RECENT CLIMATE-INFLUENCED CHANGES IN THE SAND HILLS COULEE DUNE FIELD, EAST-CENTRAL WASHINGTON

ArcGIS-assisted analysis of a partially stabilized sand dune field in the Channeled Scabland of east-central Washington reveals recent, climate-influenced changes in parabolic dune mobility as evaluated from variations in slip-face position, geomorphic form, and vegetation density. The Sand Hills Coulee dune field lies within an elongate, NE-trending channel excavated into the upland portions of a large loess island by glacial outburst flooding. Located >120 m above surrounding deep scabland channelways, this loess island was eroded during a large volume outburst flood or floods that originated from glacial Lake Missoula during the Pleistocene. Sand Hills Coulee dune sands are generally fine grained, quartz- and feldspar-enriched and basalt-lithic depleted, illustrating a primary provenance from the Pliocene Ringold Formation rather than only from glacial outburst flood sediment. The dunes are up to 10-m-high and characterized by active and stabilized parabolic forms with limbs that roughly parallel the NE orientation of the coulee. Dune stratigraphy reveals episodes of likely late Pleistocene and Holocene activity and stability and continued modern dune migration following the 1980 Mount St. Helens eruption.

Sand Hills Coulee lies in the rain shadow of the Cascade Mountains and receives ~250 mm of annual precipitation with the bulk occurring during the late fall to spring. Prevailing winds are from the WSW and SW but effective sand-moving winds may come from any direction. During the past decade the Sand Hills Coulee dunes have been subject to persistent drought interrupted by punctuated episodes of increased late spring to early summer precipitation during 2004 and 2005; in addition, episodically strong winds contributed to dune remobilization. In spite of generally favorable long-term climatic conditions for dune mobility, stability has increased and active sand dune migration, where it has occurred, has been to the east rather than immediately downwind. Preliminary findings suggest that sand dune mobility has been most strongly influenced by only a few anomalous precipitation and wind events. This insight is relevant to efforts to assess the susceptibility of the sensitive, dry agricultural lands surrounding the dunes to future eolian deflation.