HYDROLOGIC AND EROSIONAL RESPONSES OF A GRANITIC WATERSHED TO HELICOPTER LOGGING AND BROADCAST BURNING

Forest land managers are concerned about the effects of logging and si te preparation on erosion, site productivity, streamflow, and water qu ality. Effects of helicopter logging and prescribed burning on streamf low and sediment yields from headwater drainages in the Idaho Batholit h were evaluated, using paired watersheds monitored from 1966 to 1986. In the fall of 1976, 23% of a 162 ha watershed was clearcut. All the cutting units were located on south-facing slopes. Helicopter logging was followed by broadcast burning on the cutting units. Streamflow par ameters showed little change in response to the logging and burning. H owever, total annual sediment yields on the treated watershed increase d an average of 97% in the 10 yr following logging, with the largest i ncreases occurring in the years of highest sediment yields. Increased sediment yields did not appear to result from accelerated channel eros ion; rather, about 94% was attributed to accelerated surface erosion o n the cutting units, and 6% was contributed by a single mass erosion s ite. Accelerated erosion persisted on the cutting units throughout the study period. The accelerated surface erosion occurred primarily as a result of the prescribed burning (rather than the helicopter logging) ; surface erosion rates on the burned areas were about 66 times greate r than those on undisturbed slopes. The accelerated rates of erosion a nd sedimentation have potentially serious implications for on-site pro ductivity and downstream resources.