David Ignell best describes the threat to Juneau residents living in Juneau’s Mendenhall Valley caused by recent jokulhlaup flooding events. It is well researched. First Things First reposts Ignell’s article due to its public education value.
Bill Wildes, a former lieutenant in the Glacier Valley Fire Department is worried the next jokulhlaup in the Mendenhall Valley could result in substantial loss of life.
Wildes, who was instrumental in the formation of the Juneau Mountain Rescue team back in the 1980’s, was also active in carrying out local water rescue operations back then. He recalls attending a meeting in the 1980’s where a couple scientists warned that a catastrophic jokulhlaup in the Valley wasn’t a question of “if”. Rather, it was a question of “when”.
Wildes can’t recall the names of the scientists or even the location of the meeting, but two dreadful scenarios they outlined stand out in his mind 40 years later. One was a ten to twelve foot “wall of water” sweeping through the Valley. The other was the Mendenhall River carving a new channel in the vicinity of Killewich Drive and heading towards Duck Creek, which about 100 years ago was the main drainage channel for our receding glacier.
Wildes recalls the scientists saying the potential loss of life could be in the thousands.
Scientists have studied and published reports on jokulhlaups in Alaska for well over 50 years. In 1971, Austin Post and Lawrence Mayo wrote a report for the US Geological Survey (“USGS”) to present a then-current assessment of hazardous outburst floods in our state.
Post & Mayo called these outbursts “serious”, saying the major hazard presented by glacier dammed lakes is “catastrophic flooding”. They also noted that large quantities of water can be stored in or under glaciers and may create serious floods even though no surface lake is visible.
Post and Mayo’s report identified the many ways in which the outburst can be caused. Once a leak is established in the ice dam, the initial opening expands rapidly by the heat of the rushing water melting the ice. Sometimes that increase can be exponential.
The extremely abrupt release of water means that even small glacier dammed lakes can present serious hazards. The scientists wrote, “Areas flooded by glacier dammed lake releases are subject to serious damage. Wide flood plains are inundated to unusual depths and the high discharge rates can produce rapid erosion, deposition and stream channel changes.”
Their report concluded that “floods resulting from Alaska glacier dammed lakes present a serious and increasing hazard to the growing population of the State.” They recommended all structures in hazardous zones should be engineered to withstand stream erosion and severe flooding.
A growing number of Juneau families now painfully understand what Post and Mayo were talking about 50 years ago. Hundreds more are nervous about what next summer’s flood will look like. On everybody’s mind is the question, “Will it be worse?”
Trying to predict the impact of a jokulhlaup is extremely difficult. Post and Mayo wrote that “the hydrologic characteristics of the drainage basin may change suddenly and discontinuously.”
During the 1980s, Alaska scientists Matthew Sturm and Carl Benson studied outburst floods from Strandline Lake, dammed by the Triumvirate Glacier west of Anchorage. They found that hydrostatic pressure from Strandline lifted the glacier ice and the escaping water followed complex channels under the ice which shifted over the years. For instance, the exit tunnel in the ice during a 1982 event was 1000 feet away from where it had been in 1974.
The receding glacier enabled Sturm and Benson to discover that the force of the released water from Strandline was powerful enough to carve multiple water channels out of the bedrock beneath the glacier. In two places, V shaped channels more than 60 feet deep were incised into the bedrock.
Above the bedrock, the size of the tunnels bored through the ice by the escaping water can become massive. Sturm and Benson calculated that if all the potential energy released by Strandline went into melting ice, a single tunnel with a radius of 60 feet could result. After the 1984 flood, the exit tunnel in the Triumvirate glacier had a 30’ foot radius. The scientists concluded the water was dispersed along several paths, some which were abandoned before the final drainage.
A Juneau based scientist who has substantial experience studying the impacts of jokulhlaups on local rivers is Ed Neal. Neal was with the US Geological Survey for 20 years before forming his own research company.
From 2005 to 2007 Neal studied the impact of jokulhlaups on the Taku River originating from two glacier dammed lakes along the margin of the Tulsequah Glacier. In my interview with Neal this past week, he confirmed the studies of Post, Mayo, Sturm and Benson are still highly relevant today and the names of those scientists are highly regarded in the scientific community.
Neal said the amount of water released by these lakes above Taku River is generally the same year after year. The critical and somewhat unpredictable variable is the length of time it takes for those lakes to drain. The drainage period can vary from a few days to several days. Neal said the resulting impact on the Taku river flow can fluctuate from 60,000 to 120,000 cubic feet per second. The impact of a jokulhlaup is largely driven by what is happening underneath the ice in any given year.
