Effects of timber harvest on suspended sediment loads in Mica Creek, Idaho

Long-standing concerns exist regarding timber harvest and subsequent sediment impacts on aquatic resources. Intensive studies on contemporary harvest practices remain rare, particularly in the interior Pacific Northwest. To investigate this, a network of seven automated stream monitoring flumes was installed in the Mica Creek Experimental Watershed, in northern Idaho. Beginning in 1991, water samples collected at each flume under both flow-based and stream-stage storm rise conditions have been analyzed for total suspended solids (TSS). This period of record encompasses a pretreatment time interval from 1991 to 1997, and two treatment time intervals: post-road from 1998 to 2001 and post-road/post-harvest from 2001 through 2004. Treated and control catchments were statistically compared using a paired watershed approach for immediate and recovery time intervals corresponding to each treatment activity, road construction and timber harvest. The impacts corresponding to road construction remain difficult to discern. The impacts corresponding to timber harvest differ based on harvest treatment and time period of analysis. Results suggest a correlation between increased sediment loads and clearcutting for a brief period following the harvest. No significant correlation was found in the partial cut watershed. Continued monitoring at these sites is planned to evaluate trends over subsequent years.