Watershed Evaluation and Habitat Response to Recent Storms : Annual Report for 1999.

Large and powerful storm systems moved through the Pacific Northwest during the wet season of 1995--96, triggering flooding, mass erosion, and, alteration of salmon habitats in affected watersheds. This project study was initiated to assess whether watershed conditions are causing damage, triggered by storm events, to salmon habitat on public lands in the Snake River basin. The storms and flooding in 1995--96 provide a prime opportunity to examine whether habitat conditions are improving, because the effects of land management activities on streams and salmon habitat are often not fully expressed until triggered by storms and floods. To address these issues, they are studying the recent storm responses of watersheds and salmon habitat in systematically selected subbasins and watersheds within the Snake River system. The study watersheds include several in the Wenaha and Tucannon subbasins in Washington and Oregon, and the watersheds of Squaw Creek (roaded) and Weir Creek (unroaded) in the Lochsa River subbasin, Idaho. The study was designed to examine possible differences in the effects of the storms in broadly comparable watersheds with differing magnitudes or types of disturbance. Watershed response is examined by comparing storm response mechanisms, such as rates of mass failure, among watersheds with similar attributes, but different levels of land management. The response of salmon habitat conditions is being examined by comparing habitat conditions before and after the storms in a stream and among streams in watersheds with similar attributes but different levels of land management. If appropriate to the results, the study will identify priority measures for reducing the severity of storm responses in watersheds within the Snake River Basin with habitat for at-risk salmon. This annual report describes the attributes of the study watersheds and the criteria and methods used to select them. The report also describes the watershed and fish habitat attributes evaluated and the methods used to evaluate them. Watershed responses and attributes evaluated include mass failures, historic soil loss, the integration of roads with the drainage network, estimated flood recurrence intervals, and headwater channel morphology. Habitat attributes evaluated include large woody debris, pool frequency and depth, substrate conditions, and bank stability. Multiple analyses of habitat data in the Tucannon and Wenaha subbasins remain to be completed due to difficulties stemming from data characteristics that indicated that some of the pre-existing data may have be of questionable accuracy. Diagnostic attributes of the questionable data included a change in monitoring protocols during the pre- to post-flood analysis period, physically implausible temporal trends in some habitat attributes at some sites, and conflicting results for the same attribute at the same locations from different data sources. Since unreliable data can lead to spurious results, criteria were developed to screen the data for analysis, as described in this report. It is anticipated that while the data screening will prevent spurious results, it will also truncate some of the planned analysis in the Tucannon and Wenaha systems.