Large Wood in Rivers

Abstract

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.