Rain in the coming days may determine whether Cooper Landing will see flooding.
The glacier-dammed lake above the Snow River has begun draining, an unusual but predictable natural phenomenon that adds extra water to the Kenai River system.
Kenai Lake is expected to crest Sunday above flood stage at its outflow under the Sterling Highway bridge, according to the Alaska River Forecast Center.
Hydrologists tracking the event and projecting its future course predict that the "jokulhlaup" (an Icelandic term for the rupture and draining of a glacier dam) will cause minor flooding in Cooper Landing but not downstream in Soldotna or Kenai.
"Likely, it should reach its maximum outflow rate early this weekend," said hydrologist David Streubel.
Once the lake's final burst of water drains and the Snow River level peaks, the pulse will take about 12 hours to traverse Kenai Lake. The river source is expected to flow about six inches above flood stage.
"The water level in Kenai Lake was already that high at least twice already this summer," he said.
His colleague hydrologist Larry Rundquist elaborated.
"It is primarily water on the road going in to Primrose Campground," he said.
"Without additional precipitation thrown in, it is pretty hard for the glacier-dammed lake to cause appreciable flooding on the lower river."
They expect the river to crest in Soldotna on Sept. 25 with about one foot of additional water.
More rain during the next week could change the picture. The current forecast calls for showers, which probably will not add much to the river system. But if the rains turn heavy, they could swell the upper Kenai River further and add to the water level downstream.
The off-and-on lake is in the head waters of the Snow River drainage north of Seward, in a remote spot above the Snow Glacier. The lake forms in a valley blocked by the glacier and, when it gets enough water in it, forces its way under the ice four or five miles to emerge in the Snow River.
The lake forms and drains on a semi-regular cycle taking two or three years, emptying in the autumn. The last jokulhlaup at the site was in October of 1998.
This summer, the hydrologists and volunteer pilot Lyman Nichols have been monitoring the lake levels. A flight Sept. 5 showed the lake at its maximum level for the year and a return flight Sept. 9 found it had dropped about four feet. They estimate the lake began draining sometime between those two dates.
"The initiation date is a hindcast. And it's not hard and fast either," Rundquist said.
The forecasts are still uncertain of exactly when and how high the waters will crest, because the lake is in the early stages of draining. As of Monday evening, it had lost about a quarter of its water, Streubel said.
The pattern is that the lake drains slowly at first, then the process picks up speed. The further it is into the process, the more accurate the forecasts become.
Rundquist and Streubel expect to have firmer numbers and times on Wednesday, they said.
In the meantime, if the weather is clear, anyone with a small plane might be able to watch a rare show this weekend.
At the end of the process, the water is expected to drain rapidly out of the lake like water down a bathtub drain, leaving a gaping tunnel under the glacier. Streubel said it could be quite a sight.
"We've heard its about 500 feet deep," he said.
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