Landslide of the Week – Koontzville Landslide

July 13, 2009

Each week we will feature a new landslide in Washington State. Washington State is covered with dynamic and sometimes quirky landslides.

Koontzville Landslide, Okanogan County
This landslide is part of the 1961 USGS publication Landslides along the Columbia River valley, northeastern Washington.

Location Map for the Koontzville Landslide

Location Map for the Koontzville Landslide

Koontzville Landslide Map

Koontzville Landslide Map

An excerpt from USGS Professional Paper 367 on the Koontzville Landslide:

“The Koontzville landslide involved the entire village of about 35 houses, one store, and a section of State Highway lOA [replaced by State Highway 155]. The village was built in 1934 and 1935. … Old landslide materials extend from river level almost to the top of the terrace, or to an altitude of about 1,600 feet. Little or no landslide activity was noticed before the 1948 flood. There may have been some slight highway settlements or minor movements owing to irrigation and river-bank erosion below the highway, but no property damage from landslides was reported. In the fall of 1948 (about the time of the Seatons landslide movement) one resident of the area had trouble with water pipes parting and resorted to flexible hose connections to keep his water system operating. So far as is known, this marked the beginning of reactivation of the ancient slide. The slide has moved many times since. Movements are recorded on the following dates:

December 23, 1951;
November 10 or 11, 1952;
November 27, 1952;
and January 10, 1953.

In contrast to the diminishing rate of movement observed in the Seaton slide since 1948, the Koontzville slide seemed to move more and at more frequent intervals in successive years to and including the spring of 1953. Local residents have noticed that their houses cracked and moved each weekend during low stages of the Columbia River, which corresponded to drops in river level due to power operations at Grand Coulee Dam. Many houses and the store have been severely damaged, the springs have changed their courses, large fissures have crossed the village area, and each year the slide has worked farther back into the hillside. In 1952, a fissure connected the Koontzville slide with the Seaton slide along the silt-granite contact (fig. 20). The displacement in 1955 extended all along the bedrock outcrops between Seatons Grove and Koontzville. Vertical movement along this bedrock scarp ranges from a few inches to 5 feet. Before the 1948 movement there was a light-colored zone on the granite immediately above the contact with the surficial deposits which ranged in width from 0 to 15 feet. Above this zone, all the granite wall is much darker due to weathering and organic growths. This light-colored zone may represent the amount these slides moved down following an earlier Columbia River flood such as the one in 1896. Geologically, Koontzville is in a setting where the sequence of Pleistocene deposits is the most favorable for landsliding. A preglacial channel of Peter Dan Creek underlies Koontzville and because of this geologic setting ground-water conditions are very high.

Figure 20 from USGS Professional Paper 367

Figure 20 from USGS Professional Paper 367

Conditions similar to this have been described in the Reed terrace area, and they can be anticipated, almost without exception, where deposits of silt and clay now occupy the area of confluence of preglacial valleys with the main valley. The causes of the initial reactivation of this ancient landslide seem to parallel those outlined for the Seatons landslide. The causes of the periodic movements, however, are not well understood. In 1953, the Corps of Engineers drilled three test holes in the slide to obtain undisturbed samples of the soil and to install gauges to record pore-water pressures throughout the year.”

The factors pertaining to the Seatons Landslide movement were:

“Many factors influenced the renewed landslide action in this area, of which the following seem the most important:
1. The unusually heavy rainfall during the spring and summer of 1948.
2. The high water in the Columbia River during the flood of May and June, 1948, undoubtedly resulted in a higher water table throughout the entire slide area.
3. The flood eroded and unloaded the toe of the slide, which is on the outside of a bend in the river where erosion would be greatest.
4. Melt water from the heavy snowfall in the winter of 1948 and 1949 kept the slide lubricated and moving after sliding began.
5. Very deep freezing in the winter of 1948 and 1949 may have had some effect in extending old slide cracks and in damming ground water.
6. Seatons Lake was created in a key position at the head of the ancient slide so that it kept much of the lower part of the ground saturated. Springs on the lower slopes of the hill produced more water when the level of Seatons Lake was higher, and the lake surface was raised purposely at times to make the springs at lower altitudes flow at a greater rate for irrigation.
7. The material at the toe of the slide consisted of silt J and clay thinly mantled with sand, gravel, and boulders. Silt and clay could be observed pushing through the gravels at several places along the toe of the slide. Since the construction of Grand Coulee Dam, a replacement supply of sand and gravel to cover and protect the silt and clay from erosion had been largely cut off.
8. The extensive use of this area for homes, gardens, irrigated tracts, and roads had undoubtedly been a factor in encouraging the renewed activity of the slide. Renewed activity might have been postponed if the natural cover of grass and sagebrush had not been removed and if the streams had been kept in their natural channels. The principal spring, which flowed a full stream through a 2%-inch pipe, supplied the entire area with domestic water. The other two springs in the drainage above were about the same size. The small stream, which was seasonally diverted into Seatons Lake, flowed between 0.5 and 0.6 cfs, even in dry years. The stream probably flowed about 1 cfs in the early spring and during unusually wet seasons. The supply of water to the main spring was cut off during the slide of November 1948, but the flow of water was restored to about normal by driving a pipe into a small seep which broke out near the spring. The spring water was milky for several days before it cleared.”

Looking at the 10m DEM, it looks like they missed a rather large earthflow that came down.

Earthflow on the Koontzville Landslide

Earthflow on the Koontzville Landslide

Although this probably didn’t play much of a role in the Koontzville movement in the 1950’s, except for the overall instability in the area. Finding information on this landslide has been difficult. I would only assume it has mostly stabilized out, as houses still dot the landscape.

Reference:
Jones, Fred Oscar; Embody, Daniel R.; Peterson, Warren Lee, 1961, Landslides along the Columbia River valley, northeastern Washington: U.S. Geological Survey Professional Paper 367, 98 p., 6 plates.

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