RAPID GULLY FORMATION IN RESPONSE TO FOREST ROAD CONSTRUCTION: REACTIVATION OF A DORMANT GEOMORPHIC FEATURE

In the early 1950's, an old growth forest was harvested on a large deep-seated, dormant-mature, translational landslide. The landslide is approximately 700 meters wide and 1000 meters in length. The landslide contains large-scale hummocky morphology that creates several sub-basins within the body of the landslide. The harvesting of the area was apparently conducted in two phases. The initial phase removed timber from the majority of the open slope areas. A second phase removed the remainder timber from generally the stream corridors. During the harvest, a dense network of yarding furrows, skid trails and truck roads were constructed across the landslide that generally converged on a deeply incised ephemeral watercourse along the left-lateral margin of the landslide. These roads crossed two sub-basins that contained springs and near-surface water flow that were diverted into a broad swale upslope of the ephemeral stream. Because of the lack of spring type vegetation observable in the early aerial photographs, the lack of modern vegetation indicative of spring conditions and the apparent detachment of the swale from the incised stream network downslope, we interpret this landform to be a dormant groundwater sapping swale. Aerial photographs show that within the next 9 years a deep gully incised about 50 meters upslope towards the source of diverted water. During this period an unseasonably large, 1955-rain event occurred. Present conditions reveal emergent groundwater within the gully sidewalls and an apparent reduction in the rate of headward erosion of the gully. As evidenced by the lack of recent surface rilling, regeneration of the tree canopy over the last 50 years has essentially removed the surface water delivery to the swale but elevated groundwater conditions still exist. Our interpretations of site conditions suggest road construction within the area diverted surface and groundwater flow into an area of dormant groundwater sapping creating emergent water conditions that rapidly incised through the ephemeral stream channel into the area of dormant groundwater sapping. Gully incision of the dormant groundwater sapping swale has created year-round emergent spring conditions and headward regression of the gully limited to high intensity rainstorms.