Jökulhlaups from Glacial Lake Puyallup, Pierce County, WA
Kathy Troost, UW and Troost Geosciences, Inc.
For decades, geologists thought that normal surface
flooding caused the surface channels and cobble
deposits in Pierce County. However, work by Ms.
Troost shows that the channeled landscape and
extensive open-work gravel/cobble deposits are the
result of repeated jökulhlaups from Glacial Lake
Puyallup during the retreat of the Vashon Ice Sheet
about 13,500 years ago. The Icelandic-term
jökulhlaups is used to describe any large and abrupt
release of water from a subglacial or proglacial
lake/reservoir.
The affected landscape shows evidence of large
outburst floods, such as kettle lakes from the grounding of large ice blocks, large-scale
bed forms, waning-flow deposits with bogs, and thick Gilbert-type deltas at the coast.
American, Steilacoom, and Gravelly Lakes occupy kettles and are aligned parallel to the
coast. The 200-foot-thick Steilacoom and DuPont deltas provided much of region’s
supply of sand and gravel, an importance local resource. The channeled landscape,
herein called the Steilacoom Plain, is formed in the Steilacoom Gravel, a porous
gravel/cobble deposit 30–60 feet thick. This deposit offers little resistance to infiltration
of rainwater and contaminants, but hosts the remnants of a formerly much larger oak
prairie.
Kathy Troost is a Licensed Geologist with 34 years of experience in geological research
and investigations focused on the Pacific Northwest. Kathy has published many
geological maps and papers about Quaternary geology and deposits. She teaches at
the University of Washington in Seattle, has her own consulting company, is currently
the President of the Northwest Geological Society, and will complete her doctorate at
UW later this year.
Large foreset beds in
the DuPont delta at the
mouth of the jökulhlaup
channels.
Hillshade DEM of the Tacoma area showing jökulhlaup channels, west of Orting, cutting
across the north-south striated topography. Chains of kettle lakes, adjacent/west of HW
507/7 and adjacent/west of I-5, were likely grounding lines for ice blocks carried in the
flood waters.