Editor’s note: The glacial outburst flood that began on Aug. 4 rapidly intensified in the afternoon and evening of Aug. 5, leading to record flooding along the Mendenhall River.
The National Weather Service has issued a flood warning through Sunday morning for Mendenhall Lake and Mendenhall River after Suicide Basin began draining earlier this week.
Flood waters are expected to crest sometime on Saturday evening, with a four-to-six foot increase for Mendenhall Lake. That could cause flooding for Mendenhall Lake campground and Skater’s Cabin Road, along with some potential for erosion and tree falls along the Mendenhall River.
National Weather Service hydrologist Aaron Jacobs said glacial outburst flooding happens gradually.
“It’s not like a flash flood event, or like a wall of water,” Jacobs said. “Usually it will take about two days, give or take.”
A webcam above the basin recorded a significant drop in water levels on Friday morning. Gauges in Mendenhall Lake confirmed that water levels are rising downstream.
Glacial outburst flooding — also known as jökulhlaups — have been an annual summer phenomenon since 2011. Thinning ice in the mountains above the terminus of Mendenhall Glacier created an ice dam, which traps water in an area known as Suicide Basin.
The dam traps rain and meltwater in the basin, which fills up throughout the spring and summer. This year, it filled all the way up, surpassing levels recorded in 2021 and 2022.
But Jacobs says that higher water levels in the basin do not always mean there will be high flood waters downstream. Water from the basin travels under Mendenhall Glacier through cracks in the ice, which can grow in unpredictable ways that affect how fast the basin drains.
“So, the more efficiently that water can get from the Basin to the face of Mendenhall Glacier and into the lake, the higher the peak [flooding] can be,” Jacobs said.
Jacobs said that meteorologists will keep monitoring downstream water gauges and updating the flood information throughout the weekend.
