Lithologic controls on orogen and reach scale variability in bedrock river morphology and sediment cover across the Taiwan Central Range

Abstract:

The strength of potential orogen-scale feedbacks between surface and deep Earth processes is sensitive to how rock strength influences bedrock river incision. 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 rock strength controls on channel morphology at the orogen scale is difficult, as rock strength can vary at a range of scales and measuring channel morphology and sediment grain size is difficult in steep landscapes. Unmanned aerial vehicle (UAV) surveys enable mapping at high enough resolution to capture channel geometry and sediment cover, while at large enough spatial scales to account for local heterogeneity.

Here, we focus on the Taiwan Central Range, where both the metamorphic grade and the degree and orientation of metamorphic foliation and fracturing vary systematically depending on rock burial and exhumation history. Paired UAV photogrammetry and field surveys span 60 km of bedrock river channel in 21 reaches, and sample systematically across all formations and lithologies present in the range. We directly measured channel morphology and sediment cover in each surveyed channel and tributary from 1-5 cm resolution 3D models and orthoimagery and used these data to analyze patterns in clast mobility. These measurements of morphology reveal that channel slope, width, and high-water depth are highly variable both within individual reaches and across the orogen. At the reach scale, channel slope, width, and depth vary at different length scales in response to rock strength controls. This results in initial motion thresholds that are spatially variable, implying that sediment transport timescales could be buffered by local storage at lithologic boundaries. Across the orogen, we observe similarly high ranges of variability in channel morphology. However, the size of hillslope derived coarse sediment systematically increases with metamorphic grade, which can lead to otherwise similar morphologies eroding at very different time scales. While natural systems have abundant local variability, data rich approaches like these can help tease out systematic variations at large scales.