This unknown variable is the cause of substantial angst for scientists and Mendenall Valley residents alike. The rate of discharge at Suicide Basin has dangerously accelerated over the last two years. In their abbreviated report on the 2023 flood, the National Weather Service wrote, “throughout the event, the rate of rise was unprecedented”.
Using data available on the NWS website, I created my own hour by hour analysis of the 2023 and the 2024 floods. For comparative purposes I also analyzed the 2016 jokulhlaup which until the last two years held the record for the highest flooding of the Mendenhall River, 11.99 feet on July 1, 2016.
In 2016, peak discharge of 16,200 cubic feet per second was reached 58 hours after Mendenhall Lake began to rise from the draining of Suicide Basin. The highest the river rose in any of those hours was 3.12 inches and the largest hourly rate of increase was 2.6%. The total rise of the river was 6.86 feet.
The outburst in 2023 obliterated the records set in 2016. A peak discharge of 25,200 CFS was reached in 57 hours and the river crested at 14.97 feet. The river rose as much as 4.08 inches per hour and for 14 consecutive hours, the hourly increase was greater than the highest increase in 2016. The total rise of the river was 9.49 feet, an increase of 38% over that of 2016.
2024 continued the upward trend. Peak discharge of 31,800 CFS, double the 2016 record, was achieved in just 44 hours. The river crested at 15.99 feet. The river rose as much as 5.4 inches per hour, and for 16 consecutive hours, the hourly increase was greater than the highest hourly increase in 2023. The total rise of the river was 10.97 feet, an increase of 16% from 2023.
The increased velocity of the water flow in 2024 was exponential. From 4pm to 5pm on August 5, the velocity of the river increased a whopping 11.3%. The tunnels being carved in the ice and bedrock were undoubtedly growing at an extremely rapid and unprecedented rate.
The critical question for 2025 is whether Suicide Basin will drain even faster than in 2024. For instance, what flood levels could be expected if peak flow occurs in just 30 hours? I called up one of the USGS scientists studying the Mendenhall jokulhlaups and asked them that question. The scientist, who asked not to be named in my article, didn’t know the answer but acknowledged they’re currently studying the question and hope to have some answers soon.
In a subsequent call with Neal, he confirmed the accuracy of my calculations and the high importance of the question. He said that when the 2024 flood was in its early stages, he analyzed the data in real time and noticed an exponential rate of increase in the river flow. He alerted a former colleague and the NWS subsequently increased their estimated flood level.
As bad as the 2024 event was, Mendenhall Valley residents were fortunate. The flood would have been much worse had it been raining.
This year’s jokulhlaup started about 7:00 am on August 4. The last measurable precipitation was the morning of August 1. Because of the good weather, the river level was relatively low at 5.02 feet when the flooding began.
Three weeks earlier, on July 16 the river had been at 8.38 feet. Had Suicide Basin begun to drain then, imagine how much worse the resulting flood from an extra 3.5 feet of water could have been. Not only in terms of the depth of water inundating people’s homes but also in increasing the likelihood that the downstream channel changes Post and Mayo identified could alter the course of the Mendenhall River. Could that have led to the type of forecast Bill Wildes heard 40 years ago?
In 2010 Neal did a study on the incision of the Mendenhall River and the lowering of groundwater in the basin. His report described a river that is hungry, its water deprived of sediment by the containment characteristics of Mendenhall Lake. The water released through the moraine opening adjacent to the Mendenhall Campground seeks to incise the existing sides and bottom of the river. During the 2024 flood Neal observed weakening of rip rap reinforcements which owners have recently placed along the banks of the river in attempts to protect their homes.
In my conversations with Neal, he didn’t rule out the possibility the river could jump the bank at a location like Killewich Drive. When I asked him how much velocity it would take to carve a new channel, he said a study is needed that estimates the sheer force at specific locations assuming varying flow rates. He tried to initiate a study of the river channel last year after the 2023 flood, but the USGS didn’t authorize it.
The last two jokulhlaups started on August 3 and 4. We must assume that 2025’s flood is less than 48 weeks away. Is there anything we can do to help save the Mendenhall Valley from this imminent danger?
From a construction standpoint, the answer is yes if preventative work begins soon. Many people fear our biggest challenge is overcoming the challenges of government bureaucracy. Someone needs to take charge and make the critical decisions now, as winter is just around the corner.
