Geohazards in Kenai Fjords National Park

Graphic showing mantle convection as described in the accompanying text.
Conceptual drawing of assumed convection cells in the mantle (follow link for more details). Below a depth of about 700 km, the descending slab begins to soften and flow, losing its form.

Drawing courtesy of USGS.

What’s a Geohazard?

Just like you’d prepare for potentially dangerous encounters with wildlife and bad weather in a park, you should be aware of the geohazards that are common in active landscapes such as the mountainous and coastal areas of Southcentral Alaska.

Geohazards are naturally-occurring events that arise from geologic processes and have the potential to cause damage or threaten lives. While it’s often not easily apparent, the geology of our planet is in constant motion: like a pot of boiling water, the hot mantle (the rock layer surrounding earth’s core) underneath earth’s crust rises and falls in a convective motion, slowly moving the tectonic plates of earth’s surface with them. Though barely perceptible to us, this constant motion is submerging mountain ranges, lifting land masses, and shifting tectonic plates and with it, creating potentially dangerous situations like earthquakes, landslides, volcanic eruptions, and tsunamis.

A red warning sign next to a roped area, in front of ice from a glacier.
Hazardous warning signs at Exit Glacier in July 2011.

NPS Photo / P. Calamari

In Kenai Fjords, calving glaciers, unpredictable coastal weather, and dramatic mountains make for a breathtaking experience, but also a potentially dangerous one. Staying informed and making smart choices in the backcountry is important for a safe and enjoyable visit to the park. Read on to learn more about geohazards in the park.

Steep slope with minimal vegetation and loose rock.
Active slope at Bear Glacier Lagoon.

NPS Photo / D. Kurtz

Landslide Hazards in Kenai Fjords

It’s easy to forget that geologic processes are in constant motion around us until a sudden, dramatic event, such as a landslide reminds us. It might seem out of nowhere, but really, hundreds of years of incremental motion and destabilization have led to that moment.

Along Alaska’s dramatic southern coastline, these events can happen every day, seen or unseen, with glacial calving, coastal subduction, earthquakes, and more. One such reminder of the power of natural hazards occurred in October 2015, when the largest landslide in North America since the collapse of Mount St. Helens in 1980, occurred (unwitnessed) in Wrangell-St. Elias National Park. If a landslide of 200 million metric tons (about 440 billion pounds) of rock debris falls into a bay, and no one is there to hear it, does it make a sound? Well, it certainly makes some waves-- enough to affect tide gauges nearly 100 miles away. The massive landslide triggered a tsunami over 600 feet high, which scoured trees and vegetation clear off the slope across the fjord and deposited sediment into Taan Fjord. Seismologists detected the echoes of the slide and pieced together what had happened. Since this event occurred, the impact site has been a beacon for hazard geologists who can study the debris to learn more about major landslide events in the past, and hopefully, to provide insight into ones that may occur in the future. In June 2016, a 4,000-foot-high mountainside collapsed in Glacier Bay National Park, dumping over 100 million tons of rock onto Lamplugh Glacier below. The landslide stopped short of Johns Hopkins Bay, ten miles away, a popular stop for cruise ships observing calving at the famous tidewater glacier’s terminus.

Kenai Fjords shares a similar geologic makeup to its unstable counterparts in southeastern Alaska as part of the Alaska-Aleutian subduction zone. The Alaska-Aleutian subduction zone, spanning 2,900 km (1800 miles) from the Gulf of Alaska to Kamchatka, Russia, is the area where the Pacific plate is plunging beneath the North American plate. No major events have been recorded in the park since the Great Alaska Earthquake of 1964, when underwater landslides created tsunamis that hit coastal areas of the park and wreaked havoc on the town of Seward. Understanding the mechanisms and dangers of landslides in Kenai Fjords can help park staff and visitors to prepare for future incidents.

