Durability of Pacific Northwest Instream Structures Following Floods

Abstract

The durability of 3,946 instream structures in 94 streams that had floods with return intervals exceeding 5 years were assessed. Overall structure durability (defined as the degree to which a structure remained at its original location) was high; less than 20% of the sampled structures had been removed from the site of original placement. The magnitude of flood events had a significant effect on structure durability with higher magnitude floods reducing durability. Stream order also affected structure durability; structures in large streams were 20 times more likely to have been removed from the site of original placement than structures in small streams. Other conditions that affected structure durability included location of the structure within the stream channel, whether the structure was anchored or not, structure material, and upslope landslide frequency. Instream structures are most appropriate when used as short-term tools to improve degraded stream conditions while activities that caused the habitat degradation are simultaneously modified. When instream structures are part of a properly sequenced watershed restoration strategy, they can improve habitat conditions through a range of flow conditions including large floods.