LANDSLIDE INVENTORY MAPPING BETWEEN KELSO AND WOODLAND, COWLITZ COUNTY, WASHINGTON STATE

In response to the Aldercrest-Banyon (1998) and other recent damaging landslides in Cowlitz County, the Washington Division of Geology and Earth Resources began a GIS-based landslide inventory and slope stability mapping project in 2000. ~ 275 km² of urbanizing lands between Kelso and Woodland have been mapped for landslides. More than 300 deep-seated and 115 shallow landslides have been identified. Landslides were identified from 1951 (1:48,000), 1974, 1993, and 1999 (all 1:12,000) aerial photographs. Of the deep-seated landslides identified in aerial photos, 75% have been field checked, and 20% exhibit evidence of movement within the past ~ 10 years. Deep-seated landslides range in size from 0.0001 to 1.4 km². Landslide movement has been documented on slopes with gradients as low as ~ 10% (6°).

The study area is characterized by high-plasticity clay-rich soils and deeply weathered Tertiary bedrock (saprolites). Landslides occur within Tertiary volcanic and sedimentary units, at the interface between Tertiary bedrock and overlying younger late Tertiary to Quaternary unconsolidated fluvial units, and within the younger unconsolidated fluvial deposits. The dominant form of landslide movement is slow, rotational to translational rock and/or earth sliding. The majority of landslides appear to have moved in response to natural causes, such as above-average annual precipitation. Some of the now-dormant deep-seated landslides may have been seismically triggered. Lower elevation landslides (below ~ 75 meters) may have initiated in response to rapid drawdown of late-Pleistocene glacial outburst floodwaters along the Columbia River and tributaries. Many of the now-active landslides are reactivated portions of pre-existing landslides. Minor, but not insignificant, numbers of landslides appear to have been influenced by human activities.

The updateable landslide inventory includes specific information about each landslide such as level of certainty and activity, dimensions, inferred cause, type of movement, geologic unit(s), impacted infrastructure, and previously reported identification. The publicly available inventory map and database will be useful for a variety of county and city land-use planning and development purposes.