A Regional and Geomorphic Reference for Quantities and Volumes of Instream Wood in Unmanaged Forested Basins of Washington State

Abstract

We collected field data on instream wood quantities and volumes from 150 stream segments draining unmanaged basins within Washington State to develop reference conditions for restoration and management. The wood loads in these streams provide a reference for management since it is assumed that they incorporate the range of conditions to which salmonids and other species have adapted. We also used these data to evaluate existing standards for large wood in streams. Large wood is an important component of salmonid habitat, and stream channel assessments and restoration and enhancement efforts often associate habitat quality for salmon Oncorhynchus spp. with the quantity and volume of woody debris; however, the wood targets currently used to assist resource managers typically do not account for variations in quantity or volume owing to differences in geomorphology, forest zones, or disturbance regimes. For restoring the appropriate range of conditions in salmon habitat, we offer a percentile wood distribution of natural and unmanaged wood-loading ranges based on regional and geomorphic variation for the purpose of reestablishing central tendencies. We recommend that streams in a degraded state (e.g., below the 25th percentile) be managed for an interim target at or above the 75th percentile until the basin-scale wood loads achieve these central tendencies. Based on the sample distribution, these reference conditions are applicable to streams with bank-full widths between 1 and 100 m, gradients between 0.1% and 47%, elevations between 91 and 1,906 m, drainage areas between 0.4 and 325 km², glacial and rain- or snow-dominated origins, forest types common to the Pacific Northwest, and several other distinguishing physical and regional classifications.