Andrew C. Wilcox

Most research into post-fire geomorphology has been conducted in regions with frequent fire, such as in dry Mediterranean and temperate climates and less has been conducted in wet forests where wildfire is now expanding with climate change. Because climate, soils, topography, vegetation, management legacies and fire history influence post-fire geomorphic processes, research into post-fire erosion and debris flow hazards across a broader set of climatologies is needed. The western Pacific Northwest (wPNW) of the United States is susceptible to multiple types of post-fire debris flows (i.e. shallow landslide-initiated and runoff-generated) that can be initiated by longer-duration lower-intensity rainfall compared to other regions in the western US. Therefore, knowledge of how wildfire impacts geomorphic processes in drier, more frequently burned regions does not necessarily apply to the wetter wPNW climate. Significant gaps in data coverage remain a key limitation for predictive modeling and empirical studies in this region. This review synthesizes the current state of post-fire geomorphic knowledge and identifies gaps in data and understanding for the wPNW. These gaps and opportunities for future work can help expand knowledge and evaluate assumptions about post-fire geomorphology in other regions, such as those with similar pyro-climatology and others not yet well represented by current post-fire geomorphology research.

Intensification of hydroclimatic volatility increases the potential for large floods to reshape rivers and threaten infrastructure. Fluvial geomorphic resilience, defined as a river's resistance to, and recovery from, flood disturbance, offers a framework for understanding river response to extreme events and guiding management. We assessed resilience at two scales: (1) a detailed geomorphic and hydraulic analysis of the June 2022 flood on East Rosebud Creek, Montana, which was part of broader flooding in the Greater Yellowstone Ecosystem, and (2) a multiriver comparison of 13 flood‐impacted alluvial mountain rivers (East Rosebud Creek plus 12 additional sites across North America and Europe). We quantified resistance using active‐channel width ratios () and recovery using postflood channel evolution () and recovery rate (). East Rosebud Creek showed strong spatial variability in disturbance among two defined segments, with an upstream segment shifting to new forms via widening, coarsening and/or incision, whereas a downstream segment that experienced similar flow strengths adjusted more modestly, within a dynamic geomorphic equilibrium. Slope, confinement and grain size were key influences on flood response in East Rosebud Creek. In the multiriver analysis, multilevel regression showed a positive log‐transformed relationship between normalized flood peak exceedance above the () and mean and a negative linear relationship between mean slope and mean . Multiriver results suggest that slope‐driven influences on vegetation growth shape recovery. Understanding patterns of resilience can improve flood adaptation strategies and inform targeted river management.

Understanding and predicting flood‐induced geomorphic change, and the relative influences of fluvial forces and valley‐bottom geometry on system response, are persistent quandaries in geomorphic‐process studies. We combined field surveys, remote sensing, and hydraulic modeling to assess the geomorphic effects of the June 2022 flooding in northern Yellowstone National Park, in the western United States, which was by far the largest recorded flood at nearby gaging stations and caused extensive damage to park infrastructure. We evaluate geomorphic response relative to two ways of calculating the hydraulic forcing produced by the flood: unit stream power and impulse, a metric that incorporates flow duration, grain size, channel‐bed slope, flood depth, and a threshold based on threshold channel theory. Measurements of pre‐ and post‐flood active‐channel width change captured the geomorphic response associated with deposition‐ and erosion‐related channel migration. The geomorphic response to flooding showed substantial variability. Notable lateral channel erosion in some cases occurred in areas of high flow strength, such as high‐gradient reaches, but elsewhere was concentrated along erodible channel margins and/or downstream of confluences, where calculated flow forces were unremarkable. Channel widening was greater in partially confined reaches than in unconfined or confined reaches, and channel areas of extensive deposition were associated with decreases in valley‐bottom confinement. Low‐gradient, unconfined reaches where flood flow spread across floodplains showed little evidence of the passage of a historic flood. As climate change increases the propensity for hydroclimatic extremes and events such as the Yellowstone flood, investigation of links between flood forcings and geomorphic response is particularly important.

