GSA Connects 2021 in Portland, Oregon

Paper No. 27-8
Presentation Time: 9:00 AM-1:00 PM


TAYLOR, Stephen, Earth and Environmental Science Dept., Western Oregon University, 345 N. Monmouth Ave, Monmouth, OR 97361

Studies in the Oregon Coast Range (OCR) have yielded numerous contributions to the understanding of fluvial processes in mountainous watersheds. This research examines the relationships between variable bedrock substrate and hydrogeomorphic factors that influence channel morphology in the Luckiamute River Basin (Ad ~815 km2).

A synthetic channel network model was derived from LiDAR-based 1-m DEMs using TerrainWorks NetMap software. Model results include a parameterized network divided into 100-m reach segments, with four higher-order sub-basins delineated: Upper Luckiamute (389 km2), Little Luckiamute (206 km2), Lower Luckiamute (80 km2), and Soap Creek (142 km2). Each sub-basin is in turn underlain by variable percentages of bedrock substrate: Upper Luckiamute >60% Tyee Fm sandstone, Little Luckiamute >75% Yamhill Fm with abundant mafic intrusions, Lower Luckiamute mosaic of Spencer Fm sandstone and Quaternary Alluvium, and Soap Creek 50/50 mix of Siletz River Volcanics (upper) and Quaternary Alluvium (lower).

Mainstem channels from each sub-basin were extracted via the terrain model for subsequent comparative analysis. Average values of morphometric parameters for Upper Luckiamute, Little Luckiamute, Lower Luckiamute and Soap Creek include, respectively: (1) drainage density [1.67, 2.27, 1.33, 1.9 km-1], (2) relief [732, 876, 11, 401 m], (3) gradient [0.0093, 0.0182, 0.0003, 0.0138], (4) channel width:depth ratio [19.6, 18.5, 21.2, 13.0], (5) sinuosity [1.44, 1.36, 1.94, 1.32], (6) bankfull stream power [2154, 5904, 1062, 762 watts/m], (7) floodplain width [38, 39, 45, 91 m], (8) floodplain width:channel width ratio [2.0, 2.7, 1.3, 8.5], and (9) debris flow susceptibility (relative probability) [0.0029, 0.0046, 0.0007, 0.0015].

Data suggest that composition of bedrock substrate exerts a significant influence on channel morphology. Little Luckiamute is characterized by higher values of drainage density, relief, gradient, stream power and debris flow susceptibility. Lower Luckiamute is associated with lower values of drainage density, relief, gradient and debris flow susceptibility; combined with high channel width:depth ratios and sinuosity. Results suggest that resistant mafic-intrusive substrates in the Luckiamute Basin form higher gradient channels with increased debris flow potential. The Lower Luckiamute is characterized by low-relief, low-gradient channels associated with non-resistant substrate. Results have important implications for guiding river restoration strategies in the OCR.