Steelhead Haven Landslide
Details are slowly emerging of the landslide history of this site. It is clear that major landslides have occurred here on many previous occasions; indeed so much so that the landslide is known as either the Hazel landslide or the Steelhead landslide.
BEFORE AND AFTER:
This aerial photograph shows the devastation of the Steelhead Haven Landslide Monday afternoon near Oso, Snohomish County. In this view to the west, Highway 530 runs along the Stillaguamish River until the highway disappears into the debris field.
The pile of mud and debris at center, 20 feet deep or more in some areas, blankets Steelhead Drive and surrounding homes. On the other side of the river, which is cutting a new channel, stands the cliff that was left behind, 1,500 feet wide and 600 feet tall.
*Use the toggle in the center of the image in order to see the actual extent of the slide.
The Steelhead Haven Landslide occurred at the southeastern edge of Whitman Bench, a land terrace about 800 ft (240 m) above the valley floor and consisting of gravel and sand deposited during the most recent glaciation.
When the Puget Lobe of the Cordilleran Ice Sheet moved south from British Columbia, Canada filling the Puget Lowland, various mountain valleys were dammed and lakes were formed. Sediment washed down from the higher mountains settled in the lake bottoms forming a layer of clay. As the glacial ice pressed higher against the western end of Mount Frailey, water flowing around the edge of the ice from the north was forced around the mountain, eventually pouring in through the long valley extending to the northwest and now occupied by Lake Cavanaugh. Sand and gravel carried by the flow and entering the glacial lake dropped out to form a delta, the remnant of which is now known as Whitman Bench.
Following the glacier’s retreat and allowing for the lake to be released, the river carved out most of the clay and silt deposits, leaving the former delta “hanging” approximately 650 ft (200 m) above the current valley floor. When the sand portion of a deposit has very little clay or “fines” to cement it together, it is structurally weak, leaving the area around it vulnerable. Such an area is also sensitive to water accumulation, increasing the internal “pore” pressure and subsequently contributing to ground failure. Water infiltrating from the surface will flow through the surface, save for contact with the less permeable clay, allowing the water to accumulate and form a zone of stability weakness.
Such variations in pore pressure and water flux are one of the primary factors leading to slope failure. In case of the area of the Stillaguamish River where the March 2014 slide occurred, erosion at the base of the slope from the river flow further contributes to slope instability. Such conditions have created an extensive series of landslide complexes on both sides of the Stillaguamish valley. Additional benches on the margin of Whitman Bench are due to deep-seated slumping of large blocks, which also creates planes of weakness for future slippage and channels for water infiltration.
According to a 1999 report submitted to the Army Corps of Engineers by geologist Daniel J. Miller, PhD:
The Hazel landslide has been active for over half a century. Thorsen (1996) noted a tight river bend impinging on the north bank with active landslides visible in 1937 aerial photographs. The next 60 years involves two periods of relatively low landslide activity, and two periods of relatively high activity, the last of which extends to this day .
Known activity at this specific site includes the following:
1937: aerial photographs show active landslides.
1951: mudflow from a side channel briefly blocked the river.
1952: movement of large, intact blocks, leaving head scarps 70 ft (21 m) high. Later photographs show persistent activity through the next decade.
1967 January: slump of a large block and accompanying mud flows push the river channel about 700 ft (210 m) south. This protects the toe from erosion, activity is minor for about two decades.
1988 November: erosion of the toe leads to another slide, and the river is again moved south, but not as far as in 1967.
2006 January 25: large slide blocks the river, new channel is cut to alleviate flooding.