The Return of the Jökulhlaup… Or is it? (Photo Credits: Sharon Sullivan, NWS Juneau, Alaska- Pacific River Forecast Center)
Water begins to cover the Skater’s Cabin walkway to Mendenhall Lake Thursday morning July 19, after reports that Suicide Basin to the side of Mendenhall Glacier had released. Mendenhall Glacier in the background (Sharon Sullivan)
The jökulhlaup has struck again. Jökulhlaup (“yo-kel-yawp”) comes from the Icelandic word meaning “glacial run”. This unique annual flooding event in Southeast Alaska results from a glacial dam outburst from Suicide Basin, located about 2 miles up from the eastern side of the Mendenhall Glacier. The basin is like a bowl with an opening on the bottom. The exit is normally sealed with ice, allowing rainwater and glacial melt to collect during the spring and summer months. Once the weight of the water becomes too great for the icy seal, it is lets loose and sends a surge of water through the drainage system below the glacier into the Mendenhall Lake. Floods can occur all at once or have multiple releases. The first jökulhlaup occurred in 2011, with 2016 being the largest.
An abrupt change in the basin’s water level near the end of June 2018 indicated that Suicide Basin was beginning to drain. Upon further examination, a calving event with a chunk of ice 1/5 of a mile wide disrupted the cycle by splashing into the basin and displacing the sensor, similar to dropping an ice cube into a glass of water. The calving event seemed to strengthen the dam holding the water in place, but a flood watch was issued as a precaution. By July 17, the water level in Suicide Basin had dropped overnight, but there was no response from Mendenhall Lake. Waters began responding by rising accordingly for the lake and the Mendenhall River on Wednesday, July 18 as glacial water flowed overtop the wall of ice surrounding the Mendenhall Glacier. By 10 am, a flood watch was in effect for the Mendenhall Lake area, with an expected 12 foot crest by Thursday evening and the potential for record flooding. At 8 am on July 19th, a flood warning was issued by NWS Juneau when the current stage level reached 8.99 ft. The flood stage for Mendenhall Lake is 9 feet.
Major flood stage of Mendenhall Lake occurs at 14 ft. The highest recorded water levels on the lake occurred in 2016, when the jökulhlaup caused the lake to reach heights of 11.99 feet (Alaska- Pacific River Forecast Center)
The crest occurred at 4:30 pm at 10.92 feet, tying for the 3rd highest lake level (11.99 feet in 2016), which was lower than the crest that was predicted. Significant flooding was reported along view drive and water inundated the Mendenhall Campground. According to the NWS Hydrologic Prediction Center, water levels begin dropping at a rate of about 3.5 inches per hour. The Mendenhall Campground, which had been evacuated the previous day was back open as of 9 am Friday. As of 11 am Friday, people were once again setting up their tents and enjoying the sunshine.
It is difficult to predict when the jökulhlaup will happen during the summer and if significant flooding will occur, but it is important that people are aware of the threat and prepare accordingly. In any flooding event, residents are advised to keep an emergency bag with important documents, dry foods, tools, and medication to take on the go. If you see water on the roadways, do not proceed to drive through it, as water may be deeper/ faster than you estimate. And, in the case of a jökulhlaup event, glacial waters can reach below freezing so hypothermia may become a threat. The response to the flooding went generally well due to a collaborative effort between the National Weather Service office in Juneau, the River Forecast Center, the Forest Service to evacuate the campgrounds, USGS river gauges, and the Department of Transportation’s thorough inspections on important bridges and roadways every year. Dynamics of a glacier in a changing climate, unknowns in the basin water levels, and small sample sizes may lead to varying crest heights and timing, but significant progress has been made since 2011 to better understand, forecast, and monitor this phenomena.
(Follow-up: On August 8th, the Mendenhall Lake and River were once again under Flash Flood Warnings as a significant heavy rain event over the northern panhandle and some additional contributions from Suicide Basin increased waters past flood stage).
To learn more about other flooding events, please click here!
©2018 Meteorologist Sharon Sullivan
“The Flooded Stairway to Nowhere” (Sharon Sullivan)