Bedrock river bathymetry is dynamic, with incision rates dependent on sediment cover, supply, and mobility in the channel. However, the scale and fluctuation of this dynamic sediment storage is not well understood, particularly in large bedrock rivers where the bed is not visible at low flows. We used repeat, high resolution, multibeam bathymetric surveys from 2021-2023 to characterize bed and bank topography in nine bedrock canyons that are representative of a wide range of width, depth, slope, and velocity observed through the 375 km long Fraser Canyon in British Columbia. Change in elevation as high as 15 m is identified between surveys. We characterize patches of contiguous change to measure changes in sediment storage volume. Our observations reveal that channel morphology determines where storage occurs. We find that sediment is "staged" through canyons, initially being deposited in a canyon near a sediment supply site, then moving downstream as the initial deposit declines. Substantial changes in storage volume occur without substantial changes in patch footprint. These findings provide key context for interpreting the reach-scale structure of bedrock erosion, the long-term evolution of mountain river networks, and the moderation of sediment delivery to lowland environments.

Abstract Laboratory flumes are powerful tools for investigating riverine processes in a controlled environment. However, the proximity of flume sidewalls modifies the flow relative to natural river channels, and sidewall corrections are crucial for obtaining accurate estimates of bed stress. Sidewall corrections are well‐studied for flumes with aspect ratios (width/depth) greater than one but not for narrower flumes. There has been an increase in grain‐scale dynamics research that uses flumes with widths of just a few grain diameters and aspect ratios down to 0.1 (very narrow flumes). We investigate whether sediment transport experiments in very narrow flumes can be generalized and compared to flow and sediment transport in wider channels. Using a compilation of 400 flume experiments, including 100 with width/depth <1, we find that the influence of sidewalls can be explained by the same theories in very narrow flumes as in wider flumes. However, the classic Vanoni‐Brooks sidewall correction makes an approximation that does not work as well in very narrow flumes. For very narrow flumes, we recommend a more recent sidewall correction. We also investigated whether the flume aspect ratio influences grain‐scale dynamics by applying the sidewall correction method to experiments on grain shape in bed load transport. Our results suggest that there is no effect of the flume aspect ratio on bulk sediment flux relations or on the influence of grain shapes, indicating that sidewalls do not strongly affect grain‐scale dynamics in bed load sediment transport.

In bed load sediment transport, grains are moved by turbulent flow and are in nearly continuous contact with other grains, resulting in sediment flux that is intermittent, displaying bursts of activity in both space and time. Understanding the dynamical origin of these fluctuations is a challenge. Grain-scale models resolve the grain-fluid coupling and provide insight into the grain-scale sources of fluctuations, but are impractical to apply at the channel scale. On the other hand, landscape evolution models and other continuum treatments of alluvial channels ignore fluctuations by averaging over grain-scale processes. We introduce an intermediate-complexity lattice model, or cellular automaton, of bed load sediment transport with grain dynamics based on a simple set of rules. Mimicking collision interactions, grains in our model are entrained by their mobile neighbors with a probability that depends on the local bed slope. When a grain is entrained from or deposited onto the bed, it changes the bed elevation, providing a feedback between entrainment and bed topography. Despite this simplified representation of grain dynamics, the model reproduces the intermittent statistics of grain activity observed in previous studies. Numerical experiments further reveal how intermittency depends on bed width and point to a physical explanation. Finally, we use our model to derive a stochastic partial differential equation describing the system, providing a direct connection between collective entrainment and a nonlinear source term for grain entrainment, which is responsible for the intermittent dynamics.

Non-uniform flow dynamics in bedrock-bound channel morphologies play a critical role in landscape evolution because these reaches are locations along river long profiles where active bedrock incision occurs. Field observations indicate that plunging flows, characterized by velocity inversions within bedrock-bound constriction-pool-widening (CPW) channel morphologies, drive incision at the local scale. These flows generate high shear stresses that promote sediment transport and contribute to the development and maintenance of CPW morphology. Previous studies of plunging flows have relied on coarse-scale field observations and labor-intensive laboratory experiments to investigate their dynamics. Here, we use eddy-resolving computational fluid dynamics models to examine plunging-flow behavior, building on experimental evidence that lateral channel constriction induces plunging flows. Using large-eddy simulations (LES) of laboratory-scale flows, we found that the optimal constriction for generating plunging flows is approximately 35

