LAUSR

Variations in uplift, erosion, climate, and bedrock are commonly invoked as key controls on drainage basin morphology, yet the scale of landforms that define changes in regional drainage networks has not been addressed, limiting our ability to predict their planform evolution. Here we use two‐dimensional (2D) continuous wavelet transforms of topography in Cascadia to highlight dominant topographic features at different scales. Surprisingly, our wavelet analysis shows that for wavelengths >30 km, the Cascadia Forearc Lowland (CFL) spans the entire margin. Separately, we compare observed catchment boundaries with synthetic boundaries generated on topography filtered with 2D Gaussian functions. We observe reorganization of synthetic drainage networks from an arc‐to‐coast drainage system into arc‐spanning, margin‐parallel river systems, akin to the modern Willamette River and coincident with the CFL. In concert with field observations of stream capture and Willamette Valley expansion, we propose that the Cascadia forearc is actively transitioning to a predominately margin‐parallel river system.