Glacial Deep-Seated Landslides and Forest Parctice: One Example Near Index, Washington

The Washington State Department of Natural Resources (DNR) issued a mitigated determination of non significance (MDNS) for the Deer Wrap timber sale on January 12, 2015. The proposed timber sale is on DNR managed trust lands and the sale would, if a high enough bid comes forward, generate revenue to the state and local governments (Snohomish County in this case). The proposed timber sale area is presented on the map below and this map and related documents including the MDNS can be found at http://www.dnr.wa.gov/ResearchScience/Topics/SEPATimberSales/Pages/amp_sepa_nw_ts_deerwrao_feb.aspx.

The City of Index and the Forest Law Center submitted some comments during the SEPA (State Environmental Policy Act) comment period. The comment period on these proposals is rather short. In this case DNR responded to the comments and then issued the MDNS.

One of the concerns raised was that the DNR's own geology maps included a large landslide feature just below a portion of the proposed harvest area. The Forest Practices Board very recently passed new Forest Practice Guidance for timber harvests in areas that are considered groundwater recharge areas to deep-seated glacial landslide areas. This was in part and certainly sped by the Oso/Hazel Landslide.

The DNR geology map appears to be based on the Skykomish 1:100,000 Geologic map by Tabor and others (1993). I marked up the Skykomish map to illustrate a few pertinent features: the proposed harvest, the mapped landslide area, the edge of the Puget ice lobe which blocked the Skykomish River forming a lake, the location of the lake outflow, and the location of a nasty landslide that took place last winter, as well as a personal life note.

 

Part of Skykomish Map (Tabor and others, 1993) marked up

A few units: 

Ql = landslides

Qvr = glacial recessional outwash

Qvgl = glacial lake sediments

Qa = alluvial

other units are bedrock

The Skykomish Map included previous work by Booth (1990) on Quaternary deposits and land forms. That earlier work provided a detailed mapping effort that better mapped the drainage impacts of the Puget ice lobe on the mountain front on the west side of the Cascades between the Skykomish River and the Snohomish River. The Booth map shows the same landslide area but a bit more detail on the glacial units.

Part of the geologic map by Booth (1990)

Booth (1990) mapped and described how the Puget ice lobe blocked the Skykomish River and the outlet to the river flowed through now abandoned river valleys south to the Snoqualmie River. The Puget ice lobe flowed up the Skykomish and blocked the river near Index and the outlet to the river as well as lots of water flowing around the margin of the ice lobe flowed south through the pass marked on the Tabor and others (1993) map above.

Below is a marked up DEM showing how that blockage might have looked.

Booth (1990) indicated that a portion of the outlet flow along the ice margin would have been below the ice and during the maximum ice extent that is most likely to have been true based on DEMs of the area.

Booth (1990) mapped the area without the aid of LiDAR. The more recent LiDAR imagery has provided even better resolution and reveals a more refined image of the landforms in the vicinity of the proposed Deer Wrap timber sale. Kara provided me a LiDAR image of the mapped landslide area.

The landslide area based on the DNR map is outlined in orange. Based on the LiDAR it appears that the slide area is not a single landslide but multiple deep-seated landslide features of smaller scale than a single massive landslide covering the whole area.

I marked up an outcrop of bedrock that cuts across the slope. The LiDAR suggests that a layer of glacial outwash sediments overlies the bedrock on the steep slopes. The scalloped terrain above the bedrock outcrops suggests a number of convergent landslide scarps that extend to the top of the steep slope. The fluted landscape above the landslide area appears to be a river flow path which would be consistent with water flowing around the margin of the Puget ice lobe. Booth mapped the area to the west as being underlain by glacial moraine, and the LiDAR imagery appears consistent with that interpretation.

It is not possible to assess the potential landslide risk of the slide areas that appear in the LiDAR. That will take some on the ground work. It is not clear how much of that has or has not been done. The forest practice application did not include any geology reports.

The new Forest Practice guidelines for these types of slide areas suggests that geologic assessment evaluation should be provided in regards to potential impacts from increasing the groundwater flow to the slide area(s) as a result of loss of tree canopy and water interception and transpiration. It is my understanding that the DNR may have withdrawn the sale and is reevaluating or will provide a more detailed assessment of this area.