Shaped by ancient fire and ice, waterway twists to age-old tune

Posted: Friday, July 06, 2001

Everyone on the central Kenai Peninsula knows the Kenai River and where it goes. But few know where it came from and how it got the way it is.

Geologists are still piecing together the biography of the waterway.

"Six percent of the water that flows into Cook Inlet is from the Kenai River, but 40 percent of the salmon are from the river. There is something unique about the Kenai River," said Robert Ruffner, director of the Kenai Watershed Forum.

Ruffner, who teaches geology at Kenai Peninsula College, gives talks about the river's geology as part of a summer program series the forum sponsors.

"We are in about as active a geological area as can be," he said.

The foundations of the Kenai Peninsula were laid down nearly 200 million years ago to the east. When dinosaurs stalked the earth, volcanoes in the Wrangell-St. Elias area dumped ash and mud into the seas nearby, and the shifting plates of the Earth's crust moved the ocean floor westward to what is now Prince William Sound and the Kenai Peninsula, he said.

The ocean floor plate hit against the land plate in Southcentral Alaska and pressed beneath it. At the juncture, the land rose, and the deep sediments of the seafloor scraped off and piled up in mountainous ridges.

"That is what formed the Chugach range we see," Ruffner said.

Those multimillion-year-old plates are still moving at a rate of about 4 to 10 inches a year. Beneath our feet, the crust that was once the floor of the northern Gulf of Alaska angles in toward the planet's hot core. When it gets deep enough, it gets hot enough to partially melt.

The molten portions are less dense, and work their way toward the surface, pressing up and fueling the volcanoes along Cook Inlet's western shore.

We live between the place where the ocean plate dives below the land and the place where it melts, he explained.

The lower rolling hills, plains and Cook Inlet between the two mountain ranges are a type of land feature scientists call a fore-arc basin, Ruffner said.

"They are really cool. We don't understand them," he said.

For the last 65 million years or so, since about the time when mammals and birds replaced dinosaurs, the Kenai Peninsula and Cook Inlet basin have been filling up with soil and slowly sinking under the accumulated weight. Near the towns of Kenai and Soldotna, the blanket of sediments is about three miles thick. Beneath it lie fossils -- and fossil fuels.

More recently, in the geological eye blink of the past 25,000 or so years, ice ages shaped the Kenai Peninsula.

"All of what is now Cook Inlet was probably filled with ice," Ruffner said.

Ice pushing dirt around formed the hills and ridges that vary the plateau of the western peninsula today.

Dick Reger, a retired state geologist who is studying the Kenai River as part of a project for the watershed forum, picked up the story from there.

"In order to understand the history of the Kenai River, you have to understand the glaciation, he said.

The last major glaciation in this area began about 25,000 years ago, he explained. At that time, all of Cook Inlet and the peninsula were covered with ice. Looking at clues to how high the land has rebounded after the weight of ice was removed, geologists estimate that the site of the city of Kenai was buried beneath a glacier about 1,000 feet thick.

As the ice waxed and waned, it eventually separated into different glaciers. As those glaciers retreated, meltwater between them gave rise to rivers.

On the peninsula, Reger dates the parting of the ice sea to about 18,000 years ago. At that time, a line appeared between the glaciers from the Kenai Mountains to the southeast and those from the north and west. The shifting ice produced a series of glacially dammed lakes running from about the Mystery Hills southwest to about Anchor Point.

"What happened then was basically the beginning of the Kenai River," he said.

A lake over what is now Sterling eventually overflowed and cut a course southwest across the newly opened land, joining with meltwater from what is now the Tustumena Glacier in a lake near upper Coal Creek northeast of Kasilof. Traces of this oldest Kenai River channel remain in the Kenai National Wildlife Refuge south of Funny River Road and include Headquarters Lake, he said.

But the young Kenai River had a wandering urge.

As the ice pulled back, the river moved northwest and flowed along the front of the northern glaciers.

Skilak Glacier used to block the area where the river now flows into that lake. Scientists believe the river flowed north of the present course, through Engineer Lake and others in the area and via what is now the East Fork of the Moose River.

The river and the peninsula looked very different from what they are now.

Sea level relative to the land was higher then, and at one point the river met the sea near what is now Poppy Lane. The landscape was bare and frigid. Geologic traces show wind-blown dunes and volcanic ash, but little vegetation until about 10,000 years ago.

"It was a pretty inhospitable place," Reger said.

"One of the interesting problems I have is trying to date the material in this area because, until recently, there were no trees. ... Spruce didn't get into this area until about 8,000 years ago."

The young river moved massive amounts of meltwater and gravel in a wide valley of braided glacial streams. Over about 16,000 years of erosion, it remade the face of the central peninsula and gradually converted to an entrenched, meandering river when the Ice Age ended about 10,000 years ago.

"It has been doing a lot of work. It has cut out a lot of sand and gravel," Reger said.

That process left behind the biggest rocks.

"Most of the Kenai River channel is lined with big boulders that don't move. And of course they make a great nursery for salmon," he said.

Ruffner noted that the river carries far less water than it did during the retreat of the massive ice sheets. The Kenai River today is what geologists call an "underfit stream," he explained.

"It means its course was set by a huge amount of water and now it has a lot less."

That is why the river valley is so broad compared with the size of the modern waterway. One effect of the shrinkage is that the Kenai River is stable and unlikely to move around.

That reduces the likelihood of unpredictable erosion, such as that seen at the Matanuska River.

The retreat of the glaciers lightened the weight of the land, causing it to rise. As the land rose, the river and sea cut deeper into it.

That is why there are bluffs near the river mouth and beaches, he said.

That glacial rebound lifting has, in the long run, more than compensated for the subsidence caused by earthquakes such as the 1964 tremor, Ruffner said.

But the future of the river and the towns along it do not really rest on firm ground.

"We are kidding ourselves if we think 1964 was a long time ago and it won't happen again," Ruffner said. "We should expect dramatic things to happen, given where we live."

Subscribe to Peninsula Clarion

Trending this week:


© 2018. All Rights Reserved. | Contact Us