Adult Pacific salmon ( Oncorhynchus spp.) in the Fraser River, British Columbia, can die trying to retrace and ascend the river network to their natal spawning grounds due to hydraulic barriers, where encounter velocities exceed swim speeds of adult salmon. We evaluated river hydraulics, river morphology, and swimming ability to better understand these potential hydraulic barriers. A 375 km centreline velocity survey of the Fraser Canyon identified 22 high velocity locations where the distribution of velocity within the reach could produce a hydraulic barrier. We identified and studied three flow types associated with these 22 high velocity locations: (1) plunging flows, (2) rapids, and (3) overfalls, using drone footage at various discharges to examine flow structure and compare surface velocities with swimming modes. Complex flow within the major hydraulic features highlights the spatial locations requiring anaerobic swimming and areas of potential recovery that change with discharge. This approach can be used to improve the understanding of fish migration limits in a natural river system and aid in future mitigation.

Landscape scale bedrock erosion is the integration of bedrock erosion at the reach scale, which is driven by particle impacts from sediment transport caused by near-bed hydraulics. Plunging flow hydraulics have been identified in bedrock canyons and cause velocity profile inversions, which enhance near-bed velocities, sediment transport, and the potential for bedrock erosion. Observations of plunging flows are limited, and the frequency and statistical properties of this hydraulic phenomenon have not been investigated. Here, we define metrics to identify velocity inversions and use them to detect instances of plunging flows through a 375 km reach of the Fraser River where channel morphology is controlled by bedrock. Isolated plunging flows are identified as well as plunging flow complexes where a series of plunges cause the core of maximum velocity to remain depressed in the water column for a prolonged distance. A significant relationship between plunging flows and bedrock exposure is identified, and plunging flows occupy more than half of the bedrock confined reaches. Stronger plunging flows are correlated with deeper and narrower channels with higher maximum shear stresses. Plunging flows are also concentrated in steeper reaches, which likely represent knickzones in the river profile. We use particle abrasion-based bedrock erosion models to show that plunging flows drive reach-scale incisions in bedrock rivers, creating deep bedrock pools. These pools dominate the incision into the bedrock, which sets the base level for their drainage areas and in turn sets the pace of landscape evolution.

Landslides are important natural hazards to infrastructure and humans, but they also pose a hazard to the biosphere and constitute an ‘ecohazard’. Recent and prehistoric landslides in southwest British Columbia have significantly impacted local fluvial processes and nearby ecosystems. For example, the 2018 Big Bar landslide caused a major hydraulic barrier in the Fraser River, which impeded upstream salmon migration for 2 years. To better understand the impact of landslides on the biosphere, defined here as ’ecohazard’ impacts, we assessed the distribution, size and character of landslides near the Fraser River over the past 12 to 15 ka. We created a landslide inventory using 2560 km 2 of airborne lidar along the Fraser Canyon Corridor, a 375‐km stretch of the Fraser River, which flows through varied topography and bedrock. We mapped 274 landslides with planform areas between 2 × 10 3 and 2 × 10 6 m 2 and estimated volumes that range from 600 to 1.05 × 10 8 m 3 . While the landslides with the largest area were flows/spreads in volcanic rocks, landslides in intrusive and metamorphic rocks had the highest median area. Area–volume relations were consistent across lithology and failure styles. Area–frequency and volume–frequency distributions have a positive skew, in agreement with other landslide inventories. Area–runout relations were also consistent across classifications, meaning the likelihood of a given landslide event reaching the Fraser River is primarily controlled by valley morphology rather than lithology or specific failure mechanics. We propose an ecohazard impact framework to classify the risk posed to salmon migration for the different classes of landslides documented in our inventory. We found that landslides of similar size and character to the Big Bar landslide are relatively common within the Fraser River Corridor; however, their location within the valley will determine how likely they are to significantly impact the ecosystem.

