This story is Part II of a two-part series on the Tanana River Watershed. See Part I here.
Long-term monitoring has consistently shown that winter flow levels in Interior Alaska rivers are rising. But why? Precipitation is not systematically increasing, and most source waters are frozen in winter. According to a new paper in Geophysical Research Letters, the answer comes from far away: as mountain glaciers melt during the summer, melt water percolates into aquifers and later resurfaces downstream, making streams flow fuller in the coldest months.
For University of Alaska Fairbanks researcher Anna Lilijedahl and her coauthors, arriving at this conclusion meant uniting two different disciplines. “I came to realize by talking to colleagues and friends and reading papers that in the Arctic, the glaciology and hydrology communities are often doing the same things in parallel,” Lilijedahl told GlacierHub. “I saw an opportunity to link the two—what happens when water leaves a glacier and travels hundreds of miles to the ocean?” she wondered.
As her study area, Lilijedahl chose the Jarvis Creek watershed, which she called “accessible by Alaska standards,” meaning it runs parallel to the Richardson Highway and can be reached without a helicopter. The discontinuous permafrost that underlies this basin is characteristic of Interior Alaska, and Lilijedahl considers the region a proxy of other glaciated watersheds north of the Alaska range that lack road access.
To examine the relationship between glaciers, permafrost, and streams, Lilijedahl took a trip back to an older, colder time in Interior Alaska. She examined the Tanana River winter discharge record, which began in Fairbanks in the 1970s. It shows a steady increase in wintertime flow, as do similar records of streams across the Arctic and subarctic. She also analyzed satellite data and found that glacier coverage in the Tanana River watershed (of which Jarvis Creek is part) decreased by a remarkable 12 percent between 1950 and 2010.
Armed with this historical data, Lilijedahl and her team turned toward the present conditions of Jarvis Glacier and the Tanana River watershed. Scientists from the U.S. Geological Survey (USGS), the Army’s Cold Regions Research and Engineering Laboratory, and the Salcha-Delta Soil and Water Conservation District worked together to measure how much ice and snow Jarvis Glacier lost over the summer. They placed two gauges in the outflow stream, 55 kilometers apart.
“This is the only study that I’m aware of that put two stream gauges in front of a glacier,” USGS glaciologist Shad O’Neel told GlacierHub. This innovative method yielded interesting results: the stream lost roughly 46 percent of discharge between gauges. Such staggering loss is probably happening globally, according to O’Neel. “Around the world, glaciers turn into braided rivers with an abundance of sand and gravel that’s particularly good at soaking up water,” he said.
The scientists don’t yet know how long this water remains in the aquifer, but it clearly stays at depth long enough to be warmed by the tectonically active ground that underlies the Alaska Range. The 6 degree Celsius temperature of the groundwater is an “indicator that water has traveled pretty deep before coming back up. The deeper you go into earth, the warmer it gets,” said Lilijedahl.
From the aquifer, the water leaks into Jarvis Creek through the winter. “Everything is frozen except groundwater, so that’s definitely the source,” added O’Neel.
As glacial melt increases, the water melts the surrounding permafrost, enlarging the aquifer. Though this means that storage capacity is higher, input will ultimately decrease as the watershed’s glaciers shrink in a warming climate. Eventually, local wells may run dry, and Lilijedahl believes that lowland areas of Jarvis Creek and other similar rivers could eventually go dry in the summer.
Drying streams would threaten an animal important to Alaskan culture and economy—salmon. According to the Alaska Seafood Marketing Group, “Salmon are responsible for the greatest economic impact (jobs, income, and total value) among all species in the Alaska seafood industry.” In 2013 and 2014, this industry generated an average of 5.9 million dollars of total economic activity. Salmon is also a crucial resource for people living subsistence lifestyles. “A lot of Arctic communities rely on chum salmon for food and pet food. Some species may do well, others won’t fare so well,” said O’Neel.
The health of salmon populations depends on a complex web of factors, including water temperatures in areas where eggs develop and stream levels where adult salmon migrate. Glacial melt and groundwater reserves shape both factors. Chum salmon eggs develop faster where warm water, between 3 and 6 degrees Celsius, bleeds into rivers. Salmon lay their eggs where the 6 degree Celsius groundwater upwells in the Tanana and Yukon rivers, according to Lilijedahl.
Glacial contributions to streamflow can also help support adult salmon. Glacial melt accounted for over 15 percent of Jarvis Creek’s annual discharge. For streams in the Alaska Range that aren’t glacially fed, said Lilijedahl, flow near mountains may yield to dry river beds a few miles downstream. “Imagine that if you’re a fish!” she said.
Thinking like a fish is key to many scientists and decision-makers in Alaska. The ancient migration habits of salmon have for millennia guided them between the ocean and their freshwater birthplaces, where they spawn before dying. Favorable conditions, such as water temperature and sufficient stream flow, constitute the difference between life and death, a poor year for Alaskan fisheries and a booming one.
The findings from this study will help in management of salmon populations. It will also help with understanding the impacts of climate change on a much broader scale. “In the past, we assumed all loss from glaciers ended up in ocean. But hey, wait—at least 30 percent in this system is lost in the aquifer. It introduces more complexities into this sea-level rise thing,” said O’Neel.
Faced with the complexities of a changing climate, myriad impacts of which are already shaping Arctic and subarctic landscapes, further collaborations between the hydrology and glaciology communities will be essential. “We’ve drawn a linkage between the physical systems of glaciers and the biological system of salmon. No one would have guessed that existed, not that long ago,” said O’Neel.