Forestry influences on instream salmon habitat and the implications for watershed management

Freshwater ecosystems support important species, such as salmon, but can be degraded by various human pressures such as forestry. Forestry activities can alter the delivery and storage of water, nutrients, wood, and sediment in streams, resulting in changes to habitat. Forestry can also alter water temperatures, leading to changes in growth and survival of juvenile salmon. Previous research on forestry impacts on habitat has focused on small scale, intensively monitored coastal systems, challenging the application of this science to management decisions in larger watersheds. Here I examined forestry impacts, watershed characteristics, physical habitat, and stream temperature for 28 tributaries of the North Thompson River to examine relationships between forestry and juvenile coho stream habitat. For each watershed, I quantified natural components (elevation, area, and stream gradient), forestry impacts (harvest area, road density, and stream crossing density), and habitat (water level, temperature, pool cover, pool depth, woody debris abundance, sediment cover, and bank dimensions). Forest harvest had a positive correlation to maximum summer stream temperature. Streams with 35% of the riparian area harvested since 1970 had maximum summer temperatures that were 3.7°C higher on average than those with 5% harvested. Stream gradient explained most of the variation in physical habitat and had negative correlations to pool cover, pool depth, and fine sediment cover. Streams with 1% gradient had on average 23% pool area and 41% fine sediment cover, while streams with 3% gradient which had 12% pool area and 16% fine sediment cover. Taken together, these results indicate that watershed characteristics drive physical habitat, but forestry harvest can be a primary driver of water temperatures. This study advances understanding of impacts and monitoring of forestry on stream environments.