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Wildfire and post-fire rainfall have resounding effects on hillslope processes and sediment yields of mountainous landscapes. Yet, it remains unclear how fire–flood sequences influence downstream coastal littoral systems. It is timely to examine terrestrial–coastal connections because climate change is increasing the frequency, size, and intensity of wildfires, altering precipitation rates, and accelerating sea-level rise; and these factors can be understood as contrasting accretionary and erosive agents for coastal systems. Here we provide new satellite-derived shoreline measurements of Big Sur, California and show how river sediment discharge significantly influenced shoreline positions during the past several decades. A 2016 wildfire followed by record precipitation increased sediment discharge in the Big Sur River and resulted in almost half of the total river sediment load of the past 50 years (~ 2.2 of ~ 4.8 Mt). Roughly 30% of this river sediment was inferred to be littoral-grade sand and was incorporated into the littoral cell, causing the widest beaches in the 37-year satellite record and spreading downcoast over timescales of years. Hence, the impact of fire–flood events on coastal sediment budgets may be substantial, and these impacts may increase with time considering projected intensification of wildfires and extreme rain events under global warming.

Increasing hydrologic volatility—more extreme rain, and larger variations between wet and dry years—has become apparent in some regions, but few data exist to determine how intensifying hydrologic extremes affect sedimentary systems. Using uniquely high‐resolution records of fluvial suspended sediment and coastal morphology, we quantify sedimentary responses from a steep, 357‐km2 watershed in California under extreme wet and dry hydrologic conditions. In years with multiple 2‐ to 10‐year floods, fluvial sediment coarsened significantly as the wet season progressed, with late‐season floods delivering dominantly sand‐sized material to the coast. Greater and coarser sediment supply under wetter antecedent conditions affected nearshore geomorphic evolution for 4–5 years. The watershed and coastal changes we documented point to an increasing role of sediment‐related hazards (flooding and hillslope erosion) and resources (nearshore accretion) as wet seasons intensify. Plain Language Summary Some regions are experiencing more extreme rain and a stronger contrast between extreme drought and extreme wet years. Understanding how those extremes affect sediment moving through rivers is important for assessing hazards, risk to ecosystems, and managing resources in rivers and along coastlines where rivers deliver sediment, but not much is known about how river sediment is responding to shifts in hydrologic extremes. We studied river and coastal sediment over more than a decade in the San Lorenzo River area (California coast), including collecting data during extreme wet years and extreme dry years. During extreme wet years when multiple floods occurred, the river started carrying much more sand‐sized material, consistent with many landslides dumping new sediment into the river and its tributary creeks. The greater proportion of river sand delivered to the coast late in those wet seasons (in contrast to finer‐grained, mostly mud‐sized material in non‐extreme‐wet years) means those extreme wet years have an outsized influence on the sand supply to beaches and other nearshore sediment deposits. The extra sediment from extreme wet years stays in the nearby coastal region for 4–5 years and could help counteract beach losses expected from sea‐level rise. Key Points Sand proportion in river flood sediment is significantly greater with wetter antecedent conditions, consistent with more hillslope supply Extreme wet years have substantial, years‐long influence on coastal morphology Future climate likely will enhance coarse fluvial sediment delivery to U.S. west coast

Increasing wildfire and declining snowpacks in mountain regions threaten water availability. We combine satellite‐based fire detections with snow seasonality classifications to examine fire activity in California's seasonal and ephemeral snow zones. We find a nearly tenfold increase in fire activity during 2020–2021 versus 2001–2019. Accumulation season broadband snow albedo declined 25%–71% at two burned sites (2021 and 2022) according to in‐situ data relative to un‐burned conditions, with greater declines associated with increased burn severity. By enhancing snowpack susceptibility to melt, both decreased snow albedo and canopy drove midwinter melt during a multi‐week dry spell in 2022. Despite similar meteorological conditions in December–February 2013 and 2022–linked to persistent high pressure weather regimes–minimal melt occurred in 2013. Post‐fire snowpack differences are confirmed with satellite measurements. With growing geographical overlap between wildfire and snow, our findings suggest California's snowpack is increasingly vulnerable to the compounding effects of dry spells and wildfire. Plain Language Summary Satellite fire detections indicate substantial increases in wildfire activity in California's snow‐covered landscapes during 2020 and 2021, suggesting wildfire is increasingly altering mountain hydrology. During 2022, a multi‐week mid‐winter drought, or dry spell, occurred. A meteorologically‐similar dry spell occurred in 2013, and the 2022 event provides a test case to examine how post‐fire changes (canopy loss and deposition of burned dark material on snowpack) alter snowmelt patterns. Using field observations, weather station data, and satellite remote sensing of snow, we find large reductions in snow albedo and canopy cover drove rapid melt during the 2022 dry spell in burned areas whereas during 2013, minimal melt occurred. The societal connection between mountains and humans will be strained as mountains face increasing climate‐related stressors. Midwinter drought, snow loss, and increasing wildfire are expectations of a warming world. Addressing these challenges requires innovative water and forest management paradigms. Our findings motivate additional research into assessing and planning for post‐fire hydrologic changes in snow‐dominated landscapes as both wildfire and dry spells will increase in frequency with climate warming. Key Points A 9.8x increase in satellite fire detections in California's snow zones in 2020–2021 versus 2001–2019 implies growing overlap in fire and snow Post‐fire accumulation season broadband snow albedo declined 25%–71%, driving fewer snow‐covered days and lower snow‐cover fraction Compared with the meteorologically similar 2013 dry spell, albedo and canopy declines led to rapid midwinter melt in 2022

