Adding wood to streams: when is it worthwhile?

Posted on March 23, 2016

Placing woody material into waterways with the aim of improving habitat quality is a common stream restoration technique. However, debate continues around the extent to which wood placement actually leads to significant increases in fish abundance. In a wide-ranging literature review, American ecologists note that many researchers reported that the installation of instream structures was followed by `large (>50%) increases in habitat diversity, the frequency and depth of pools and the retention of sediment, spawning gravel and organic matter, especially in mountain streams. Correlated biological responses included increases in the abundance, biomass, survival and species diversity of salmonids. However, results for non-salmonids were relatively rare and highly variable. Long-term studies suggest that the addition of wood to streams does lead to a real increase in fish production, rather than just providing focal points for local aggregation, while responses of macroinvertebrates to the introduction of wood tend to be more limited to the structures themselves. The stability of wood structures was reported to be high in 78% of 22 restoration projects examined. Only two studies reviewed stability 20 or more years after installation, but one of them reported that after 50 years almost half of the placed structures were still in place. The authors of the review conclude that, to help decide when wood placement is worthwhile, natural resource managers need answers to the following research questions: How much and what type of wood is needed for a physical responses in different sizes of stream? How quickly will the physical response take to occur, and how long will it last? When is wood placement counter-productive (i.e., when is it more likely to increase erosion and habitat degradation?)

Reference: Roni, P. et al. 2015. Wood placement in river restoration: fact, fiction, and future direction. Canadian Journal of Fisheries & Aquatic Sciences 72, 466–478.