Mendenhall River carves a new course

Posted: Monday, August 09, 2004

The retreating Mendenhall Glacier is creating potential hazards for users of a popular trail and threatening the water level of the nearby airplane float pond.

The Mendenhall River cut through a riverbank just downstream from the Airport Dike trail last week, changing the river's course.

The change was due to warmer temperatures, which have caused the Mendenhall Glacier to retreat and the land around it to rise, said area experts who have been monitoring the erosion.

While the changing river has attracted onlookers fascinated by the acts of Mother Nature, airport officials are scrambling to put in motion plans to stabilize the dike that runs between the river and the float pond, where floatplanes are kept.

"There's been a significant amount of erosion on the embankment between the airport and the river," Airport Manager Allan Heese said Thursday. "Primarily the float pond is at risk right now."

Although the cut is downstream from the dike, experts say the change in the river will have upstream effects, as well. They don't know how far-reaching those effects will be. The rising land is causing the river to entrench itself and create a meandering effect, they said.

The airport is not in an emergency, Heese said, but he has started the process of getting appropriate permits to stabilize the dike. Heese is looking to place 500- to 1,000-pound boulders against the dike to prevent the river from eroding it and affecting the float pond.

At higher tides, a valve automatically opens and allows water into the float pond, Heese said. At lower tides, the valve remains shut to maintain the water level in the pond. But if the erosion goes deep enough, it could drain the float pond at low tide, he said.

Heese said he does not know yet how much the project will cost but will tap an emergency airport maintenance account. If the airport does not have enough money, he will look to the city for money.

Emergency vehicles can still use the dike trail in case of an airplane accident in the Mendenhall wetlands, Heese said. The trail and river run through part of the Mendenhall Wetlands State Game Refuge.

If the trail becomes too unstable, airport officials would cut the airport fence or go through a crash gate on the east end of the float pond.

Airport officials have decided to keep an airfield rescue fire-fighting truck off the trail as a precautionary measure. The truck is used on the trail for training.

"I don't want to take a chance because the path is starting to wash away a bit," said Jerry Mahle, the field maintenance manager at the airport.

Airport officials posted signs warning trail users of the unstable riverbank but were still allowing pedestrian use as of Friday. Heese especially cautions parents to keep track of their children. He witnessed a little girl chase a dog that ran up to the edge of the riverbank.

Laurie Ferguson Craig of Juneau has walked the trail almost every day for the past 20 years. She said the riverbank was still intact at noon last Monday but had cut through by the time she returned at 9 a.m. Tuesday.

"A week ago you could have walked right over to that and now it's an island," Craig said, pointing to the cut Wednesday.

The Mendenhall River probably was formed 100 to 250 years ago, when the Mendenhall Glacier began its most recent retreat, according to a 1999 study, "Hydrology, Geomorphology and Flood Profiles of the Mendenhall River, Juneau Alaska."

The study was done by the U.S. Geological Survey in cooperation with the city and state. The river runs 5.5 miles from Mendenhall Lake to the mouth at Gastineau Channel, past the dike trail.

Naturalist Richard Carstensen of Juneau predicted the cut would occur in 2005, he said Thursday. Carstensen has an air photo collection that shows that a portion of the riverbank that jutted into the river, forming an oxbow shape for the water to run around, was 800 feet wide in 1948 and eroded to 60 feet wide by 2001. The river has now cut through the neck of the oxbow and formed an island.

A cut in a riverbank is not unusual, but the Mendenhall River has become more meandering over time due to glacial melting, said Roman Motyka, a professor of geology and geophysics for the Geological Institute in Fairbanks. Motyka, who is stationed in Juneau, has studied glaciers of Southeast Alaska for more than 20 years.

The glacier retreated about 2 miles during the 20th century, he said.

The region running from Sitka to Yakutat to Skagway has glaciers that are melting, which is causing land to rise, Motyka said. The land is rising about one-half inch per year. Over the last 200 years, 10 feet of land has risen, he said. When glaciers retreat, they release pressure on the Earth's crust. The Back Loop Road area of the Mendenhall Valley was under ice 200 years ago, he said.

The cut at the Airport Dike Trail had officials talking about a similar second cut they expect to happen. In 1962, a section of the river's bank upstream from Vintage Business Park was 240 feet wide and is about 35 feet wide today, said Rorie Watt, the city's chief capital improvement projects engineer.

"The river is a dynamic beast," Watt said. "It's always changing. It's always moving gravel and depositing it somewhere else."

A gravel bar exists behind the business park, but Watt said it could be washed downstream at any time. Several houses are downstream from the business park, as well. The city is not responsible for protection of property along the river that is not city-owned, Watt said.

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