A local non-profit group called First Things First (“FTF”) has evaluated a number of proposals to resolve the crisis by next summer. The FTF board includes long-time Juneau residents Joseph Kahklen, Neil MacKinnon, Wayne Coogan, Dennis DeWitt, Mac Meiners and Roger Callaway. Its President is Frank Bergstrom, a fluvial geomorphologist, and its Executive Director is Don Habeger, whose home is on the Mendenhall River and has suffered devastating flooding the last two years.
I’ve been invited to attend several FTF board meetings the last two years and am impressed by the wealth of local knowledge and experience of its members. I was in attendance for the April 2024 meeting when a scientist from USGS was invited to discuss their studies of Suicide Basin and the potential of a flood in the coming months.
Bergstrom and Coogan have conferred extensively with local contractor Dave Hanna and others to determine the most practical solution to alleviate next year’s probable flooding of Mendenhall Valley. Their favored solution is to build a levee around the existing shoreline of Mendenhall Lake with a gate to safely release the rising water into the river.
The existing surface area of the lake is over 1000 acres. Building a 15-foot high levee would increase the storage capacity of the lake by 15,000 acre-feet, which is roughly 40% to 50% of the estimated volume of water released by Suicide Basin. Dredging the lake for the levee material would further increase its holding capacity.
Support for the levee concept can be derived from the 1989 report by Sturm and Benson on Strandline Lake. Below the Triumvirate Glacier lies the Upper and Lower Beluga Lakes. The scientists wrote the lakes “act as a natural flood-control system and mitigate destruction from the floods downstream near Cook Inlet where roadways, bridges, power lines, and gas and oil installations are located.”
Sturm and Benson detailed how during a 1979 jokulhlaup the lakes rose approximately 30 feet in 40 hours. They calculated that 2/3 of the total water volume released was ponded temporarily in the lakes and discharged more slowly through the Beluga River. As a result, a flood crest of just 13’ feet was observed at a downstream bridge.
The downstream geography from Suicide Basin is similar to that of Strandline Lake. Like the Beluga Lakes, Mendenhall Lake already acts as a natural flood control system. By increasing its capacity, it seems logical that we can reduce the downstream flood crests.
I asked Neal what he thought of the levee idea. He thinks it could be viable and its effectiveness could be potentially increased in conjunction with lowering the elevation of the outlet at Mendenhall Lake. Neal said the key issue is calculating the flow into and out of the lake under proposed levee designs, factoring in the size and location of the gate or weir which could safely release the water. Neal said such a study could be done in 2-4 weeks.
This study should commence immediately. While we’re waiting for its completion, others should be fine tuning cost estimates, lining up the funds and government approvals so that construction can begin this fall. Hanna believes the construction of such a levee could proceed throughout most of the winter and could be completed before next summer.
An important additional benefit of a levee is that it will help control outburst flooding from locations other than Suicide Basin. As noted earlier, Post and Mayo concluded back in 1971 that glaciers can hold and release vast amounts of water that aren’t noticeable on the surface. In 1989 Sturm and Benson concluded that supraglacial pools were filled via subglacial hydraulic connections with Strandline which then collapsed and connected with the main drainage channel during a jokulhlaup.
In 2003 a team of scientists drew on 90 years of observations of drainage of Hidden Creek Lake dammed by the Kennicott Glacier. Their report suggested there may be a linkage between timing of outbursts and seasonal “upglacier” extensions of the “subglacial conduit system”. It was suggested that subglacial conduits and the snowline move “upglacier” in tandem.
In other words, there’s a lot we don’t know about the jokulhlaups exiting under Mendenhall Glacier. Water from other sources besides Suicide Basin could contribute to Mendenhall River flooding. Both my USGS source and Neal referenced a suspected jokulhlaup that impacted the Mendenhall River back in the 1960s but little is known about it.
The bottom line is that we can’t accurately predict what Mother Nature is going to do. However, there are things that we can do to reduce the impact of catastrophic outburst floods and minimize the risk they pose.
Think of the levee and the lowering of the river at its source as an insurance policy. Then, think of the scenario that Wildes recalls hearing 40 years ago. Let’s set a goal of starting construction in October at the latest. Let’s not wait to see if the next flood results in the loss of human life. Time is of the essence.
David Ignell was born and raised in Juneau, where he currently resides. He holds a law degree from the University of San Diego and was formerly a licensed attorney in California. He has experience as a volunteer analyst for the California Innocence Project and is currently a forensic journalist and author of a recent book on the Alaska Grand Jury.
First Things First Alaska Foundation
P.O. Box 240605
Douglas, AK 99824-0605
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