UPLIFT AND INCISION OF THE YAKIMA RIVER CANYON FROM CHANNEL PLANFORM MAPPING AND COSMOGENIC 26AL/10BE ISOCHRON DATING

The Yakima Fold and Thrust Belt (YFTB) comprises a series of northwest-southeast and east-west trending anticlinal ridges and associated reverse faults deforming Columbia River Basalt flows immediately east of the Washington Cascades. Despite decades of geologic study, the relative contribution of Late-Quaternary deformation to the topographic and structural development of the YFTB remains largely unknown. Here, we characterize the geomorphology of the Yakima River between Kittitas Valley and Yakima, WA to evaluate the potential for uplift-driven incision across several major YFTB structures. Our dataset consists of 1) high-resolution channel planform measurements derived from GeoEarthScope lidar and Google Earth imagery, 2) detailed 1:5000-scale fluvial terrace mapping, and 3) ²⁶Al and ¹⁰Be cosmogenic isochron dates for alluvium directly overlying three strath terraces within the canyon. Mapping of the river planform reveals anomalous narrowing across the Manastash Ridge, Umtanum Ridge, and Selah Butte anticlines, despite relatively constant channel slopes and uniform substrate. The discontinuous nature of terrace remnants inhibits precise correlation, although tread elevations extracted from lidar data for lowest surfaces are roughly consistent with broad, low-amplitude warping centered near Umtanum Ridge. ²⁶Al and ¹⁰Be cosmogenic isochron dating of quartz-rich, Cascade-sourced terrace cover gravels suggests a young age, indistinguishable from modern, for the lowest surface. However, early-to-mid Quaternary ages (1.08 ± 0.29 Ma and 1.59 ± 0.25 Ma, 1σ) are suggested for a terrace ~50 m above the river within the Manastash Ridge anticline and downstream for a lower surface (~16 m) within a syncline north of Umtanum Ridge. While the uncertainty in these ages reflects both the low content and narrow range of ¹⁰Be concentrations for analyzed samples, the presence of similar-age clasts at notably different elevations is consistent with Quaternary uplift of the Manastash and Umtanum anticlines relative to interfold channel reaches. Taken together, our results support the notion that anticlinal growth locally controls incision of the Yakima River, and that at least a portion of the total canyon relief represents the product of Quaternary uplift and continued structural development of the YFTB.