Last week, Jim Coe witnessed signs of a phenomenon that he wouldn’t usually expect to occur until after the blueberries had fully ripened and hares had begun to grow white coats. U.S. Geological Survey hydrographs charting water levels in the Kenai River indicated that after peaking on Wednesday, water levels in the Kenai River at Cooper Landing had begun to drop, but river water downstream of Skilak Lake was not following suit.
Instead, river water downstream of Skilak Lake climbed on Thursday, Friday and again Saturday. Finally on Saturday, Coe, a National Oceanic and Atmospheric Administration river forecast center senior hydrologist, began to suspect a glacier-dammed lake outburst had sent water gushing into the Kenai River.
About once every two to three years, runoff water collects in a valley perpendicular to the side of Skilak Glacier grows into a lake too big for the glacier to hold back. The lake’s water bursts out below the glacier, flows into Skilak Lake and into the Kenai River. But records going back to 1985 indicate that all of the outbursts since 1985 have occurred in either the fall or the winter. Neither precipitation nor snowmelt, however, could account for the water level trends the hydrographs had recorded, and on Saturday, Coe began making some calls.
As Coe called other researchers in the area, he received reports further suggesting that a glacial-dammed lake outburst had occurred, including a report that the water color in Skilak Lake had recently changed.
Also, hydrographs charting water levels in the lower Kenai River were typical of those documenting a glacial-dammed lake outburst. Such a hydrograph initially shows a slow increase in water levels, which then begin to increase at a faster and faster rate. Outbursts occur when a glacial-dammed lake grows too deep for the glacier to hold back.
“When the water level gets to a certain point, it exerts pressure on the underside of the glacier and will actually lift it enough for the water to start flowing under and through the tunnels in the glacier,” Coe said.
The range of lake depth that will trigger a release depends on multiple factors, including temperature. Once a lake gets to a certain height and the bottom of a lake builds up enough pressure, it forces its way out.
Scientists do not know for certain why the outbursts at the lake trapped by Skilak Glacier typically occur in the fall and winter. But with the wet season usually occurring in mid-July, August and September, late summer and early fall rainfall accumulation may give the lake enough depth to force its way out, Coe said. This year the lake apparently got deep enough in July.
Many factors could be involved in answering why the lake’s outburst occurred in the summer. NOAA has not yet been able to get a flight up to the lake to quantify how high the water was before the release and doesn’t know if the lake’s water was higher or lower than it usually would be when it releases. NOAA is hoping to arrange a flight to the site before the end of the week.
Because river levels have already been somewhat high due to snowmelt and recent rainfall, researchers have had a difficult time pinpointing when the outburst began, but suspect it began July 21. As of Monday, the water level data suggested the lake was continuing to drain, and researchers believed it might reach a peak later in the week.
“It’s strictly guesswork, hopefully educated guesswork, but we’re anticipating that it will peak on Wednesday (July 29), which would be eight to nine days since it started,” Coe said.
NOAA doesn’t know how much water is coming out of the lake, and historically the time it has taken the lake to drain has varied widely. The flow of water from an outburst tends to peak just as the lake is nearly through draining, and the number of days it has taken the Skilak glacial-dammed lake to peak has ranged from five days in 1995 to 11 days in 1985.
“When the lake is empty it shuts off and goes down very quickly,” Coe said. “So it doesn’t look like a typical rainfall, snowmelt hydrograph at all.”
The lake trapped by Skilak Glacier is not the only glacial-dammed lake in the Kenai River watershed. A lake trapped by the Snow River Glacier above Kenai Lake also occasionally outbursts, pouring even more water into the Kenai River than the lake trapped by Skilak Glacier. The Snow River Glacier-dammed lake also outbursts every two or three years, and in 60 years of record keeping has only outburst three times during the summer. Since 1949 the lake has always gone out in September, October or November, except for one outburst in July and two outbursts in August.
“Most of the time they’re both fall events, but Mother Nature does what it wants to do when it wants to do it,” Coe said. “The internal mechanisms of glaciers, let alone the glacial outbursts, are not well-known.”