Severe wildfire may alter steep mountain streams by increasing peak discharges, elevating sediment and wood inputs into channels, and increasing susceptibility to landslides and debris flows. In the Pacific Northwest, where mean annual precipitation is high and mean fire‐return intervals range from decades to centuries, understanding of steep stream response to fire is limited. We evaluate the hydrologic and geomorphic response of ~100‐m‐long steep stream reaches to the large‐scale and severe 2020 fires in the Western Cascade Range, Oregon. In the two runoff seasons after the fires, peak flows in burned reaches were below the 2‐year recurrence interval flood, a level sufficient to mobilize the median grain size of bed material, but not large enough to mobilize coarser material and reorganize channel morphology. Sediment inputs to study streams consisted of two road‐fill failure landslides, slumps, sheetwash, and minor bank erosion; precipitation thresholds to trigger debris flows were not exceeded in our sites. There was a 50% increase in the number of large wood pieces in burned reaches after the fires. Changes in fluxes of water, sediment, and wood induced shifts in the balance of sediment supply to transport capacity, initiating a sequence of sediment aggradation and bed‐material fining followed by erosion and bed‐material coarsening. Gross channel form showed resilience to change, and an unburned reference reach exhibited little morphologic change. Post‐fire recruitment of large wood will likely have long‐term implications for channel morphology and habitat heterogeneity. Below‐average precipitation during the study period, combined with an absence of extreme precipitation events, was an important control on channel responses. Climate change may have a complex effect on stream response to wildfire by increasing the propensity for both drought and extreme rain events and by altering vegetation recovery patterns.

Estimating sediment transport in mountain rivers is challenging because of sediment supply limitation, broad grain size distributions, complex flow hydraulics, and large form drag. Consequently, sediment transport equations are lacking for application in rivers where the bed is coarse and largely immobile, but small fractions of finer, transportable sized material contribute disproportionately to bedload transport. We introduce a framework for estimating sediment transport in mountain rivers that addresses two limitations: estimating the shear stress acting on mobile grains, and accounting for the difference between mobility of size fractions, that is, whether or not a specific grain size can move at a given flow, and transportability, which we define as how much of that size present in the bed will be recruited into transport. We use two bedload data sets to develop equations for predicting incipient motion and transport rates of each grain size fraction present in the bed. We tested the new equations against incipient motion and sediment transport data we collected from streams in the Rocky Mountains, USA, and against published regional sediment yield data. Using this method results in transport estimates where the finer fractions, despite being a small fraction of the bed surface, make up a large part of the total yield. Fractions greater than the median bed grain size are mobile only during peak flood flows, consistent with the existing mountain river bedload data sets. The approach is parsimonious, requiring only data that are often readily available or obtainable: a bed grain size distribution, hydraulic geometry measurements, and discharge.

The strength of interactions between plants and river processes is mediated by plant traits and fluvial conditions, including above‐ground biomass, stem density and flexibility, channel and bed‐material properties, and flow and sediment regimes. In many rivers, concurrent changes in (1) the composition of riparian vegetation communities as a result of exotic species invasion and (2) shifts in hydrology have altered physical and ecological conditions in a manner that has been mediated by feedbacks between vegetation and morphodynamic processes. We review how Tamarix, which has invaded many southwestern US waterways, and Populus species, woody pioneer trees that are native to the region, differentially affect hydraulics, sediment transport, and river morphology. We draw on flume, field, and modelling approaches spanning the individual seedling to river‐corridor scales. In a flume study, we found that differences in the crown morphology, stem density, and flexibility of Tamarix compared to Populus influenced near‐bed flow velocities in a manner that favoured aggradation associated with Tamarix. Similarly, at the patch and corridor scales, observations confirmed increased aggradation with increased vegetation density. Furthermore, long‐term channel adjustments were different for Tamarix‐ versus Populus‐dominated reaches, with faster and greater geomorphic adjustments for Tamarix. Collectively, our studies show how plant‐trait differences between Tamarix and Populus, from individual seedlings to larger spatial and temporal scales, influence the co‐adjustment of rivers and riparian plant communities. These findings provide a basis for predicting changes in alluvial riverine systems which we conceptualize as a Green New Balance model that considers how channels may adjust to changes in plant traits and community structure, in addition to alterations in flow and sediment supply. We offer suggestions regarding how the Green New Balance can be used in management and invasive species management.