It may be hard to believe that the mountains of Southcentral Alaska are, to this day, still reacting to events from the Pleistocene, which lasted from 2.6 million to 11,700 years ago. As glaciers in Kenai Fjords have retreated rapidly since the Pleistocene and then the Little Ice Age, the steep slopes they left behind did not have much time to stabilize without the support of ice walls. Known as glacial debuttressing, this rapid deglaciation causes geologic instability that means slopes are more prone to landslide events. Just as glaciers formed the fjords and valleys of Southcentral Alaska, their absence shapes the landscape as well. During periods of glaciation, the weight of the ice carved out mountains and turned the sediment beneath it into rock. In the absence of ice, the walls that were once held up by glacial ice grow more and more vulnerable to landslides. Also, areas once burdened by the weight of glaciers are now experiencing uplift, or rebounding, much like a ship after cargo is unloaded. Alaska’s mountains are some of the fastest growing mountains in the world, at three to four millimeters a year in some cases, but this also makes them highly unstable and at risk of landslides (Cossart et al. 2014). There are many areas with active slopes in Kenai Fjords. For example, on the south side of Bear Glacier Lagoon, active slopes pose a potential risk for landslides into the waters of a popular recreational area.

Throughout Southcentral Alaska, including Kenai Fjords, unstable slopes often lie in fjords and coastal areas where, should they fail, the mass of land crashing into the ocean could trigger a tsunami.

Loose rock forms multiple minor landslides from steep slopes into the water.

Landslide Safety

Whenever you hike or recreate near an active mountain or glacier, the most important thing you can do is be aware of your surroundings. Though there are often no warning signs before an event like an earthquake or landslide, staying alert and avoiding potentially dangerous situations can make all the difference.

  • Listen for unusual sounds that might indicate moving debris. Falling mud or sediment may precede a larger event, such as an earthquake or landslide.
  • When near a stream channel, be aware of any sudden changes in water flow, level, or clarity. This could indicate landslide activity upstream.
  • Strong shaking from earthquakes can potentially trigger or intensify the effects of landslides.
  • Take note of your surroundings when selecting a site to camp. Avoid steep or unstable slopes, keep a safe margin of distance from the high tide line, and be sure to secure kayaks and other water vessels properly.
  • If a major earthquake or landslide occurs in a coastal area, tsunami action could follow. Be familiar with tsunami procedure in the area and relocate inland or to higher ground.
Chunks of ice fall into the ocean off the front of a glacier.
A calving glacier in Aialik Bay in June 2010.

NPS Photo / J. Pfeiffenberger

Ice Calving Hazards in Kenai Fjords

One of the most unique experiences that Kenai Fjords has to offer is witnessing the thunderous calving of a tidewater or lake-terminating glacier. Tidewater glaciers, those that terminate in the ocean, periodically break apart at the terminus and release icy blocks into the water. This occurs as glacial ice flows down-glacier, breaks off from the terminus, and crashes into the water. Glacial lagoons and tidewater glaciers offer visitors an opportunity to experience the dramatic interaction of ocean and ice, but all glaciers (even land-terminating glaciers like Exit Glacier) pose a risk of sudden calving if you venture too close. Whether kayaking, paddleboarding, camping on the shores of a glacial lagoon, or merely approaching the edge of an active glacier from what may seem like a safe distance; unexpected and unpredictable calving can put you in harm’s way.

In Kenai Fjords, calving is a daily occurrence at many of our glaciers. Occasionally, seismic activity may cause a more significant calving to occur. Imagine paddling through the aquamarine waters of a glacial bay, only to look up and witness an area of ice the size of more than 17 football fields crashing into the lagoon. In July of 2015, that’s what park visitors witnessed when a 6.3 magnitude earthquake occurred 120 miles west of Bear Glacier (Kurtz 2015). Visitors reported that the quake triggered a one mile swath of ice to calve from the glacier, issuing waves and chaos throughout the lagoon. Luckily, there were no injuries.

