How does the Suicide Basin glacial outburst flood occur and how can the flood be controlled?

November 2024

Juneau is perched on the edge of the Juneau Icefield, which is the third largest icefield in the United States, covering 1,500 square miles and including 40 large valley glaciers and 100 smaller glaciers (https://en.wikipedia.org/wiki/Juneau_Icefield). The most recent maximum extent of ice occurred during the Little Ice Age in the 1700s. Since that time, many of the glaciers have been in retreat.

Glaciers consist of accumulated snow that has survived summer melt and been compressed into ice. Snow accumulates in the higher elevation portion of the glacier known as the accumulation zone. When ice depth becomes sufficient it can begin to behave plastically and flow downhill. When ice begins to flow it is called a glacier.

Mass balance is a term that describes the relationship between snow that accumulates in the winter versus ice and snow that melts in the summer. The upper elevation portion of the glacier tends to accumulate snow, while the lower portion tends to lose snow (and ice). The point in elevation where the two are equal over a year’s time is called the Equilibrium Line Altitude. If that line tends to migrate down the glacier, the glacier tends to advance. If the line tends to migrate up the glacier, the glacier tends to retreat. That is, If the glacier experiences more snowmelt in the summer than accumulates in the winter, it will begin to shrink – both in length via a retreating terminus and in thickness.

The Mendenhall Glacier is a valley glacier draining a western portion of the Juneau icefield. It has been in retreat while the upper portion of the icefield is in advance (Taku Glacier). Glaciers on the periphery of the icefield – such as the Suicide alpine glacier on Mount Wrather and Spencer Peak – have retreated more rapidly and become ice free (excluding icebergs). However, Suicide Basin is dammed by the Mendenhall Glacier so runoff into the valley accumulates as a lake (green area in Figure 1).

Since the Mendenhall Glacier is a moving river of ice, it is a transitory dam, and it leaks. In recent years the leak at Suicide Basin has become more frequent and catastrophic.

We have seen Suicide Basin fill from late summer through mid-July to early August. At a critical point the forces resisting release are no longer sufficient, and the water pressure of over 450 feet of impounded water begins to flow under or through the glacier. The actual mechanism is not well understood, but this process has occurred repeatedly and once initiated tends to drain Suicide Basin fully or nearly so. Roughly 35,000 acre feet of water rapidly drains from Suicide Basin (Figure 2).

Currently Suicide Basin holds roughly 25 percent of its volume as icebergs, which have been shed from the side of the Mendenhall Glacier. These icebergs accumulate on the floor of Suicide Basin as the water is drained away forming a massive jumble of ice (Figure 3) https://www.weather.gov/ajk/suicidebasin. Perhaps these icebergs plug the drain through the Mendenhall Glacier as Suicide Basin drains, but this is pure conjecture.

Mendenhall Lake acts as a flood control basin for the Suicide Basin glacial outburst flood. Water leaves Suicide Basin, traverses the glacier, and enters the head of Mendenhall lake. This then raises the water level in Mendenhall Lake, which in turn increases the outflow into the Mendenhall River. But, this process takes time and results in the temporary storage of flood water in the lake. The rise in level at the head of the lake takes time to be felt at the outlet of the lake, which is necessary before an increase in outlet flowrate can occur. This results in an attenuation of the flood hydrograph. That is, by temporarily holding back some of the water, the rate of flow in the Mendenhall River does not rise as high as it would if Mendenhall Lake did not exist.

The natural effect of Mendenhall Lake to store water can be seen in the aerial photograph of the 2014 glacial outburst flood (Figure 4). The Dredge Lake area is partially flooded upstream of the Back Loop Road. The campground is almost completely flooded.

We can enhance this effect of the lake to further reduce Mendenhall River flow rate during a glacial outburst flood. If we raise the outlet elevation of the lake by building a levee or dam, and thus increase the available temporary storage volume of the lake, this effect is enhanced. We can raise the outlet elevation by constructing a levee (dam) across the river and through the woods to a suitable elevation determined by a flood routing study. More water can then be temporarily stored. This further detains the flood, and lowers the Mendenhall River flow rate downstream. This is the basic concept of flood routing through a flood detention basin practiced around the world. A levee of less than 25 feet maximum height would be constructed just downstream of the lake.  The levee would be high and dry for the entire year except during the glacial outburst flood. At all other times, the levee would be above the typical lake water level (Figure 5).

As the outburst flood drains Suicide Basin, the inflow to the lake subsequently decreases and stops. Over the following day or two the accumulated water temporarily detained in the lake drains down the river, but always at a rate that protects downstream people and property (Figure 6).