|Abstract or Summary
- Severe scouring from splash damming was one of the earliest reported forms of widespread anthropogenic disturbance in streams of the Pacific Northwest, USA. Splash damming was a common method of log transport in western Oregon from the 1880s through the 1950s. Before being released in large freshets to downstream lumber mills, water and logs were stored in reservoirs behind splash dams. Further protocol called for dynamiting downstream obstacles such as large boulders and natural logjams. In recent literature, the legacy effect of historical splash damming is proposed as contributing to currently poor habitat conditions for lotic species, such as Pacific salmon (Oncorhynchus spp.), but this has never been formally evaluated at a regional scale. In this study, all known splash-dam sites and log drives in western Oregon were recorded in a geo-database and mapped in ArcGIS 9.3 at the 1:24,000 scale. Splash-dam sites were located through intense archival, historical aerial photograph and field searches. The final splash-dam map was overlaid with regionally available continuous and probabilistic stream surveys. After accounting for basin area and channel slope, in-channel variables were compared between reaches upstream and downstream of splash dams (within-basin analysis) and between reaches in splashed
and not-splashed basins (among-basin analysis). Only data from sites located in a forested land cover and sedimentary rock type in the Oregon Coastal Province were analyzed. A significant difference (α = 0.1) was seen in either within- or among-basin analyses for each evaluated category of in-channel variable (geomorphology, substrates, pools, and channel complexity). Both analyses demonstrated significantly more bedrock and fewer deep pools in splashed reaches. In the among-basin analysis, three times fewer pieces of key large wood were found in splashed reaches (p = 0.07). Many of the in-channel variables that demonstrated significant differences are regarded as indicators of salmon habitat quality. This is the first regional study to document that splash-dam legacy effects still persist on evaluated stream reaches 50-130 years after the practice ceased. Further, I detected a splash-damming signal in widely used regional monitoring datasets, which suggests that legacy effects should be considered in future applications of these datasets. Splash-damming impacts are pervasive and persistent throughout the Oregon Coastal Province; consequently, extensive and intensive restoration measures may be necessary to accelerate recovery of certain stream habitat characteristics in streams where splash damming and log drives occurred. This study demonstrates the importance of including archival information in modern-day studies, and that history can account for significant variation in the stream environment.