Large Wood in Rivers

by

Ellen Wohl

• LAST REVIEWED: 27 May 2020
• LAST MODIFIED: 27 May 2020
• DOI: 10.1093/obo/9780199363445-0079

Introduction

Large wood consists of downed, dead pieces of wood. Although different size definitions have been proposed, the most widely used is pieces ³ 10 cm diameter and 1 m length. Many works refer to large woody debris, but because debris typically has negative connotations, investigators increasingly use instream wood or large wood. Systematic scientific studies of large wood in river channels and floodplains began during the late 1970s, mostly in the northwestern United States and southwestern Canada. Knowledge of wood characteristics in these regions still exceeds understanding of wood in other environments. The historical development of large wood studies partly reflects the continued existence of extensive forest cover in these portions of North America and partly reflects the concern with endangered populations of salmonid fishes in many western coastal rivers of North America. Like many other fish species, salmonids benefit from the habitat associated with large wood, and this connection was a focus of the earliest wood studies. Increases in aquatic habitat abundance and diversity are one of many environmental benefits associated with large wood in channels and floodplains. Wood in channels creates flow resistance and deflects current toward the channel bed and banks, enhancing pool volume and the diversity of hydraulics and bed substrate and increasing the retention of organic matter and dissolved and particulate nutrients. Wood can enhance the exchange of water between the channel and underlying hyporheic zone, with attendant benefits for water temperature and chemistry and the abundance of macroinvertebrates. Wood creates spatial heterogeneity of channel form and enhances the connectivity between channels and floodplains by deflecting flow and suspended sediment beyond the channel and onto the floodplain. Historical descriptions of rivers throughout forested regions emphasize the enormous volumes of wood within channels and floodplains and the associated abundance of features such as floodplain wetlands. Most of the wood historically present throughout forested rivers of the temperate zone has been removed for flood control, navigation, and enhanced conveyance of cut logs being floated downstream to sawmills. This ubiquitous and sustained removal has led to widespread river metamorphosis and loss of river diversity and resilience to disturbances. Consequently, reintroduction and/or retention of wood, along with minimizing hazards from mobile wood, is now emphasized in river management. The number and geographic diversity of wood-related studies has exploded in the early 21st century. This tremendous increase has been facilitated by advances in ground- and space-based remote sensing imagery, active and passive techniques for tracing the movement of individual pieces of large wood, and increasingly complex numerical models that simulate diverse processes by which wood enters rivers and moves within river networks. Important gaps remain between public perceptions of wood in rivers, which tend to be largely negative, and scientific appreciation of wood in rivers. The classification of works cited here into specific headings and subheadings is partly arbitrary and subjective because many of the works address multiple aspects of large wood and could be placed under multiple subheadings.

General Overviews

The works cited here provide comprehensive summaries of some aspect of large wood in rivers. Harmon, et al. 1986 is a classic foundational study that discusses all aspects of wood in rivers. Maser, et al. 1988 provides a particularly comprehensive discussion of wood from headwater streams to marine environments. Gurnell, et al. 2002 reviews many aspects of wood and proposes a size-based classification that has subsequently been widely used. Gregory, et al. 2003 is an edited volume that summarizes knowledge up to that point for many physical and ecological aspects of wood in rivers. Hassan, et al. 2005 reviews work on wood in headwater streams of the Pacific Northwest region of North America. Gurnell 2013 and Le Lay, et al. 2013 are both chapters from volumes in a treatise on geomorphology that review knowledge of wood transport, storage, and physical effects in rivers. Ruiz-Villanueva, et al. 2016 and Wohl 2017 are more recent syntheses, each of which emphasizes areas of limited knowledge and likely future research. Wohl, et al. 2019 draws understanding of wood budgets and wood process domains to propose the conceptual framework of a natural wood regime in rivers.

• Gregory, S. V., K. L. Boyer, and A. M. Gurnell, eds. 2003. The ecology and management of wood in world rivers: International Conference on Wood in World Rivers, held at Oregon State Univ., Corvallis, Oregon, 23–27 October 2000. American Fisheries Society Symposium 37. Bethesda, MD: American Fisheries Society.

