Research shows impacts of melting glacial watersheds

New research shows how accelerated melting of glaciers in the Cascade Mountain Range could dwindle late summer stream flows 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 study states.

“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 in 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 said.

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 affected 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 volumes.”