The particles in natural bedload transport processes are usually aspherical and span a range of shapes and sizes, which is challenging to be represented in numerical simulations. We assemble existing numerical methods to simulate the transport of natural gravel (NG). Starting with computerized tomographic scans of natural grains, our method approximates the shapes of these grains by "gluing" spheres (SP) of different sizes together with overlaps. The conglomerated SP move using a Discrete Element Method which is coupled with a Lattice Boltzmann Method fluid solver, forming the first complete workflow from particle shape measurement to high-resolution simulations with hundreds of distinct shapes. The simulations are quantitatively benchmarked by flume experiments. Beyond the flume, in a more generalized wide wall-free geometry, the numerical tool is used to further test a recently proposed modified sediment transport relation, which takes particle shape effects into account, including the competition between hydrodynamic drag and material friction. Unlike a physical experiment, our simulations allow us to vary the hydrodynamic drag coefficient of the NG independently of the material friction. The results support the modified sediment transport relation. The simulations also provide insights into particle-level kinematics, such as particle orientations. Though particles below the bed surface prefer to orient with their shortest axes perpendicular to the bed surface, with a decaying tendency with an increasing height above the bed surface, the orientational preferences in transport processes are much weaker than those in settling processes. NG rotates relatively freely during bedload transport.

The legacy of glaciation persists for tens to hundreds of thousands of years in postglacial landscapes, where transient storage and release of paraglacial sediment masks signals of primary landscape denudation (i.e., bedrock incision). The timescales over which glacial legacies persist are difficult to quantify without detailed information on fluvial sediment load or landscape denudation. Here, we present 33 new detrital 10 Be cosmogenic radionuclide analyses from the Fraser River basin in western Canada. We combine 10 Be concentrations with paraglacial terrace distributions and present a sediment mixing model to assess the extent to which paraglacial terrace sediment contributes to fluvial sediment loads. Estimated basin‐averaged denudation rates are ~0.23 mm yr −1 and largely invariable along the Fraser Canyon, a 375‐km bedrock influenced reach, despite a doubling in drainage area and extensive paraglacial terrace distribution. Denudation rates are poorly correlated with landscape morphometry (slope, channel steepness) and climate. To reproduce patterns in 10 Be concentrations along the canyon, terrace sediment contributions must be limited to ~15% of the total flux. We attribute the low terrace inputs to limited connectivity between the terrace fill and the Fraser River channel, which is now incised into bedrock. Longitudinally invariant specific sediment yields are likely a consequence of limited floodplain storage for sediment that is instead transported through the Fraser Canyon to the delta. Sediment derived from bedrock denudation exceeds sediment inputs from paraglacial terrace deposits in the lower Fraser River. Despite widespread prevalence of paraglacial terrace fill in the landscape, our results indicate that basin‐scale paraglacial effects diminish once channels re‐incise into bedrock and terrace fills become disconnected from lateral river channel erosion. These findings shed new light on the processes controlling the timescales of paraglacial effects on modern river sediment loads and wider postglacial landscape evolution.

Landscapes are shaped by the interaction of tectonics, climate, and rock erosion dynamics. Active incision in bedrock rivers sets the pace of landscape evolution because river incision cuts deep valleys and canyons into bedrock, transporting that material to the sea. This unburdens Earth's surface, allowing uplift of majestic mountain peaks in tectonically active settings. Bedrock-bound rivers, where the banks and bed are mostly bedrock, are hard points in the landscape that set the upstream base level of drainage basins and that must be vertically incised to lower landscape elevation and balance erosion against tectonic uplift. There are four distinct bedrock-bound channel morphologies that do not occur in alluvial channels—constriction-pool-widenings, rapids, overfalls, and waterfalls—each of which has a distinct flow structure. Our ability to predict bedrock-bound channel morphodynamics is nascent, but the discovery of mechanistic lateral bedrock erosion models, coupled with existing vertical incision models, allow prediction of bedrock river geometry and adjustments due to changes in water flux, sediment supply, and regional uplift. ▪ Coupled lateral and vertical erosion models reveal that the geometry of bedrock rivers is dominantly controlled by sediment supply, not discharge. ▪ Coupling observations of nonuniform flow structures and erosion models confirm that bedrock-bound channels are loci of intense erosion along a river's profile. ▪ Prediction of the 3D shape of bedrock-bound rivers is possible by combining models for flow, sediment transport, and bedrock erosion. ▪ Morphodynamic predictions are limited by poor understanding of nonuniform flow structures, flow resistance, and sediment transport in bedrock-bound channels.