  Proceedings of the International Conference on Wood in World Rivers held in Corvallis, Oregon. Volume remains a key resource, with chapters reviewing all aspects of large wood in rivers. Numerous chapters from the book are cited throughout this article.

• Gurnell, A. M. 2013. Wood in fluvial systems. In Fluvial geomorphology. Edited by E. Wohl, 163–188. San Diego, CA: Academic Press.

  Thorough review of definitions of large wood, transport and storage of wood, wood budgets, geomorphic effects of wood, and implications of wood for understanding and managing river process and form.

• Gurnell, A. M., H. Piégay, F. J. Swanson, et al. 2002. Large wood and fluvial processes. Freshwater Biology 47.4: 601–619.

  DOI: 10.1046/j.1365-2427.2002.00916.x

  Reviews diverse aspects of wood in rivers, including wood supply, transport, storage, geomorphic effects, and dynamics of wood in differently sized rivers and rivers with different planforms.

• Harmon, M. E., J. F. Franklin, F. J. Swanson, et al. 1986. Ecology of coarse woody debris in temperate ecosystems. Advances in Ecological Research 15:133–302.

  DOI: 10.1016/S0065-2504(08)60121-X

  Extremely thorough, comprehensive, and still-relevant review of numerous aspects of large wood; covers recruitment, decay, wood loads, physical and ecological effects, historical alterations of wood loads in forest and river environments.

• Hassan, M. A., D. L. Hogan, S. A. Bird, et al. 2005. Spatial and temporal dynamics of wood in headwater streams of the Pacific Northwest. Journal of the American Water Resources Association 41.4: 899–919.

  DOI: 10.1111/j.1752-1688.2005.tb04469.x

  Useful synthesis of understanding of wood dynamics in the most well-studied environment, with respect to large wood in rivers, in the world; addresses wood budgets, temporal and spatial variability of wood loads, models of wood dynamics, effects of timber harvest on wood dynamics, and identifies knowledge gaps.

• Le Lay, Y. -F., H. Piégay, and B. Moulin. 2013. Wood entrance, deposition, transfer and effects on fluvial forms and processes: Problem statements and challenging issues. In Ecogeomorphology. Edited by D. R. Butler and C. R. Hupp, 20–36. San Diego, CA: Academic Press.

  Review of wood dynamics, including recruitment, transport, deposition, decay, and geomorphic effects, as well as management issues. Discusses differences with respect to position in drainage network and between geographic regions.

• Maser, C., R. F. Tarrant, J. M. Trappe, et al. 1988. From the forest to the sea: A story of fallen trees. USDA Forest Service General Technical Report PNW-GTR-229. Portland, OR: USDA Forest Service.

  Comprehensive treatment of all aspects of wood in rivers, from wood recruitment and decay in terrestrial and aquatic environments through geomorphic and ecological effects of wood in rivers to physical and ecological effects of river-transported wood in estuaries, beach, and nearshore environments.

• Ruiz-Villanueva, V., H. Piégay, A. M. Gurnell, et al. 2016. Recent advances quantifying the large wood dynamics in river basins: New methods and remaining challenges. Reviews of Geophysics 54.3: 611–652.

  DOI: 10.1002/2015RG000514

  Summarizes advances in understanding large wood dynamics, including wood budgets and wood mechanics. Emphasizes integration of measurements and modeling and develops meta-analyses. Also highlights areas of research importance and likely future trajectories.

• Wohl, E. 2017. Bridging the gaps: An overview of wood across time and space in diverse rivers. Geomorphology 279:3–26.

  DOI: 10.1016/j.geomorph.2016.04.014

  Covers all components of wood budgets, geographic location of wood field studies, physical and ecological effects of wood, and management of wood. Emphasizes what is known but also gaps in contemporary knowledge.

• Wohl, E., N. Kramer, V. Ruiz-Villanueva, et al. 2019. The natural wood regime in rivers. BioScience 69.4: 259–273.

  Proposes that wood recruitment, transport, and storage be conceptualized as a part of a natural wood regime in rivers, analogous to the natural flow regime and natural sediment regime conceptual frameworks. Discusses how wood budgets and wood process domains can be used to characterize a wood regime.