Sediment pulses can cause widespread, complex changes to rivers and coastal regions. Quantifying landscape response to sediment-supply changes is a long-standing problem in geomorphology, but the unanticipated nature of most sediment pulses rarely allows for detailed measurement of associated landscape processes and evolution. The intentional removal of two large dams on the Elwha River (Washington, USA) exposed ~30 Mt of impounded sediment to fluvial erosion, presenting a unique opportunity to quantify source-to-sink river and coastal responses to a massive sediment-source perturbation. Here we evaluate geomorphic evolution during and after the sediment pulse, presenting a 5-year sediment budget and morphodynamic analysis of the Elwha River and its delta. Approximately 65% of the sediment was eroded, of which only ~10% was deposited in the fluvial system. This restored fluvial supply of sand, gravel, and wood substantially changed the channel morphology. The remaining ~90% of the released sediment was transported to the coast, causing ~60 ha of delta growth. Although metrics of geomorphic change did not follow simple time-coherent paths, many signals peaked 1–2 years after the start of dam removal, indicating combined impulse and step-change disturbance responses.

Understanding long-term coastal evolution requires historical data, yet accessing reliable information becomes increasingly challenging for extended periods. While vertical aerial imagery has been extensively used in coastal studies since the mid-20th century, and satellite-derived shoreline measurements are now revolutionizing shoreline change studies, ground-based images, such as historical photographs and picture postcards, provide an alternative source of shoreline data for earlier periods when other datasets are scarce. Despite their frequent use for documenting qualitative morphological changes, these valuable historical data sources have rarely supported quantitative assessments of coastal evolution. This study demonstrates the potential of historical ground-oblique images for quantitatively assessing shoreline position and long-term change. Using Conceição-Duquesa Beach (Cascais, Portugal) as a case study, we analyze shoreline evolution over 92 years by applying a novel methodology to historical photographs and postcards. The approach combines image registration, shoreline detection, coordinate transformation, and rectification while accounting for positional uncertainty. Results reveal a significant counterclockwise rotation of the shoreline between the 20th and 21st centuries, exceeding estimated uncertainty thresholds. This study highlights the feasibility of using historical ground-based imagery to reconstruct shoreline positions and quantify long-term coastal change. The methodology is straightforward, adaptable, and offers a promising avenue for extending the temporal range of shoreline datasets, advancing our understanding of coastal evolution.

