The Kennicott Glacier Jokulhlaup
When Hidden Lake Drains and McCarthy Celebrates
Alaska’s Annual Glacial Flood and the Boaters Who Can’t Wait for It
Living and working in Wrangell-St. Elias National Park means your schedule gets dictated by powerful geologic forces. Most of the time, geology is a patient sculptor, reshaping landscapes over thousands or millions of years. But every once in a while, the earth conspires to alter everything in a sudden, dramatic event.
Earthquakes. Floods. Volcanoes. The abrupt, sometimes violent reminders that we live on a dynamic planet.
In McCarthy, we get our own version of this drama every summer: the jokulhlaup.
What Is a Jokulhlaup?
Pronounced “yo-kull-hop,” this Icelandic word translates to “glacier run” and describes what happens when a glacial dam suddenly fails, releasing massive volumes of trapped water into surrounding valleys.
Jokulhlaups have been reshaping Alaska for hundreds of thousands of years. During the last glacial period, much of what is now Wrangell-St. Elias National Park sat beneath a massive ice-dammed lake that periodically released catastrophic floods. The largest of these ancient jokulhlaups reached an estimated peak discharge of 2 million cubic feet per second (cfs), making it one of the most powerful floods ever documented on Earth.
The jokulhlaup we experience in the Kennicott Valley is tiny by comparison. But it dramatically affects the river we work on, the landscape we live in, and provides the one occasion each summer when the entire community gathers in the same place at the same time to watch water do something spectacular.
It starts in Hidden Valley, a beautiful drainage just west of the Kennicott Glacier, about 11 miles north of McCarthy. Hidden Creek flows through this valley, but instead of passing under the glacier like many streams do, the water pools at the glacier’s edge, forming Hidden Lake.
As summer progresses and snowmelt from surrounding mountains continues, the lake grows larger. And larger. The water has no outlet. Hydraulic pressure builds behind the glacial dam.
Then one day, usually in July, the lake drains.
The accumulated pressure finally lifts the glacier that’s been trapping all that water. Millions of gallons flow through channels beneath the ice, traveling down-glacier toward the terminus. Eventually, the flood reaches Kennicott Lake, raising the lake level by several feet, jostling icebergs around, and then continuing its journey down the Kennicott River toward the sea.
The entire drainage event typically happens over 24-48 hours.
Hidden Lake looking east, full, a week before the 2018 jokulhlaup. Photo: Elvie Underwood
Hidden Lake looking west, empty, just hours after the 2019 jokulhlaup. Photo: Caitlyn Tetterton
What This Means for River Guides
For those of us who spend nearly every day on the Kennicott River, a cataclysmic flood that temporarily transforms the entire character of our workplace presents some logistical challenges. River features change. Banks erode. New channels open. Old rapids develop new personalities.
Every jokulhlaup is different. Water levels, timing, and duration vary based on how much snow accumulated in the Hidden Creek drainage, how warm the summer has been, and the specific conditions of the glacial dam.
We always assess river changes carefully and reschedule trips as necessary during and immediately after the flood. Safety protocols require familiarity with current conditions, and a river flowing at twice its normal volume with entirely new features demands respect and reconnaissance.
But in the first 24 hours after Hidden Lake breaks? We react in typical McCarthy fashion. We celebrate.
The 2019 Jokulhlaup: A Case Study
The gauge at the footbridge measuring cubic feet of water per second during the 2019 jokulhlaup. Photo: USGS
On the morning of July 7, 2019, the USGS gauge measuring Kennicott River flow showed a slight but steady increase from a normal summer flow of around 10,000 cfs to 13,000 cfs. The previous week had been one of the hottest on record for the valley (some thermometers reading 90°F in direct sun), so increased glacial melt wasn’t particularly surprising.
Then around 4:00 PM, a pilot flying over Hidden Valley radioed down with news: Hidden Lake had drained.
Between 4:00 PM and 10:00 PM that evening, the Kennicott River rose nearly three feet and reached a flow of 20,000 cfs. The gauge continued climbing through the night, eventually peaking at 24,000 cfs before beginning to drop.
The gauge reading height of the river during the jokulhlaup. Photo: USGS
Why Boaters Love the Jokulhlaup
High water on any river can pose serious risks. Trees wash into channels, creating dangerous strainers. Powerful hydraulics develop that can trap swimmers. Familiar features become unrecognizable and potentially hazardous.
The jokulhlaup presents different conditions. Because this is glacial flood water rather than rain-driven flooding, it doesn’t wash vegetation into the river. Trees stay rooted. The occasional iceberg rolls downstream, but ice behaves predictably (like a very cold rock) and is easily avoided by experienced paddlers.
What the flood does create is powerful, swollen whitewater. Rapids that normally provide splashy fun become genuinely engaging. Wave trains grow massive. Hydraulics develop complexity. The river transforms into a temporary playground for skilled paddlers who’ve been waiting all summer for this exact moment.
When the 2019 gauge hit 20,000 cfs, the McCarthy River Tours crew assembled and started inflating rafts.
