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Wildfires act as potent agents of weathering and erosion, triggering the mobilization of dry ravel sediment and often resulting in temporary increases in sediment transport rates and associated debris-flow hazards. Existing hillslope sediment flux models fail to adequately capture the complex dynamics between erosion and deposition, particularly after wildfires and in landscapes dominated by processes such as bioturbation, tree falls, or other disturbances. To better understand bioturbated dry ravel and subsurface soil properties, we seek to study two hillslopes affected by the 2020 Santa Clara Unit Lightning Complex Fire at the University of California Blue Oak Ranch Reserve near San Jose, CA, by employing a novel application of short-lived radionuclides to characterize dry ravel transport processes. We used gamma spectroscopy on soil cores and recently excavated material from squirrel burrows, sampled along transects from the channel to the ridge on two opposing hillslopes to determine the concentrations of short-lived meteoric radionuclide 210Pb. Our initial findings indicate that the excess 210Pb in soil core sediment varies from the hillslope ridge to toe. In the ridge soil cores, concentrations initially increase within the top 5 cm, followed by a sharp exponential decline with depth. However, the toe soil cores show a sharp exponential decrease in concentrations from the soil surface to ~35-45 cm depth. The toe cores have a concentration of ~ 80 Bq/kg near the surface, while the ridge cores have a much lower concentration of ~ 30 Bq/kg. Based on this preliminary data, we infer that deposition of lower-concentration soil excavated from squirrel burrows leads to mixing of the upper soil layers. In contrast, the well-preserved exponential decay profile and higher surface concentrations at the steeper toe locations indicate less mixing overall. These initial findings warrant further examination of sediment characteristics at various depths through continued gamma spectroscopy and comparative analysis of shallow subsurface structure from ground-penetrating radar. These observations enhance our understanding of the roles of surface gradient and bioturbation in post-fire steepland sediment dynamics. 

Abstract Coastal mountain rivers export disproportionately high quantities of terrestrial organic carbon (OC) directly to the ocean, feeding microbial communities and altering coastal ecology. To better predict and mitigate the effects of wildfires on aquatic ecosystems and resources, we must evaluate the relationships between fire, hydrology, and carbon export, particularly in the fire‐prone western United States. This study examined the spatiotemporal export of particulate and dissolved OC (POC and DOC, respectively) and particulate and dissolved black carbon (PBC and DBC, respectively) from five coastal mountain watersheds following the 2020 CZU Lightning Complex Fires (California, USA). Despite high variability in watershed burn extent (20–98%), annual POC, DOC, PBC, and DBC concentrations remained relatively stable among the different watersheds. Instead, they correlated significantly with watershed discharge. Our findings indicate that hydrology, rather than burn extent, is a primary driver of post‐fire carbon export in coastal mountain watersheds. 

Abstract Non‐perennial rivers and streams are ubiquitous on our planet. Although several metrics have been used to statistically group or compare streamflow characteristics, there is currently no widely used definition of how many days or over what reach length surface flow must cease in order to classify a river as non‐perennial. At the same time, the breadth of climate and geographic settings for non‐perennial rivers leads to diversity in their flow regimes, such as how often or how quickly they go dry. These rivers have a rich and expanding body of literature addressing their ecologic and geomorphic features, but are often said to be ignored by hydrologists. Yet there is much we do know about their hydrology in terms of streamflow generation processes, water losses, and variability in flow. We also know that while they are prevalent in arid regions, they occur across all climate types and experience a diverse set of natural and anthropogenic controls on streamflow. Furthermore, measuring and modeling the hydrology of these rivers presents a distinct set of challenges, and there are many research directions, which still require further attention. Therefore, we present an overview of the current understanding, methodologic challenges, knowledge gaps, and research directions for hydrologic understanding of non‐perennial rivers; critical topics in light of both growing global water scarcity and ever‐changing laws and policies that dictate whether and how much environmental protection these rivers receive. This article is categorized under:Science of Water > Science of Water