Applying a new surface roughness metric to determine earthflow age, cause, and impacts on salmon habitat

Earthflows are fine grained mass movements whose activity can continue for centuries, resulting in persistent sediment delivery from hillslopes to channels. While important for basin-wide sediment dynamics and erosion, this sediment delivery may also have long-term effects on in-stream habitat, particularly for salmon. Here, we analyze the drivers of earthflow activity and effects on salmon habitat in the Teanaway basin, Washington State, USA, through development of a new surface roughness metric to date earthflows. As earthflows age, linear flow features diffuse and are dissected, leading to random roughness elements; we apply a flow directional roughness metric called MADstd that captures the loss of linear elements over time to relatively date 187 earthflows in the Teanaway basin. Six radiocarbon dates and sedimentation volumes in three earthflow-dammed lakes provide absolute earthflow ages. We find that earthflow orientation is controlled by bedrock dip direction and Eocene paleosols, but the timing of earthflows is climatic. During Pleistocene and early Holocene dry periods, frost cracking and desiccation cracks weathered bedrock to soil and produced conduits for water flow. As the regional climate grew wetter in the mid Holocene, the deep regolith mobilized into earthflows, some of which stayed active through the late Holocene. Earthflow timing is coincident with salmon habitat stabilization from Pleistocene climate and base level effects. That salmon were able to thrive coincident with earthflows suggests fine sediment delivered by earthflows, despite the possibility of clogging pore spaces and suffocating redds, had a relatively minor effect on the overall habitat. However, valley widtha landscape characteristic associated with more salmon habitatincreases upstream of valley blocking earthflows and indicates that earthflows may have had a net positive effect on channel habitat. As the Teanaway River warms and water levels drop due to climate change, floodplain heterogeneity provided by earthflows may be a focal point for habitat mitigation.