A city can't function without water and electricity.
Anchorage's primary source of water is Eklutna Lake. The reservoir, located about 30 miles northeast of downtown Anchorage, is about 7 miles long and 200 feet deep.
Eklutna Lake also provides hydroelectric power. Anchorage Municipal Light and Power (ML&P) and two other power companies own all of the water in the lake. However, the electric utilities are required to give first priority to Anchorage's drinking water supply. The lake supplies over 90 percent of the municipality's water and about 15 percent of its electricity. According to Mike Dillon, a former supervisor of the Eklutna Power Plant, when it comes to hydropower "water is fuel." Neither of the municipal entities that supply electricity and water wants to waste a drop of the water stored in Eklutna Lake. When the water level gets high, ML&P can operate the Eklutna Power Plant at its highest capacity and throw open the sluice gates leading into the municipal water system.
The lake's water level is managed so carefully that it is unusual for water to pour over the spillway. Prior to the last couple of years, the lake's storage capacity had been exceeded only eight times since the dam was built in 1965, most recently in 1997.
Judging from the past two summers, Eklutna Lake is brim full and then some. The water level has exceeded the height of the spillway by late summer, allowing millions of gallons to escape downstream. So you might think that there's more than enough water in Eklutna Lake for people and fish alike.
That happy situation is not going to last.
The incredible shrinking glacier
The lake's water comes from runoff and glacial melt in a 120-square-mile watershed. Most of the water from glaciers comes from the West Fork of Eklutna River, where the Eklutna Glacier covers about 46 percent of the basin's surface area. About 12 percent of the East Fork of Eklutna River's watershed is covered in small, remnant glaciers. In some years, Eklutna Glacier supplies most of the lake's water.
But like most Alaska glaciers, Eklutna Glacier is shrinking. A photo taken by Stephen R. Capps in 1915, the year after Anchorage was founded, showed the glacier's terminus jutting from the gorge. By comparison, a photo taken by Ron Karpilo in 2010 showed the glacier had receded substantially, more than 1-1/2 miles. The terminus and surrounding area depicted in the 1915 photograph, which appears to have consisted solely of ice and recently exposed rocks, is now largely forested.
Michael Loso, a glaciologist and associate professor of earth sciences at Alaska Pacific University, began collecting data on Eklutna Glacier six years ago. Employing the brainpower and leg power of undergraduate and graduate students, Loso has mapped the current limits of the glacier, measured the depth of its ice with radar, and monitored the annual accumulation and melting of snow that fuels glacial movements.
Obviously, the glacier lost lots of ice in its retreat up the gorge. But that's just the tip of the iceberg. Less obvious to a casual onlooker is how much thinner the glacier has become. One of Loso's graduate students, Louis Sass, now a glaciologist with the U.S. Geological Survey, measured an average 486 feet of ice in Eklutna Glacier, with the deepest ice about 1,400 feet, nearly five times the height of the tallest building in Alaska, the Conoco-Phillips Building.
Comparing 2010 data on ice surface area and thickness with the surface area and elevation of the glacier when it was first mapped in 1957 by the U.S. Geological Survey, Loso calculated the surface of the glacier has dropped as much as 200 feet in some portions of the accumulation zone. A large portion of the eastern lobe of Eklutna Glacier is still more than 800 feet thick. Unfortunately, that's also the lobe that's shrinking fastest.
Eklutna Glacier lost 23 percent of its volume between 1957 and 2009. Converting ice into a standard measure of water, the net loss was about 973,000 acre-feet. This is enough water to completely fill Eklutna Lake 2.4 times or enough to recharge its storage capacity nearly six times, because the Eklutna Power Plant's water intake tunnel is located 78 feet below the surface.
Mining the glacier for water
According to Loso, his model estimated that since 1957, on average, 8 percent of the water Anchorage has drawn annually from Eklutna Lake was "mined" from the glacier.
Glaciologists call this net loss of ice that becomes available for human use during extended periods of heavy glacial melting the "deglaciation discharge dividend."
Like other materials that are mined, coal for instance, the water from the lost glacial ice isn't a renewable resource in a time frame that is useful for civilization. Eklutna Glacier has been shrinking for millennia, but the rate of shrinkage of many glaciers has accelerated in recent decades and even if the warming trend plateaus, Loso believes that the geometry of Eklutna Glacier "almost guarantees" there will be substantial ongoing shrinkage. Anchorage's "deglaciation discharge dividend" will diminish over time.
Eventually, perhaps many decades from now, Eklutna Glacier will stop shrinking and its annual ebb and flow, which depends on snowfall and air temperatures, will settle into a more stable equilibrium. By then, the source of the bonus water the city is extracting from the glacier will be gone. Annual precipitation isn't likely to make up the difference. So Eklutna Lake isn't going to be recharged as quickly it is now.
Loso is no Henny Penny. He's not predicting that Eklutna Lake is going to be sucked dry or that the city will run out of water for electricity and drinking. But the law of supply and demand means Anchorage residents will pay more for drinking water, and electricity is liable to cost more when electricity derived from burning natural gas or coal is substituted for cheaper, cleaner hydroelectric power.
Further complicating the impending reduction in glacial runoff is the uncertainty of how to budget water use. ML&P wants the lake to be full at freeze-up to maximize the hydroelectric capacity in winter.
However, if rainfall or glacial melt exceeds expectations in late summer or fall, the water can't be drained fast enough for electricity and drinking and some water pours over the dam.
As spring turns into summer, ML&P must closely monitor how much water is in the lake and accurately predict how much more water is likely to flow into the lake before freeze up. The prediction relies on weather and runoff patterns from previous years. And there's the rub.
The warming climate and loss of the "deglaciation discharge dividend" are confounding factors. Loso speculated that this year's extreme weather patterns contributed to the water flowing over the dam. A late spring suggested that summer would be shorter than usual, with low runoff. Instead, warmer-than-anticipated air temperatures released more water from the glaciers, filling the lake to the brim. Runoff from the near-record rainfall in late summer and early fall boosted the water level several feet above flood stage.
Loso believes that we can expect more years of unpredictable variation in glacial melting before Eklutna Glacier establishes a new equilibrium with the climate.
Because other Alaska rivers are glacier fed, Loso added another piece of glaciological advice. The folks who are counting on generating hydropower by damming the Susitna River shouldn't assume that the rapidly melting glaciers at its headwaters, in the Alaska Range, will keep pouring as much water into the river as they do now. The predictable loss of their "deglaciation discharge dividend" should be taken into account.
Rick Sinnott is a former Alaska Department of Fish and Game wildlife biologist. The views expressed here are the writer's own and are not necessarily endorsed by Alaska Dispatch. Contact him at firstname.lastname@example.org