Julia Carr
Published: 2022
Total Pages: 0
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Bedrock rivers shape the surface of the planet and respond to changes in tectonic and climatic forcings. The rate of bedrock river incision is set by the local channel morphology, rock material strength, and the flux and caliber of sediment in the landscape. Both the erodibility of in-channel bedrock and the grain size of sediment supplied to the channel can be influenced by rock properties, leading to a complex suite of local and non-local lithologic controls on channel morphology. Quantifying each property at the landscape scale is challenging, as rock strength and channel morphology can vary over small spatial scales, and local sediment properties vary over time. Recent advances in small uncrewed aerial vehicle (UAV) technology and structure from motion photogrammetry enable mapping at high enough resolution to resolve patterns in sediment cover, channel geometry, and rock properties, while mapping across large enough spatial scales to integrate across reach scale heterogeneity. In addition, repeat surveys enable tracking sediment transport and channel adjustment through time. Here, I take advantage of natural gradients in metamorphic grade and exhumation depth in the Taiwan Central Range to characterize how rock material properties develop across an orogen and influence channel morphology and sediment. Paired UAV photogrammetry and field surveys span over 53 kilometers of river channel, with 30 km confined to of bedrock-bound channels, and over 10 kilometers of repeat surveys. These surveys sample systematically across all formations and lithologies present in the range. I directly measured channel morphology and sediment cover in each surveyed channel and tributary from 1-5 cm resolution 3D models and orthoimagery. I then use repeat surveys to analyze patterns in clast mobility. These measurements reveal that the size and number of boulders supplied to the channel from hillslope mass wasting systematically increase across both the west to east gradient in metamorphic grade and the south to north gradient in exhumation depth. Channel morphology (slope, width, and depth) is highly variable both within individual reaches, and across the orogen. The increase in boulder size locally steepens the channel, implying that the addition of coarse sediment increases thresholds for incision. However, while the measured grain sizes and slopes both increase from south to north, there is no observed trend in orogen scale relief. On the other hand, while Taiwan has substantial variation in channel width, boulder abundance and size do not set channel width at the reach or orogen scale. Instead, variations in channel width are primarily controlled by bedrock anisotropy and variations in bedrock wall lithology. Finally, repeat surveys reveal that boulder transport and abrasion occurs during 1-3 year storm events. Bed reorganization enhances boulder abrasion, and the selective transport of large clasts imply that boulder residence time in this system is likely shorter than the timescale of bedrock river incision. Ultimately, high resolution surveys reveal local and regional controls on bedrock river incision, set by the interplay of rock strength, sediment, and channel morphology. These observations provide a critical link between theory derived from physics at the local scale, and models of erosion at the landscape scale.