Restoring Streams with Large Wood: an Analysis of Geomorphic Changes 7 Years Post-Restoration in Small Coastal Streams

Introducing large wood (LW) into streams for restoration purposes is a common practice, as it creates habitat through processes like scouring, deposition and sediment sorting. However, while monitoring often focuses on short-term (<3 years) or long-term (>10 years) changes in habitat features, there is a lack of understanding regarding annual geomorphic changes over relatively long periods. In this study, we investigated annual geomorphic adjustments (channel geometry and substrate size) over 7 years in three tributaries of the Mill Creek watershed (Oregon, USA). The 7-year period included moderate to high flows, with peak annual flow exceeding bankfull flow (Qbf) 2-5 times and flows being above half Qbf on average 4-20 days per year. Data included topographic surveys and surface pebble counts collected from 2014 (1 year before LW) to 2021 (6 years after LW). We quantified scour and deposition and estimated sediment grain sizes and sorting from topographic surveys and pebble counts. Our analysis revealed that stream size influenced geomorphic adjustment, with smaller streams experiencing more scouring compared with larger streams over the 6 years. LW structures promoted increased scouring at the cross-section scale, with a strong relationship found between volumetric blockage ratio and scour. In our case, the most significant scouring changes were associated with volumetric blockage ratios between 35% and 50%; further research is needed to investigate scouring for higher blockage ratios. Instream changes in scour and deposition peaked around 3-4 years after LW introductions but persisted until the end of the monitoring period. Sediment size dynamics were influenced more by time since restoration than by proximity to LW jams. While LW introductions increased sediment sorting into patches, the degree of sorting declined 5-6 years post-restoration at all sites. Our findings offer insights into the long-term persistence and magnitude of instream changes associated with LW introductions.