Geoscience Reference
In-Depth Information
D i s c o v e r…
THE CHANNELED SCABLANDS
IN EASTERN WASHINGTON
The Channeled Scablands in eastern Washington are known
for the numerous dry canyons carved into the landscape. In
the early 20th century, the geologist J. Harlan Bretz hypoth-
esized that these landforms were created by catastrophic
floods caused by melting glaciers. He was ridiculed for most
of his career about this idea because most geologists be-
lieved that Earth-changing processes due to glaciers had
to be slow and gradual. By the 1970s, however, his hypoth-
esis gained increasing support because more had become
known about the way that water flows in open channels. His
hypothesis began to be accepted by the scientific commu-
nity in the 1970s. By the time Bretz died in 1981, however,
his hypothesis had been accepted. Indeed, he received ge-
ology's highest honor, the Penrose Medal, in 1979.
Geologists now believe that a lobe of the Cordilleran
Ice Sheet—the Purcell Trench Lobe—advanced far enough
into Montana that it blocked a valley through which glacial
meltwater flowed west. This blockage created an ice dam
that allowed a deep lake to form behind it, spreading back
into the mountain valleys to the east. This lake is known
to geomorphologists as
Glacial
Lake Missoula
because
it formed shorelines that are still visible around Missoula,
Montana. These shorelines indicate that the lake was as big
as today's Lake Ontario and Lake Erie combined. As the lake
deepened, it eventually began to flow over the top of the ice
dam, finally causing the dam to break catastrophically.
Torrential floodwaters, up to 300 m (1000 ft) deep,
subsequently burst onto the plains of what is now eastern
Washington, flowing at a rate 10 times the total of all the
rivers on Earth and emptying Lake Missoula in perhaps as
little as 48 h. This type of catastrophic discharge may have
happened up to 100 times and created the enormous valleys
carved into the landscape. The network of now-dry channels
is so well defined that it can be seen on satellite imagery.
Ice sheet
Ice dam
Montana
Seattle
Channeled
Scablands
Spokane
Glacial
Lake
Missoula
Missoula
Washington
Portland
u
Oregon
Idaho
0
100 km
0
100 mi
(a)
The Channeled Scablands cover a large area that includes most of southeastern Washington.