Wildfire risk has increased in recent decades over many regions, due to warming climate and other factors. Increased sediment export from recently burned landscapes can jeopardize downstream infrastructure and water resources, but physical landscape response to fire has not been quantified for some at‐risk areas, including much of northern California, USA. We measured sediment yield from three watersheds (13–29 km2) that drain to Whiskeytown Lake, California, within the area burned by the 2018 Carr Fire. Structure‐from‐Motion photogrammetry on aerial images combined with sonar bathymetric mapping of submerged areas indicated first‐year post‐fire sediment yields of 4,080 ± 598 t/km2 (Brandy Creek), 2,700 ± 527 t/km2 (Boulder Creek), and 305 ± 58.0 t/km2 (Whiskey Creek)—some of the first post‐fire yields measured in northern California and 64, 42, and 4.8 times greater than pre‐fire yields, respectively. These were measured during a wet year and resulted largely from rilling erosion and fluvial sediment transport, without post‐fire debris flows. Rilling preferentially developed in contact with dirt roads, aided by thin soils and exposed bedrock, and on slopes vegetated by chaparral pre‐fire. The second post‐fire year (a dry year) was characterized by fluvial reworking and delta progradation of the first‐year deposits and relatively little new sediment export. First‐year sedimentation of 111,000 m3 represented minor loss of storage capacity in Whiskeytown Lake but would be detrimental to smaller reservoirs; in general, increased sediment yields from western US watersheds as fire and extreme rainfall increase will likely pose risks to water quality and storage. Plain Language Summary Climate change is increasing wildfires across many regions, including California. Burned landscapes typically produce large amounts of sediment after a fire, which reduces water quality, decreases space for water storage in reservoirs, and sometimes produces hazardous debris flows. We investigated landscape response after the 2018 Carr Fire in northern California. Sediment yield measured in the first year after the fire (a wet year) varied among three study watersheds, ranging from one to two orders of magnitude greater than before the fire. The primary means of sediment mobilization was by water flowing over the land surface, instead of by landslides. These results will aid prediction of future fire response elsewhere in northern California, a growing research need given the increasing tendency for large fires to affect northern California. Key Points Aerial photogrammetric and sonar bathymetric mapping quantify sediment volume change for 2 years after the major 2018 Carr Fire, California First‐year post‐fire sediment yields in three watersheds were 64, 42, and 4.8 times greater than long‐term yields, some of the first post‐fire data from northern California Hillslope erosion was dominated by rilling rather than landslides; rilling commonly occurred downslope of dirt roads

Today, climate change is affecting virtually all terrestrial and nearshore settings. This commentary discusses the challenges of measuring climate‐driven physical landscape responses to modern global warming: short and incomplete data records, land use and seismicity masking climatic effects, biases in data availability and resolution, and signal attenuation in sedimentary systems. We identify opportunities to learn from historical and paleo data, select especially sensitive study sites, and report null results to better characterize the extent and nuances of climate‐change effects. We then discuss efforts to improve attribution practices, which will lead to better predictive capabilities. We encourage the Earth‐science community to prioritize scientific research on climate‐driven physical landscape changes so that societies will be better prepared to manage the effects on health and safety, infrastructure, water–food–energy security, economics, and ecosystems that follow from climate‐driven physical landscape change. Plain Language Summary Modern global warming will ultimately affect physical landscape processes virtually everywhere on Earth, and some of those effects are evident already. This commentary describes the challenges to measuring climate‐driven physical landscape responses to global warming: short and incomplete data records, land use and earthquakes masking climatic effects, biases in data availability and resolution, and climate signals becoming harder to read at the downstream end of a landscape. We discuss ways to collect more informative data in key locations to better understand climate‐change impacts, while also diligently reporting where impacts are not evident. Forming a more complete picture in these ways will mean societies are better prepared to predict and manage impacts on human health and safety, infrastructure, water–food–energy security, economics, and ecosystems that are linked to climate‐driven physical landscape change. Key Points Modern anthropogenic climate change affects a vast range of geomorphic settings We identify challenges of measuring physical landscape response to modern climate change and opportunities to improve studies Better understanding physical landscape impacts will prepare societies to manage hazards and economic effects of climate change

In a warming climate, an intensifying fire regime and higher likelihood of extreme rain are expected to increase watershed sediment yield in many regions. Understanding regional variability in landscape response to fire and post‐fire rainfall is essential for managing water resources and infrastructure. We measured sediment yield resulting from sequential wildfire and extreme rain and flooding in the upper Carmel River watershed (116 km2), on the central California coast, USA, using changes in sediment volume mapped in a reservoir. We determined that the sediment yield after fire and post‐fire flooding was 854–1,100 t/km2/yr, a factor of 3.5–4.6 greater than the long‐term yield from this watershed and more than an order of magnitude greater than during severe drought conditions. In this first large‐scale field validation test of the WEPPcloud/wepppy framework for the Water Erosion Prediction Project (WEPP) model on a burned landscape, WEPP predicted 81%–106% of the measured sediment yield. These findings will facilitate assessing and predicting future fire effects in steep watersheds with a Mediterranean climate and indicate that the increasingly widespread use of WEPP is appropriate for evaluating post‐fire hillslope erosion even across 100‐km2 scales under conditions without debris flows. Plain Language Summary In a warming climate, more wildfire and more extreme rain will cause more erosion, producing more sediment that will be carried downstream by rivers. Understanding how much sediment a landscape produces after fire and extreme rain is essential for managing water resources and infrastructure, because sediment fills up storage space in reservoirs and can interfere with proper functioning of water systems and roads. We measured how much sediment was produced in a watershed due to wildfire followed by extreme rain and flooding, studying the Carmel River on the central California coast, USA. We evaluated how much sediment this watershed produced by measuring the volume of sediment deposited within a reservoir that the watershed drains into. We determined that the rate of sediment exported from the study watershed due to a large fire and post‐fire floods was much greater than the long‐term rate there. Having compared our results with the amount of sediment predicted by a model called the Water Erosion Prediction Project model, we found that the model performed well and realistically predicted the amount of sediment that this landscape would shed. These findings will facilitate assessing and predicting future fire effects in steep watersheds with a Mediterranean‐type climate. Key Points A 116‐km2 central California watershed produced sediment yield of 854–1,100 t/km2/yr in the first year after a fire and extreme rain Post‐fire sediment yield in an extremely wet year without debris flows was 3.5–4.6 times greater than the long‐term rate The Water Erosion Prediction Project model predicted 81%–106% of the measured post‐fire sediment eroded from hillslopes

