King County staff investigated if and how forest conditions affect stream health. We used B-IBI scores as our measure of stream health. Our goal was to see if forest harvests impact B-IBI scores in forested basins. But it is difficult to get accurate measures of past forest harvest. Instead, this study focused on forest conditions influenced by forest harvest. We looked at different potential mechanisms and spatial scales of impact.

We found the forest conditions caused by timber harvest had a small but significantly negative effect on B-IBI scores. The effect was mainly through alterations to sediment, light and leaf litter inputs. This small effect suggests degraded forest conditions are unlikely to be the primary drivers of low B-IBI scores observed in some forested basins. The results also suggest the influence of forest conditions on in-stream processes and B-IBI scores gets smaller as distance from the stream increases. Forest conditions at both upstream and stream-reach scales influenced B-IBI scores. These results apply to forested watersheds with very little residential or commercial development.