New research shows how accelerated melting of
glaciers in the Cascade Mountain Range could dwindle late summer streamflows in
decades to come, taking previous work on glacial melt to a new level.
A peer-reviewed study is slated for
publication in Water Resources Research, a journal of the American Geophysical
Union, outlining a complex model for six drainages in the Cascades that are all
fed by glaciers in varying degrees. Dr. Chris Frans, the lead on climate
studies for the Northwest Division of the U.S. Army Corps of Engineers, led the
study while at the University of Washington.
“This is the one that finally pulls it
together,” Frans said. “There’s some very detailed work on glaciers alone, but
not necessarily the rest of the watersheds.”
The study’s abstract puts it this way: “While
changes in glacier area have been studied within the region over an extended
period of time, the hydrologic consequences of these changes are not well
defined. We applied a high-resolution glacio-hydrological model to predict
glacier mass balance, glacier area, and river discharge for the period
1960-2099. Six river basins across the region were modeled to characterize the
regional hydrological response to glacier change.
The study accounted for high, mid and low
elevation drainages on the eastern front of the Cascades and the cooler, wetter
western front, along with seasonal effects of glaciers above those drainages.
According to the study, glacial area coverage
has been shrinking as much as 50 percent in the North Cascades over the last
century, and as little as 22 percent on Mount Rainier.
“The river basins we modeled include 47
percent of the total area of glaciers and perennial snowfields in Oregon and
Washington and encompass a representative range of physical settings,” the
signature of glacier melt is strongest in late summer” when glaciers are
exposed and other sources of water tend to be declining, Frans explained.
“Results show that the rate of regional
glacier recession will increase, but the runoff from glacier melt and its
relative contribution to streamflow display both positive and negative trends,”
the study continues. “In high elevation river basins, enhanced glacier melt
will buffer strong declines in seasonal snowpack and decreased late summer
streamflow, before the glaciers become too small to support streamflow at
historic levels later in the 21st century. Conversely, in lower elevation
basins, smaller snowpacks and the shrinkage of small glaciers result in
continued reductions in summer streamflow.”
One notable finding from the study: “In
glacierized river basins experiencing more rapid retreat, declines in glacier
melt can further exacerbate negative trends in summer streamflows driven by
reductions seasonal snowmelt leading up to an 80 percent reduction in
late-summer discharge volumes by the end of the century.”
Frans explained that the 80 percent projected
reduction in late-summer discharge applies to the more sensitive Stehekin and
Cascade upland river basins that were evaluated, and he stressed that the
projected reduction would decline further down those drainages, as other
sources of water contribute to streamflows.
“That number will become smaller the further
downstream you go, the further you get away from the glacial influence,” Frans
A concept the study embraces is a “peak water”
theory, where glaciers begin melting at an accelerated rate because of their
reduced mass and increased exposure to warming temperatures.
“As things start to warm up, you’ll get
increased runoff from glaciers, but that decrease rapidly,” resulting in a
severely diminished streamflow contribution from glaciers, Frans said.
While low-elevation glaciers have already hit
their peak melt, high-elevation glacial melt is projected to peak around
mid-century, according to Frans. For areas fed by these glaciers, increased
glacier melt in the next several decades will partially buffer declining stream
flows from other sources, such as groundwater and reduced snowpack.
Frans said the theory relies on “pretty
robust” projections of declining annual snowpacks in decades to come, followed
by a lagging diminishment of glacial runoff.
“Glaciers can buffer water supplies. They melt
when it’s really warm and there aren’t many other sources of water,” Frans
said. “The buffering effects will sustain for the higher elevation areas, but
not so much for the lower elevation maritime basins.”
The research paper did not quantify
consequences of changes in summer streamflow but some of these changes may have
already begun impacting downstream systems, said Frans, noting that the study
sets the stage for future analysis of economic impacts, irrigation distribution
and impacts on recreation and fisheries that will likely be effected by a trend
of rising water temperatures.
“Glacier melt plays a more important role
during the driest periods of the year,” Frans said. “Systems downstream of
glaciers that rely on glacier melt to buffer low flow periods will suffer from
increased variability of the low flow season. This is all linked to lesser