Field evidence for the control of grain size and sediment supply on steady‐state bedrock river channel slopes in a tectonically active setting

• Published in: Earth surface processes and landforms Vol. 42; no. 14; pp. 2338 - 2349
• Main Authors: Finnegan, Noah J., Klier, Rachael A., Johnstone, Samuel, Pfeiffer, Allison M., Johnson, Kerri
• Format: Journal Article
• Language: English
• Published: Bognor Regis Wiley Subscription Services, Inc 01.11.2017
• Subjects: Alluvial channels; Basement rock; Bed load; Bedrock; Catchment area; Catchment areas; Catchments; Channel morphology; Channels; Coastal inlets; Control; Diorite; Erosion; Fluid flow; Fluvial sediments; Grain size; High flow; Landscape; Mechanical stimuli; Mountains; Movement; Particle size; River channels; Rivers; Rock; Rocks; Sediment; Sediment transport; Sedimentary rocks; Sediments; Shear flow; Shear stress; Slope; Slopes; Stresses; United States > US
• ISSN: 0197-9337; 1096-9837
• DOI: 10.1002/esp.4187

We exploit a natural experiment in Boulder Creek, a ~ 30 km2 drainage in the Santa Cruz mountains, CA, USA to explore how an abrupt increase in the caliber of bedload sediment along a bedrock channel influences channel morphology in an actively uplifting landscape. Boulder Creek's bedrock channel, which is entirely developed on weak sedimentary rock, has a high flow shear stress that is about 3.5 times greater where it transports coarse (~ 22 cm D50) diorite in the lower reaches in comparison with the upstream section of the creek that transports only relatively finer bedload (~2 cm D50) derived from weak sedimentary rocks. In addition, Boulder Creek's channel abruptly widens and shallows downstream and transitions from partial to nearly continuous alluvial cover where it begins transporting coarse diorite. Boulder Creek's tributary channels are also about three times steeper where they transport diorite bedload, and within the Santa Cruz mountains channels in sedimentary bedrock are systematically steeper when >50% of their catchment area is within crystalline basement rocks. Despite this clear control of coarse sediment size on channel slopes, the threshold of motion stress for bedload, alone, does not appear to control channel profile slopes here. Upper Boulder Creek, which is starved of coarse sediment, maintains high flow shear stresses well in excess of the threshold for motion. In contrast, lower Boulder Creek, with a greater coarse sediment supply, exerts high flow stresses much closer to the threshold for motion. We speculate that upper Boulder Creek has evolved to sustain partial alluvial cover and transfer greater energy to the bed via bedload impacts to compensate for its low coarse sediment supply. Thus bedload supply, bedrock erosion efficiency, and grain size all appear to influence channel slopes here. Copyright © 2017 John Wiley & Sons, Ltd. We exploit a natural experiment in Boulder Creek in the Santa Cruz mountains, CA USA to explore how an abrupt increase in the caliber of bedload along a bedrock channel influences channel morphology in an actively uplifting landscape. Boulder Creek's bedrock channel, which is entirely developed on weak sedimentary rock, has a high flow stress that is about 3.5 times greater where it transports coarse diorite compared to the upstream section of the creek that transports only relatively finer bedload.