EFFECTS OF BASIN-SCALE TIMBER HARVEST ON WATER YIELD AND PEAK STREAMFLOW

Streamflow changes resulting from clearcut harvest of lodgepole pine ( Pinus contorta) on a 2145 hectare drainage basin are evaluated by the paired watershed technique. Thirty years of continuous daily streamflo w records were used in the analysis, including 10 pre-harvest and 20 p ost-harvest years of data. Regression analysis was used to estimate th e effects of timber harvest on annual water yield and annual peak disc harge. Removal of 14 million hoard feet of lodgepole pine (Pinus conto rta) from about 526 hectares (25 percent of the basin) produced an ave rage of 14.7 cm additional water yield per year, or an increase of 52 percent. Mean annual daily maximum discharge also increased bq 1.6 cub ic meters per second or 66 percent. Increases occurred primarily durin g the period of May through August with little or no change in wintert ime streamflows. Results suggest that clearcutting conifers in relativ ely large watersheds (> 2000 ha) may produce significant increases in water yield and flooding. Implications of altered streamflow regimes a re important for assessing the future ecological integrity of stream e cosystems subject to large-scale timber harvest and other disturbances that remove a substantial proportion. of the forest cover.