Rates and mechanisms of bedrock incision and strath terrace formation in a forested catchment, Cascade Range, Washington

Abstract

Measurements of channel bed and bank incision into bedrock coupled with mapping and radiocarbon dating of strath terraces in the West Fork Teanaway River, Washington, provide insight into rates and mechanisms of river incision and strath terrace formation in a forested landscape. The West Fork drains 102 km 2 of the tectonically quiescent southeastern North Cascade Range, and it is rapidly eroding its bed and creating strath terraces in its lower reach. Minimum vertical incision, measured annually relative to nails embedded in the streambed, was greater in the seasonally exposed, weathering-dominated, high-flow channel (mean = 10.9 mm yr −1 ) than in the perennially wet, abrasion-dominated, low-flow channel (3.8 mm yr −1 ), documenting unsteady lowering of the channel margin. Ages of radiocarbon-dated materials from alluvium on strath terraces, 0.1 m to 5.4 m above the water surface, suggest three episodes of strath abandonment at maximum ages of ca. A.D. 830, A.D. 1560, and A.D. 1890, and average incision rates of 1.3 mm yr −1 , 1.4 mm yr −1 , and 7.4 mm yr −1 for the oldest to youngest surfaces, respectively. Weathering-promoted vertical incision in the high-flow channel provides a mechanism for “top-down” rapid lateral strath planation in which scour of alluvium on incipient strath terraces incorporates the surface into the high-flow channel, allowing rapid removal of bedrock weathered during wetting and drying cycles. Relationships among channel width, channel confinement by bedrock terrace risers, modeled bankfull shear stress, and alluvial bed cover suggest that rapid channel widening could also internally limit vertical incision by slowing incision as shear stresses decline and more alluvium is retained on the bed. The timing of the most recent (ca. A.D. 1890) strath abandonment corresponds with historic anthropogenic removal of fluvial wood, suggesting that the relative abundance of fluvial wood may influence episodes of vertical bedrock incision by affecting the retention of alluvium on streambeds.