Deep-Seated Landslides Control Valley Width and In-Channel Geomorphic Features in the Oregon Coast Range, USA

This is the first project to examine the effects of deep-seated landslides (non-catastrophic, bedrock slope deformations) on stream morphology in the Oregon Coast Range. Deep-seated landslides in the Tyee Formation of the Oregon Coast Range are structurally and lithologically controlled - associated with a lower sandstone:siltstone ratio and with areas where the hillslope aspect is in line with the bedrock dip-slope. Previous research in two basins in the Oregon Coast Range found that small, localized deep-seated landslides increase variability in valley width: when compared with the hydraulic geometry of a control basin, anomalously narrow valleys were found adjacent to the landslides and anomalously wide valleys above. This study coupled these findings with a simple intrinsic potential habitat model for coho salmon (Oncorhynchus kisutch), which linked landslides with high quality fish habitat. Here, I show that extensive deep-seated landsliding in the North Fork Siuslaw Watershed is associated with anomalously wide valleys at small drainage areas, while also increasing overall variance in valley width throughout the watershed. When compared with the stream morphology of paired control basins, the streams in landslide terrain have higher sinuosity and lower gradients. This, combined with an increased sediment load from the landslides, creates complex aquatic habitat in the form of more frequent, deeper pools and a higher ratio of secondary channel length to total channel length.