its maximum flood height of 15.99 feet at 1:30 a.m. At Brotherhood Bridge, in the middle of the valley and overlooking the Mendenhall river, several community members watched as giant trees swept by in the river. My friends and I were in awe of the raw power contained in this massive amount of moving water. As we watched the water level rise, we wondered how the floods might exacerbate Juneau’s already stressed housing market and how future floods might unfold as the climate continues changing.

That year, over 300 homes were flooded, and many people lost their belongings and suffered damage to their properties. Fear and urgency were etched on the faces of residents. This wasn’t a new event; we’d been warned and seen it before, but this one was different.

Since the first recorded outburst in 2011, floods have occurred at different scales, though their impact has progressively gotten worse. The 2024 flood was the strongest yet, reaching a river crest height of 15.99 feet, surpassing the previous record of 14.97 feet set in 2023.

More than 42,700 cubic feet per second of water flowed through the Mendenhall River, nearly 22 times greater than the average flow of the Mendenhall River in summer around half of the flow of Niagara Falls.

In 2025, the river rose even higher, but thankfully the Hesco barriers held up. These sand- and rock-filled barriers protected the community to a large degree. However, as long as the glaciers remain, the threat of outburst floods persists.

A group of environmental scientists and glaciologists – including Eran Hood, Jason Amundson and Christian Kienholz from the University of Alaska Southeast and funded by the Alaska Climate Adaptation Science Center – came together to understand the flood and plan response strategies.

The scientists have implemented a variety of forecasting techniques. They have installed cameras that monitor the basin in real time, allowing them to visually determine when the basin reaches full capacity.

Using drone surveys, the team generates high-resolution models that reveal how the basin transforms over the course of the year and the flood, providing precise measurements of the amount of water stored and released.

Along with the drones, there are in-basin sensors that provide real-time measurements of water levels. These figures help estimate the amount of water present at any given time and forecast potential flood magnitudes before a drainage event occurs.

Along with the extensive monitoring program, the Alaska CASC has recently developed a tool for the public. The Juneau Glacial Flood Dashboard is an interactive online tool designed to help residents better understand and prepare for glacial lake outburst floods from Suicide Basin.

Developed by Sean Fagan and Eran Hood with the Alaska Climate Adaptation Science Center, the dashboard includes a potential flood map that visualizes how different flood levels could impact the Mendenhall Valley.

Glacial lake outburst floods are a striking example of how rapidly glacial systems are changing, reshaping, and impacting communities.

As the Mendenhall Glacier continues to retreat and deflate, the dynamics of Suicide Basin are changing, expanding its capacity to store water and potentially increasing the magnitude of future floods. As each year passes, the basin holds more water and releases it more completely. At the same time, the calving of new icebergs into the basin and their subsequent melting contribute to the volume of water released during a flood. These processes make forecasting increasingly complex, raising questions about what future floodsmay look like and how quickly they could occur.

Despite these technological advances, significant challenges remain. Glacial systems are inherently dynamic, and their behavior cannot be predicted with complete certainty. The 2025 outburst flood underscored both this certainty and the value of preparedness: although water levels rose higher than ever before, the Hesco barriers held, protecting homes and infrastructure from severe damage. As we look ahead, continued scientific monitoring of Suicide Basin will be essential for informing local response strategies and long term planning. But research alone is not enough. Translating scientific insight into proactive mitigation measures will be critical to safeguarding Juneau’s communities and livelihoods as the climate continues to change.