Hillslope Morphometrics Calculated Using Continuous Wavelet Transforms Record Variable Uplift/Subsidence and Erosion in Oregon Coast Range and Cascadia Forearc Lowland

Abstract:

Drainage divide stability is controlled by the collective effects of uplift and erosion that act over multiple spatiotemporal scales. While numerous studies record divide transience by using geomorphic process laws describing bedrock river incision, many rivers in uplifting landscapes exhibit a diversity of bedrock exposure, making application of bedrock river metrics a challenge. Hillslopes, however, record uplift and erosion, even as bedrock river incision process laws break down. The Willamette Valley (WV), a forearc topographic low situated between the Oregon Coast Range (OCR) and Cascades, is a transient landscape where stream capture and divide migration is common as the valley grows southward, but where bedrock exposure in rivers is scarce. The adjacent OCR is a well-studied landscape, allowing for calibration of hillslope metrics, and high resolution lidar data is widespread. Here, we use high performance computing and continuous wavelet transforms of topography to map hilltop curvature, identify channel heads, and calculate dimensionless hillslope relief and erosion for the entire 1-m lidar dataset of the OCR and WV (>35,000 km²).

We observe that OCR mean hilltop curvature is ~-0.067 m^-1 (E ≈ 0.1 mm yr^-1 given K = 0.003), though significant variability exists. Hilltops are much broader and gentler on the eastern margin of the OCR along the WV, such that mean curvature is ~-0.02 m^-1 (E ≈ ~0.03 mm yr^-1). The boundary between these geomorphic regimes is often abrupt, occurring across mapped structures, suggesting that contrasting hillslopes record disparate uplift rates. Cross-divide contrasts in hilltop curvature also highlight transient regions, particularly stream captures and asymmetric divides. Importantly, these cross-divide contrasts in hilltop curvature suggest divides migrate in an opposite direction than that predicted by the χ metric. Finally, dimensionless erosion rate and relief, metrics that predict whether hillslopes are growing or decaying, suggest that OCR hillslopes on the margin of the WV are actively decaying. Such topographic decay agrees with field observations of thick and weathered soils, qualitative observations of hillslopes “sinking” into sediment, and seismic and borehole data in the southern WV that may record buried paleotopography.