March 27, 2014 in City

Spokane has lower risk for landslides than Western Washington

By The Spokesman-Review
 
PHOTO ARCHIVE photo

A mountain of earth plugs the Spokane River about 1 mile east of its confluence with the Columbia River following a massive landslide in 1969. All but tiny traces of the winter’s snowfall had been absorbed into the ground when the soggy mass broke loose from the north shore. This view looks eastward, where the blocked stream, rising slowly, began flowing again over the southern edges of the slide within hours.
(Full-size photo)

Map of this story's location

Landslides are part of Spokane’s geologic history, though the risk is slim for the type of major slide such as last weekend’s devastating mountainside collapse northeast of Seattle.

“Low risk doesn’t mean no risk,” said Thomas Frost, geologist with the U.S. Geological Survey in Spokane.

The city of Spokane has a series of slide piles left behind as a result of Ice Age floods about 12,000 years ago. Those piles are considered stable now.

The lower South Hill is probably the largest slide pile in the city. Other piles are found along the Rimrock area west of Government Way and against Five Mile bluff.

They were created when the glacial floods undercut the steeper walls of the bluffs, causing them to slump downward.

Recent history shows that landslides have occurred intermittently across the Inland Northwest, and usually in locations identified by geologists as prone to slides.

Outside Spokane, one of the largest slides in recent history occurred along the Spokane River at a location known as Miles near the river’s confluence with Lake Roosevelt on the Columbia River.

In 1969, the lake was drawn down for construction of a third power house at Grand Coulee Dam. A 300- to 400-foot cliff slid into the river on March 26, blocking the flow of the Spokane River. A wave towering 50 to 60 feet pushed downriver.

Dirt continued to fall for several days, and visitors flocked to the area for a look.

More than a decade earlier, a series of slides occurred along the Columbia River near Kettle Falls. Slides had been occurring there since 1926, but on Feb. 1, 1951, a new slide sent a 30-foot wave across the river, swamping a lumber mill. Slides continued there for several years.

Geologists later discounted the water from Lake Roosevelt as the trigger, but instead pointed to excess runoff on Sherman Creek that sent moisture into the unstable layers of ancient river deposits along the cliffs.

Bonners Ferry has seen several slides over the years, including one on March 4, 1954, that killed a couple in their Bonners Ferry home. Their three children were rescued from an upper story.

In October 1988, another landslide occurred along the North Hill widening project for U.S. Highway 95 north of Bonners Ferry. It blocked the highway for three weeks.

In Spokane, flooding on Latah Creek in 1996 and 1997 undercut a steep bank below Highland Park Estates, taking away a sizeable chunk of towering bank along several exclusive view properties. Those sediments were left behind by the Ice Age floods.

Frost said that Spokane and Spokane Valley have a lower risk because differences in Ice Age glacial patterns.

“Things on the scale of the Oso slide are not likely to happen in the immediate Spokane vicinity,” Frost said.

Flood gravels in the Spokane area were deposited in the valley bottom rather than along the adjacent hillsides, Frost said.

By contrast, thicker deposits with fine-grained material were left along mountainsides from streams flowing into Puget Sound and to a lesser extent along the Columbia River Valley east of Grand Coulee, Frost said.

Ice sheets in those locations blocked river flows and caused sediments to fill up along mountain walls in Western Washington, he said.

Once the ice melted, the rivers carved out new valleys but left the higher sediments behind. Layers of silt and sand within those sediments are relatively thick. Clay and silt impede downward movement of water following heavy rains, creating saturated soils that can become very unstable and cave away, he said.

Rock and gravel in the Spokane River Valley allows water to flow easily. Also, sediments in the Spokane region are not perched above the valley floor to the extent they are in Western Washington.

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