Glaciers even have the ability to create earthquake-like tremors when they calve. Ice quakes, as they’re known, generate unique seismic signals that can be picked up even hundreds of kilometers away in some cases. In a major calving event, ice quakes can reach a magnitude of 5 or higher. Events like the Bear Glacier calving in 2015 have happened often throughout the park’s history and will continue to occur, but education, preparedness, and caution can help to prevent worst-case scenarios from occurring.

Calving Safety

  • When kayaking near tidewater glaciers or in glacial lagoons, stay at least one-half mile away from the glacier terminus. Even at that distance, falling ice can cause large waves.
  • Try to stay in deep water where waves pass under you rather than breaking.
  • Maintain awareness when landing on beaches near tidewater or lake-terminating tidewater glaciers. Sudden waves from calving ice at the terminus can hit the shore with surprising power. Even kayaks and gear stored well above the high tide line can be swept away.
  • When camping near a glacial lagoon, be aware that wave action from calving can occur overnight and take you by surprise.
A large glacier descends from the mountains emptying into an iceberg-filled lagoon.
An aerial image of Bear Glacier Lagoon.

NPS Photo

Glacial Lake Outburst Floods in Kenai Fjords

Stumble upon a glacier-dammed lake while hiking and the scene may seem like a picture of serenity. But return a few days later, and you may find evidence of the powerful and fleeting nature of these lakes and the dangers they pose. Glacier-dammed lakes, or lakes that form from water trapped behind, below, or inside of a glacier, are common throughout Southcentral Alaska. Though they may appear stable for years at a time, these ice-dammed lakes can release dangerous floods known as glacial lake outburst floods (GLOFs), in some areas decimating infrastructure and towns below. This occurs as glacial lakes undergo a cyclical process of filling, draining, and refilling over a period of months or years. As rainfall, snow melt, and ice melt collect in a glacial lake and raise the water level, the pressure may cause water to begin flowing over or under the glacier, or force a tunnel through it. Like pulling a plug from a drain, water will often continue to drain out through the glacier until the lake is empty and the process begins again. What exactly triggers outburst floods isn’t well understood, so they can be difficult to predict.

There is only one known glacier-dammed lake in Kenai Fjords. This lake, adjacent to Bear Glacier (webcam), drains every year or two resulting in a glacial lake outburst flood in the proglacial lagoon (a lake that forms between a glacier and its moraine) at Bear Glacier’s terminus. To this date, no significant damage or human injury has resulted from this flood event. It is important to study and understand outburst flood patterns to manage and protect the park and its visitors.

Map showing the ice-dammed lake, approximately 8.8 miles to the north of glacier's terminus.
Location of the Bear Glacier source lake.

Glacial Lake Outburst Floods in Action

As a playground for kayakers, stand-up paddleboarders, and campers, Bear Glacier Lagoon draws a variety of visitors in Kenai Fjords. But the same waters that draw in recreationalists are also the final destination for much of the water displaced by outburst floods on the glacier. In recent years, park visitors have reported suddenly elevated water levels in Bear Glacier Lagoon. In October 2010, a glacial lake outburst flood was documented by equipment that recorded a 1.8 meter (5.9 foot) rise in water levels in Bear Glacier Lagoon. In August 2014, an outburst flood event caused Bear Glacier Lagoon to breach the moraine that separates it from Resurrection Bay (Kurtz 2014). After the outpouring of water and silt into the bay, lagoon levels dropped dramatically and caused increased calving at the Bear Glacier terminus. Events like this are the reason that it’s important to study outburst flood activity in Kenai Fjords. But, how do you study these floods when field access is difficult, remote imagery may be impaired by cloud coverage, and floods often occur hundreds or thousands of feet below the surface of the glacier, in subglacial tunnels? Scientists have studied the patterns of glacier-dammed lakes at Bear Glacier using a combination of in situ pressure-measuring devices to record lake levels, temperature and precipitation records, and aerial imagery spanning from 1985 to 2012 (Wilcox et al. 2013).

Two photos showing the referenced ice-dammed lake, full in 2009 and empty in 2014.
The Bear Glacier source lake pictured before a flood event in September 2009 (left) vs. after a flood event in August 2014 (right).

