The Impacts of Landslide Deposits on Streamflow Generation and Water Routing in Mountainous Headwaters
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In volcanic mountains, complex subsurface architecture can have a significant impact on subsurface water dynamics. Despite this, the degree to which geomorphic landscape features (e.g., colluvial deposits, glacial features, and active earthflows) influence subsurface water dynamics is poorly understood. Elucidating the mechanistic links between geomorphology and the movement and storage of water is critical to predictions of water quality and quantity. Here, we present a new approach to disentangle flowpath dynamics using a combination of water stable isotopes, field observations, hydrometric data, and high-resolution LiDAR. Five synoptic water sampling campaigns (2021–2023) were conducted seasonally across 12 headwater streams and two high order streams in the HJ Andrews Experimental Forest (HJA), OR. We collected 1,323 surface water grab samples to characterize the isotopic signature within and between reaches underlain by a variety of landforms. Additionally, we monitored stream temperature, water chemistry, and discharge in a selection of study streams during the summer, 2023. LiDAR data was used to identify geomorphic features, estimate colluvial thickness, and develop landscape parameters for statistical modelling. To understand the extent to which these features influence subsurface water movement, we used the isotopic data to estimate an isotopic lapse rate. This allows us to predict the expected isotopic signature in each catchment based on the mean watershed elevation. By comparing these results to the observed data, we interpreted transboundary water movement. Through spatial analysis of the isotope and hydrometric data, we identified regional and reach scale variability in water movement as dictated by geomorphic setting. This analysis has provided evidence that colluvial deposits can lead to subsurface flowpaths that deviate from movements expected from surface topography. The interannual variability of these interactions varies based on the topography of the region and the extent of the deposits. Using these findings, we developed a conceptual model to explain subsurface water dynamics in the HJA and applied this methodology to several additional locations across the Cascades to predict subsurface water dynamics at a larger scale.