Downstream Effects of Timber Harvesting on Channel Morphology in Elk River Basin, Oregon

Abstract

AbstractDownstream effects, a type of cumulative watershed effect, were identified using changes in the width and distribution of open riparian canopies measured from aerial photography taken between 1956 and 1979 in Elk River basin, southwest Oregon. Open canopies appear on aerial photographs of densely forested basins as unvegetated areas bordering stream channels. Opening occurs when large disturbances, such as landslides, debris flows, large floods, and excessive sedimentation, disrupt the vegetation in the riparian corridor. Downstream changes in channel morphology, inferred by the changing pattern of open reaches were linked to upslope forestry activities; a causal link was assumed where: (i) open reaches extended continuously downstream from clearcuts and roads or (ii) the timing and pattern of opening downstream varied in direct relation to the intensity of upslope forestry activities. Open riparian canopies were observed in first‐ through fifth‐order channels, though only 11% of open reaches in low‐order channels were spatially connected to open reaches in higher order channels. Open reaches on low‐order tributaries were attributed to landslides and surface erosion generated from clearcuts and roads; the total length of open reaches in low‐order channels increased 30‐fold during the study period. Open reaches occurred on higher‐order channels throughout the study period but did not increase in size or change location in relation to upslope harvest activities. Instead, open canopies were restricted mainly to wide and low gradient channel reaches, which comprised approximately one‐third of the length of higher‐order channels. Limited downstream change in riparian canopies associated with upslope forestry activity during the study period, which included a 100‐yr storm, was attributed to three physical factors: (i) lack of debris flows in most parts of the basin; (ii) channels constrained by competent hillslopes limiting the potential for opening; and (iii) low harvest levels over much of the basin at the time of the 100‐yr storm. While air photo interpretation proved useful in deciphering the gross disturbance history of the basin and in distinguishing the general processes which generate downstream effects, sedimentation processes that do not disturb the riparian canopy may also be active in Elk River basin but were not detected due to the coarseness of the techniques used.