TOPOGRAPHIC TRANSFORMATIONS REVEAL LONGITUDINAL LENGTHENING OF OREGON FOREARC RIVERS

The interplay of tectonics and climate is responsible for regulating the form of river networks, though the scales of topography primarily responsible for driving the evolution and setting the extent of drainage basins has remained elusive. While previous work has suggested that long-wavelength topography (~100 km low-pass filter) reasonably predicts divide migration direction, the applicability and power of long-wavelength topography to predict drainage basin evolution in a variety of landscapes, including forearcs, remains unclear.

The Willamette Valley (WV) is a forearc lowland situated between the Cascade Volcanic Arc and the Oregon Coast Range and serves as the southwestern-most drainage basin of the Columbia River. The northern WV is at least 15 Ma based on emplacement patterns of the Columbia River Basalts, though interpretation of the age and evolution of the central and southern WV is limited by a scarcity of unambiguous geologic data and geochronologic age control of alluvial stratigraphy. Previous work, however, has established that rivers that now flow into the margin-parallel Willamette River at one time flowed directly from the volcanic arc to the Pacific Ocean, a configuration more evocative of the modern southern Cascadia margin, including the Umpqua and Rogue Rivers. Here, we utilize spectral techniques, particularly continuous wavelet transforms of topographic data, to demonstrate that the Cascadia Forearc Lowland (CFL), which includes the Willamette Valley, spans the entire subduction zone, reaching as far south as Mount Shasta, California. We additionally observe in Gaussian (low pass) filtered topography that the Umpqua and Rogue Rivers reorganize into a margin-parallel river system at wavelengths >30 km, conforming with the CFL observed in wavelet-transformed topography. Intriguingly, the Klamath River remains stable through a range of wavelengths, reflecting its well-entrenched route through the Klamath Mountains. Coupled with observed growth of the Willamette Valley and field observations of ongoing stream capture in the Umpqua drainage basin, these results suggest that the southern Cascadia forearc is evolving into a margin-parallel river system, evocative of the modern WV configuration.