Landslide of the Week – Stemilt Landslide
May 5, 2009
Each week we will feature a new landslide in Washington State. Washington State is covered with dynamic and sometimes quirky landslides.
Stemilt / Malaga Landslide
The Stemilt landslide (also known as the Malaga Landslide) is the largest known landslide in Washington State, if not one of the largest terrestrial landslides the world. Stemilt is a Native American (Salish?) word loosely translated as foothills. It is located along the Columbia River, south of Wenatchee at the town of Malaga.
This map is made on a 10m DEM.
Area: ~46 sq. mi.
Width: ~3.6 mi.
Length: ~12.6 mi.
Depth: ~0.3 mi. (1,500 ft)
Volume: ~13.8 cu. mi.
Age: 20,000 years.
The Stemilt landslide is composed of Eocene sandstone and siltstone and Miocene Basalts (Atrim, 1974; McJunkin, 1969). Movement is estimated to have started at least 20,000 years ago (Atrim, 1974; McJunkin, 1969). Movement might have been caused by “low strength of sand and silt interbeds of the Columbia River Group and the Eocene sandstone and siltstone beds. Saturation of these beds by ground water from high rainfall, possibly blockage of river flow near Rock Island contributed to the massive failures” (Atrim, 1974).
Glacial meltwater flowing out of Moses Coulee at this time formed a debris dam across the modern Columbia River channel, forming a lake. McJunkin (1969) suggests that the landslide flowed into the lake (catastrophically?) and suggests a drainage of the lake near the time of the landslide failure. Perhaps the failure of the Stemilt Landslide created a seiche within the lake, although no identified deposits have been discovered. The failure of the Stemilt landslide could have also blocked the Columbia River dam, as landslide debris appears to cross the channel. Thus, even though the downstream dam failed and drained, a lake could still have existed for some time. In modern times, if the Stemilt moved and blocked the Columbia River, it might look something like this:
This map represents a 200 foot blockage across the Columbia River, something that appears realistic in the topography. The lake would flood most of Wenatchee and Cashmere. A bigger dam (although difficult to accomplish in the existing topography) would eventually extent to Leavenworth. Landslides like the Stemilt Landslide are fairly common along the Columbia River. Just east and north of Wenatchee is the Beezley Hill Landslide Complex, which is about 50 sq.mi. in size and downstream is the well known Bonneville Landslide Complex (about 12 sq.mi. or so). Many of these landslides have the potential to dam the Columbia River and create a lot of impact to communities and infrastructure. A dam burst flood as a result to any of these landslide could create a wave that would over top the Columbia River dams and create considerable damage to Portland (and smaller communities along the Columbia). Luckily most of these landslides are inactive (except perhaps the Bonneville), but as growth continues to expand onto these features, problems might arise. Artim (1974) recognized that ” present stability of these ancient slides could be upset by the mishandling of rainfall runoff, septic systems, and irrigation water”. Unfortunately, there is significant agricultural growth on the Stemilt landslide.
Artim, E. R., 1974, Genesis and stability of an ancient landslide province, Washington [abstract]: Geological Society of America Abstracts with Programs, v. 6, no. 3, p. 140-141.
McJunkin, R. D., 1969, Geology of the Malaga landslide, Chelan County, Washington [abstract]: Association of Engineering Geologists, 1969 National Meeting, Program, p. 29.