The Hetten and Tompkins Creek paired catchment study : modern U.S. Forest Service timber harvest effects on suspended sediment concentration and turbidity

Suspended sediment concentration, turbidity, and discharge were monitored in two Mad River catchments from 1985-1993 to evaluate the effect of timber harvest treatments with Best Management Practices (BMPs) on water quality. Automated measurements and statistical sampling design followed U.S. Forest Service methods introduced at Caspar Creek in Jackson State Forest (Mendocino County, California). Reference conditions were monitored at both creeks from fall, 1985 through spring, of 1989. In summer 1989, Tompkins catchment received timber harvest treatments. Treatments included: constructing 0.74 kilometers of temporary road; harvesting timber from 16% of catchment area by clearcut (40.1 hectares) and by selection (4 hectares); and burning to remove slash by pile and broadcast methods. Post-treatment conditions were monitored at the treated and control basins from fall 1989 through spring, 1993. Reference conditions were established and compared to post-treatment conditions. Analyses of the monitoring data included: (1) evaluation of pre- and post-treatment suspended sediment concentrations and turbidity as dependent variables on discharge; (2) computation of sediment yields by storms; (3) standardization of storm sediment yield by total storm water yield; (4) division of storm data into rising and falling limbs for hysteresis; and (5) evaluation of "cleansing storms" (washout storms early in the water year). Considerable variation in suspended sediment concentration and turbidity was found and no treatment-related changes were identified in techniques (1) and (4). Techniques (2), (3), and (5) indicated that suspended sediment and turbidity increased beyond reference variability. While observed increases exceeded the 20% allowed in regulatory standards, ecological effects were limited to increasing the frequency of sub-lethal exposures for juvenile anadromous salmonids.