Map of the week: instream large wood in the Pacific Northwest

Map of the week: instream large wood volumes in the interior Columbia and upper Missouri River Basins, USA. 

This week's map illustrates the distribution of large woody debris volumes in the interior Columbia and Missouri River Basins in the American Pacific Northwest. Instream wood shapes stream environments by influencing stream hydraulics and the movement of sediment. For example, wood creates hydraulic refugia downstream that sorts sediment and influences pool habitats while also shaping erosion and deposition. Because of the role that wood plays in shaping stream habitat for anadromous fishes, the geomorphic and biological processes that lead to wood deposition in streams are often studied to better inform stream and watershed management. Where large wood from individual trees is limited, other processes may be responsible for the creation of pools, like beaver dams or other geomorphic controls.

This map identifies 720 stream monitoring sites where wood was measured by the PACFISH/INFISH Biological Opinion. Wood volume is plotted by quartile thresholds that were used to look at plant community composition within the watershed. Note that there is less instream wood in hot, dry areas that both grow few large riparian tree species, but are also subject to grazing disturbance and more pronounced rangeland wildfire-cheatgrass cycles. More wood occurs in wetter and/or higher elevation watersheds with less disturbance and forested community types.

A low wood volume stream following wildfire in Idaho, USA

A high wood volume stream in western Montana, USA.