A SOIL CHRONOSEQUENCE ON A DEBRIS FLOW FAN IN DODSON, OREGON

Dodson, Oregon is a small community located approximately 35 miles east of Portland, Oregon along Interstate Highway 84 in the Columbia River Gorge National Scenic Area. Dodson was mapped as a hazard area in 1978. The area gained notoriety from the 1996 event commonly referred to as the Dodson/ Warrendale Debris Flows. Our project in Dodson focused on two simple goals: to name and date the soils in each soil pit and to establish a chronosequence along the 1996 debris flow path identified as D6. We dug seven soil pits on five properties located along NE Frontage Road in Dodson, Oregon. We used soil colors, pH, and field morphology to determine the chronosequence of the soils along debris flow path D6. The age of the soils at our sites ranged from five to 5,000 years old. We identified our soil types as three Typic Xerorthents, an Entic Haploxeroll, and three Typic Haploxerolls. We then ranked our soil profiles in age in the following order: two five-year old Typic Xerorthents from Site 3 and Site 4, an eighty-three year old Typic Xerorthent at Site 2, two 2 ka Haploxerolls (Entic at Site 5 and Typic at Site 7), and finally two 5 ka Typic Haploxerolls at Site 1 and at Site 6. The soil at Site 4, with a slightly redder Cu horizon, was older than Site 3 located on the same property. The use of chronosequencing along a geographic area to determine the age of soils could provide details about the nature and extent of debris flows in a geographic area. It could also identify the frequency and magnitude of debris flows in a geographic area. More research can be done in Dodson to create a stability map of the fan and to determine the “trigger point” of parent materials in the designated hazard area.