Feedbacks between geomorphic processes and riparian vegetation in river systems are an important control on fluvial morphodynamics and on vegetation composition and distribution. Invasion by nonnative riparian species alters these feedbacks and drives management and restoration along many rivers, highlighting a need for ecogeomorphic models to assist with understanding feedbacks between plants and fluvial processes, and with restoration planning. In this study, we coupled a network‐scale sediment model (Sediment Routing and Floodplain Exchange; SeRFE) that simulates bank erosion and sediment transport in a spatially explicit manner with a recruitment potential analysis for a species of riparian vegetation (Arundo donax) that has invaded river systems and wetlands in Mediterranean climates worldwide. We used the resulting ecogeomorphic framework to understand both network‐scale sediment balances and the spread and recruitment of A. donax in the Santa Clara River watershed of Southern California. In the coupled model, we simulated a 1‐year time period during which a 5‐year recurrence interval flood occurred in the mainstem Santa Clara River. Outputs identify key areas acting as sources of A. donax rhizomes, which are subsequently transported by flood flows and deposited in reaches downstream. These results were validated in three study reaches, where we assessed postflood geomorphic and vegetation changes. The analysis demonstrates how a coupled model approach is able to highlight basin‐scale ecogeomorphic dynamics in a manner that is useful for restoration planning and prioritization and can be adapted to analogous ecogeomorphic questions in other watersheds.

Sediment regimes, i.e., the processes that recruit, transport, and store sediment, create the physical habitats that underpin river‐floodplain ecosystems. Natural and human‐induced disturbances that alter sediment regimes can have cascading effects on river and floodplain morphology, ecosystems, and a river's ability to provide ecosystem services, yet prediction of the response of sediment dynamics to disturbance is challenging. We developed the Sediment Routing and Floodplain Exchange (SeRFE) model, which is a network‐based, spatially explicit framework for modeling sediment recruitment to and subsequent transport through drainage networks. SeRFE additionally tracks the spatially and temporally variable balance between sediment supply and transport capacity. Simulations using SeRFE can account for various types of watershed disturbance and for channel‐floodplain sediment exchange. SeRFE is simple, adaptable, and can be run with widely available geospatial data and limited field data. The model is driven by real or user‐generated hydrographs, allowing the user to assess the combined effects of disturbance, channel‐floodplain interactions and particular flow scenarios on the propagation of disturbances throughout a drainage network, and the resulting impacts to reaches of interest. We tested the model in the Santa Clara River basin, Southern California, in subbasins affected by large dams and wildfire. Model results highlight the importance of hydrologic conditions on postwildfire sediment yield and illustrate the spatial extent of dam‐induced sediment deficit during a flood. SeRFE can provide contextual information on reach‐scale sediment balance conditions, sensitivity to altered sediment regimes, and potential for morphologic change for managers and practitioners working in disturbed watersheds.

Point bars influence hydraulics, morphodynamics, and channel geometry in alluvial rivers. Woody riparian vegetation often establishes on point bars and may cause changes in channel-bend hydraulics as a function of vegetation density, morphology, and flow conditions. We used a two-dimensional hydraulic model that accounts for vegetation drag to predict how channel-bend hydraulics are affected by vegetation recruitment on a point bar in a gravel-bed river (Bitterroot River, Montana, United States). The calibrated model shows steep changes in flow hydraulics with vegetation compared to bare-bar conditions for flows greater than bankfull up to a 10-year flow (Q₁₀), with limited additional changes thereafter. Vegetation morphology effects on hydraulics were more pronounced for sparse vegetation compared to dense vegetation. The main effects were (1) reduced flow velocities upstream of the bar, (2) flow steered away from the vegetation patch with up to a 30 % increase in thalweg velocity, and (3) a shift of the high-velocity core of flow toward the cut bank, creating a large cross-stream gradient in streamwise velocity. These modeled results are consistent with a feedback in channels whereby vegetation on point bars steers flow towards the opposite bank, potentially increasing bank erosion at the mid- and downstream ends of the bend while simultaneously increasing rates of bar accretion.