NPS Photo / D. Kurtz

Outburst floods at Bear Glacier have been documented multiple times throughout the past decade. The source of the Bear Glacier GLOFs sits 3.5 km (2.2 miles) up a tributary glacier of Bear Glacier and drains gradually, with an incremental rising of water levels over a period from several days to several months. A flood event may take campers by surprise, flooding campsites and dislodging improperly secured kayaks or other water vessels.

Other glacier-dammed lakes on the Kenai Peninsula include the Skilak and Snow Glacier glacial lakes, which drain every 2-3 years, often in the fall (Wilcox et al. 2014). The Skilak glacier-dammed lake, located on a lobe of the Harding Icefield, periodically releases floods that impact water levels on the Skilak River and Skilak Lake. The Snow glacier-dammed lake drains beneath the glacier into Snow River, Kenai Lake, and the Kenai River. Outburst floods from both systems have caused downstream ice jams and hazards to infrastructure in the past.

Climate Connection

Though the effects of climate variability on glacial lake outburst flood activity are challenging to predict, studies in the Himalayas have found an increase in outburst flood frequency and extent in previous decades (Parry et al. 2007). In Alaska, scientists have found that glacial lakes are forming at higher elevations than previously recorded. They predict that glacial lakes that weren’t at risk in the past may begin issuing floods and known GLOF-emitting lakes may release larger or smaller floods, at unpredictable times (Wolfe et al. 2014).

Glacial retreat and advance has an effect on outburst flood behavior as well. As glaciers retreat and thin, the “tipping point” for floods to occur is lowered, causing more frequent but smaller floods. In 2016, there were no reports of flooding in Bear Glacier Lagoon resulting from an outburst flood, but mid-summer observations by park staff indicated that it had drained. As glaciers advance, and more ice is available to impound glacial lakes, the amount of water necessary to breach the ice and cause a flooding event increases, meaning that outburst floods will be less frequent but potentially more devastating when they occur. Retreat and advance also cause glacier-dammed lakes to migrate. Since the 1990s, the Bear Glacier source lake has migrated down valley to the south, likely due to the thinning of the glacial tributary that dams it (Wilcox et al. 2014).

Glacial Lake Outburst Flood Safety

  • When near a glacial outflow channel, be aware of any sudden changes in water flow, level, or clarity. This could indicate outburst flood activity upstream.
  • Be prepared for rising waters when landing on beaches near lake-terminating glaciers. Kayaks and gear stored well above the high tide line can be swept away.
  • Changing water levels from a flood could increase iceberg calving activity in proglacial lagoons. Remember to stay at least one half-mile away from the glacier terminus when kayaking.


Cossart, E., D. Mercier, A. Decaulne, T. Feuillet, H.P. Jónsson, and, Þ. Sæmundsson. 2014. Impacts of post-glacial rebound on landslide spatial distribution at a regional scale in northern Iceland (Skagafjörður). Earth Surf. Process. Landforms, 39: 336–350. doi: 10.1002/esp.3450

Kurtz, D. 2014. Bear Glacier: Glacier Lake Outburst Flood. National Park Service Brief.

Kurtz, D. 2015. Earthquake triggered calving at Bear Glacier. Personal communication.

Parry M.L., O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson. 2007. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press.

Wilcox, A., A. A. Wade, and E. G. Evans. 2014. Drainage events from a glacier-dammed lake, Bear Glacier, Alaska: Remote sensing and field observations. Geomorphology 220 p. 41-49.

Wolfe, D., G. Leonard, J. Kargel. 2014. Alaska’s Ice-Dammed Lakes: Ephemeral, Ravaging Beauty. Summary of Global Land Ice Measurements from Space (GLIMS) Chapter 12.

Wilcox, A., A. A. Wade, and E. G. Evans. 2013. Glacial Outburst Flooding, Bear Glacier, Kenai Fjords National Park, Alaska. University of Montana.