HYDROLOGIC AND GEOMORPHIC RESPONSES TO FOREST HARVEST DISTURBANCES

Streamflow and channel geomorphology respond both locally – i.e. in 1st-order basins, and globally -- at the scale of large basins, to forest harvest, which occurs in patches of a few hectares to hundreds of square kilometers. Forest harvest spans a range of severities from selective to complete removal of trees, and may be accompanied by slash-burning or the construction of forest roads. In intensively managed landscapes, clearcutting occurs at regular intervals followed by replanting, but in publicly owned landscapes, forest harvest may occur rarely, or partially, and may rely upon natural vegetation succession. Forest harvest occurs in many climates and types of landforms. Canopy removal, slash burning, and road construction influence multiple hydrologic and geomorphic mechanisms. Hydrologic and geomorphic responses depend upon the severity and extent of canopy removal, the type and landscape position of roads, time of year, the time elapsed since the last major disturbance, and the climate. In steep experimentally-harvested 1st-order basins, small (<0.5 yr return-period) peak discharges and water yield in fall, spring, and summer are controlled primarily by evapotranspiration, and recovery is timed according to the changing water use of successional vegetation. Large (1-yr to 50-yr return-period) peak discharges and winter water yield in experimentally-harvested 1st-order basins are controlled primarily by interception and snowpack dynamics, as well as changes in subsurface flow routing from roads. Large peak discharges in large basins may be persistently augmented by the combination of forest harvest effects on snowpack dynamics, and road construction effects on subsurface and surface flow partitioning and routing. Geomorphic responses in steep forested landscapes are controlled by hydrologic mechanisms associated with snow and roads, augmenting channel change, landslides, and debris flow rates in both small and large basins.