Kayaker Hailey Thompson was the first to paddle the 2019 jokulhlaup. Photo: Paul Scannell
The MRTO crew enters the first rapid of the river while icebergs loom in the lake behind them. Photo: Paul Scannell
The Footbridge Party
Residents of McCarthy and Kennicott gather for every jokulhlaup in the same place: the Kennicott River footbridge. Because you never know the exact timing of when the flood will arrive, it’s always a surprise party celebrating the dynamic nature of this place and watching paddlers ride the biggest water of the season.
From the bridge, you can see riverbanks actively eroding. You can hear massive boulders rolling beneath the surface, grinding and crashing with sounds like distant thunder. As boaters pass underneath, the crowd cheers wildly.
During the 2019 flood, the first kayaker to paddle beneath the bridge received a hero’s welcome. Several more packrafts and kayaks followed, splashing through waves with massive grins plastered across their faces. The crowd on the footbridge helped launch one professional whitewater kayaker into the river below by literally lowering him over the railing (with his permission and enthusiasm).
Six McCarthy River Tours guides launched two rafts at the lake and paddled into the swollen river. The waves were huge and rolling. Rapids looked almost unrecognizable at this volume. Laughing and cheering, the guides hit every rapid directly, reveling in the freezing water splashing their faces.
They found a calm eddy several hundred yards downstream from the footbridge, carried the boats back upstream, and ran it again. And again. And again. Even as the sun set and the river continued growing, boaters kept running laps while their audience cheered from shore.
Boating in the Wrangells is usually a peaceful, solitary affair. The jokulhlaup is the one occasion where you navigate with a rapt, enthusiastic audience watching every move.
The crowd on the footbridge helped to launch professional whitewater kayaker Brendan Wells into the river below.
MRTO guides Caitlyn, Stone and Johnny drop into a rapid above the footbridge. Photo: Paul Scannell
The Aftermath
After multiple celebration runs, the guides headed downstream toward their normal takeout, discovering huge water around every bend. At the takeout, they found parts of the riverside property had flooded. Picnic tables and benches floated in pools of cold, silty water at the river’s edge.
But just as suddenly as the water rose, it dropped. By July 9th, flow had returned to normal 10,000 cfs levels after peaking at 24,000 during the night.
In the following days, guides paddled the river numerous times to document changes that occurred during the flood. Banks had eroded in new patterns. Fresh channels had opened. Old familiar rapids bore new names and faces reflecting their altered character.
This reconnaissance isn’t optional. Before bringing guests back onto the water, guides need to understand exactly what changed and verify that all features remain within appropriate difficulty ranges for the trips we run.
Within a week, the Kennicott River was back to its normal fun, splashy Class III character, just with a slightly different arrangement of the same basic elements.
Why Jokulhlaups Matter
For scientists studying glacial processes, jokulhlaups provide real-time data about ice dynamics, subglacial hydrology, and how glaciers respond to changing climate conditions. The timing, duration, and volume of glacial lake outburst floods reveal information about glacier geometry, basal conditions, and seasonal melt patterns.
For residents of McCarthy, the jokulhlaup serves as an annual reminder that we live in a geologically active landscape where glaciers still dictate rhythms and routines. The flood connects us to processes that have been shaping this valley for thousands of years and will continue long after we’re gone.
For river guides, it’s professional development wrapped in a party. We learn to read high water, practice navigating challenging conditions, and get reminded that rivers are dynamic systems requiring continuous observation and respect.
And honestly? It’s just really fun to watch a lake drain, a river swell, and an entire community gather to celebrate geology happening right now instead of over millennia.
When Does It Happen?
The Kennicott Glacier jokulhlaup typically occurs in July, but the exact timing varies year to year based on:
- Snowpack in the Hidden Creek drainage
- Summer temperatures (hotter summers often trigger earlier drainage)
- Glacial dam conditions
- Total water volume accumulated in Hidden Lake
Some years it happens in early July. Other years, late July. Occasionally it arrives in early August. Predicting the exact day remains impossible despite years of observation and data collection.
The unpredictability is part of what makes the event special. Nobody can plan for it. You just have to be paying attention when it arrives.
Experiencing the Jokulhlaup as a Visitor
If you’re visiting McCarthy during July and happen to be here when Hidden Lake drains, congratulations. You’re witnessing a geological event that happens once per year and transforms the river within hours.
Head to the Kennicott River footbridge. Bring a camera. Watch the water rise. Listen to boulders rolling beneath the surface. If you see boaters preparing to launch, stick around for the show.
Just don’t expect to go rafting that day. During the actual flood, we’re running safety assessments and enjoying the celebration, not guiding commercial trips. Within a few days, after we’ve documented all the changes and verified conditions, we’ll be back on the water with guests.
But if you want to understand why guides who work glacier-fed rivers in Alaska have a particular relationship with the landscape, watching a jokulhlaup provides pretty clear explanation.
The earth is dynamic. The glaciers are active. The rivers change. And sometimes, that happens fast enough to gather a crowd and cheer.
The Science Behind Jokulhlaups
For those interested in the technical details:
Planning Your McCarthy Visit
Interested in experiencing the rivers, glaciers, and geology of Wrangell-St. Elias National Park? We offer guided rafting trips, glacier hikes, and backcountry expeditions throughout the summer season.
While we can’t guarantee you’ll witness a jokulhlaup (the timing is genuinely unpredictable), we can guarantee you’ll experience the same dynamic landscape that makes these events possible.
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