Watershed sediment yield commonly increases after wildfire, often causing negative impacts to downstream infrastructure and water resources. Post‐fire erosion is important to understand and quantify because it is increasingly placing water supplies, habitat, communities, and infrastructure at risk as fire regimes intensify in a warming climate. However, measurements of post‐fire sediment mobilization are lacking from many regions. We measured sediment yield from a forested, heavily managed 25.4‐km2 watershed in the western Sierra Nevada, California, over 2 years following the 2021 Caldor Fire, by repeat mapping of a reservoir where sediment accumulated from terrain with moderate to high soil burn severity. Sediment yield was less than the geochronology‐derived long‐term average in the first year post‐fire (conservatively estimated at 21.8–28.0 t/km2), low enough to be difficult to measure with uncrewed airborne system (UAS) and bathymetric sonar survey methods that are most effective at detecting larger sedimentary signals. In the second year post‐fire the sediment delivery was 1,560–2,010 t/km2, an order of magnitude above long‐term values, attributable to greater precipitation and intensive salvage logging. Hillslope erosion simulated by the Water Erosion Prediction Project (WEPP) model overestimated the measured amount by a factor of 90 in the first year and in the second year by a factor (1.9) that aligned with previously determined model performance in northern California. We encourage additional field studies, and validation of erosion models where feasible, to further expand the range of conditions informing post‐fire hazard assessments and management decisions. Plain Language Summary Watersheds typically produce more sediment after a wildfire because burned hillslopes erode easily, meaning that streams carry more sediment. Increased sediment delivery can cause problems for downstream infrastructure and water resources. Knowing how much sediment leaves a burned area is an important part of assessing risk to water supplies, infrastructure, and ecosystems, all of which are increasing concerns as climate change intensifies fire regimes, but measurements of sediment after wildfire are lacking from many regions. We studied these problems in a forested, heavily managed (logged) watershed in the western Sierra Nevada, California, over 2 years following the major Caldor Fire of 2021, by repeatedly mapping a reservoir where sediment accumulated from the burn zone. We detected only a small amount of sediment in the first year after the fire, but in the second year the sediment delivery was much larger due to more rain and logging activity that caused ground disturbances in the burned watershed. This study is part of a series documenting post‐fire sedimentary processes in northern California, aiming to expand the knowledge base that resource managers can use in hazard assessments and decisions about managing water‐storage reservoirs after a fire. Key Points Sediment yield was quantified for 2 years after the 2021 Caldor Fire in a forested, heavily managed Sierra Nevada watershed Wetter‐than‐average conditions and intensive logging elevated second‐year sediment yield to an order of magnitude above long‐term values Reservoir surveys have useful applications in post‐fire studies of 101‐ to 102‐km2 basins if the sedimentary signal‐to‐noise ratio is large

AGU Publications encourages research collaborations between regions, countries, and communities. When well‐resourced researchers complete research or field work in low‐resourced settings while excluding local communities or researchers from the process, this can be referred to as parachute science or helicopter research. To help address concerns of parachute science and to promote greater equity and transparency in global research collaborations, AGU Publications has updated its authorship policy across its scholarly journals. The implementation of this policy follows a successful 18‐month pilot at JGR: Biogeosciences. For research completed in low‐resourced regions, authors are encouraged to include a disclosure statement pertaining to the ethical and scientific considerations of their research collaborations. Key Points To promote greater equity, inclusion, and transparency in global collaborations, AGU has updated its authorship policy Authors are encouraged to adhere to the Global Code of Conduct for Equitable Research Partnerships Authors are also encouraged to engage with the Inclusion in Global Research policy