In rivers, the addition of finer sediment to a coarser riverbed is known to increase the mobility of the coarser fraction. Two mechanisms have been suggested for this: a geometric mechanism whereby smaller sizes smooth the bed, increasing near-bed velocity and thus mobility of the larger sizes, and a viscous mechanism whereby a transitionally smooth turbulent boundary layer forms, rendering the coarser grains more mobile. Here, we report on experiments using two sediment mixtures to better understand these proposed mechanisms. In Mixture 1, we used 0.5 and 5 mm grains, and in Mixture 2, we used 2 and 20 mm grains. If the entrainment of coarse gravel by finer sediment is a purely geometric effect, then the addition of finer material should produce the same effect on the mobility of the coarser material for both mixtures because they have the same size ratio. We show that addition of finer material has a different effect on the two mixtures. We observed an increase in the mobility of the coarse fraction for both mixtures, but the increase in coarse fraction mobility for Mixture 1 was almost twice that for Mixture 2. Our experiments show that in addition to the geometric effect, enhancement of coarse gravel transport by finer sediment is also driven by a viscous effect.

The grain size 2 mm is the conventional border between sand and gravel. This size is used extensively, and generally without much physical justification, to discriminate between such features as sedimentary deposit type (clast-supported versus matrix-supported), river type (gravel bed versus sand bed), and sediment transport relation (gravel versus sand). Here we inquire as to whether this 2 mm boundary is simply a social construct upon which the research community has decided to agree or whether there is some underlying physics. We use dimensionless arguments to show the following for typical conditions on Earth, i.e., natural clasts (e.g., granitic or limestone) in 20 ∘C water. As grain size ranges from 1 to 5 mm (a narrow band including 2 mm), sediment suspension becomes vanishingly small at normal flood conditions in alluvial rivers. We refer to this range as pea gravel. We further show that bedload movement of a clast in the pea gravel range with, for example, a size of 4 mm moving over a bed of 0.4 mm particles has an enhanced relative mobility compared to a clast with a size of 40 mm moving over a bed of the same 4 mm particles. With this in mind, we use 2 mm here as shorthand for the narrow pea gravel range of 1–5 mm over which transport behavior is distinct from both coarser and finer material. The use of viscosity allows the delineation of a generalized dimensionless bed grain size discriminator between “sand-like” and “gravel-like” rivers. The discriminator is applicable to sediment transport on Titan (ice clasts in flowing methane/ethane liquid at reduced gravity) and Mars (mafic clasts in flowing water at reduced gravity), as well as Earth.

Abstract Climate change is expected to increase the frequency and magnitude of river floods. Flood flows not only cause direct damage by inundation, but also jeopardize infrastructure as a consequence of bank failure and river bed erosion processes that are poorly understood. The increased human exposure to floods has resulted in flood safety programs mainly focused on dikes and river widening. Here, we demonstrate and explain how extreme peak floods in embanked rivers, nonuniformly engineered to avoid flooding, can cause unexpected and hazardous erosion. The extreme and unanticipated 2021 flood in the Meuse Basin caused dozens of fatalities and a multibillion damage to infrastructure. Based on a large dataset and hydrodynamic modelling of the Meuse River, we unravel how multiple factors caused massive erosion in a river that was considered safe. Crucially, a recently implemented flood safety program widened the river, but created bottlenecks where infrastructure prevented widening. There, sixteen extreme scour holes formed during the 2021 flood, one exceeding 15 m deep. We infer the extreme scours are caused by the combined effect of tectonic uplift, gravel mining and riverbed incision in response to river realignment, which resulted in thinning of the gravel layer above fine Tertiary sediments. Gravel dunes formed and exposed the easily erodible sands in the troughs, which were rapidly suspended. Our findings demonstrate that subsurface heterogeneity, climate change induced discharge extremes and human control measures lead to a growing risk of uncontrolled erosion, being a peril both for infrastructure and for people living in former floodplains.

Bedrock rivers often alternate between relatively wide unconstrained reaches and conspicuously narrow deep incised bedrock reaches (canyons). These bedrock canyons exhibit a constriction-pool-widening (CPW) morphology that consists of a lateral constriction, a deeply scoured pool formed downstream of the constriction, and a channel widening at or near the pool exit. To explore how CPWs are formed in bedrock canyons, we hypothesize that the lateral constriction at the canyon entrance forces a CPW to form allogenically with subsequent CPWs propagating further downstream. Our hypothesis was tested experimentally in a flume channel with a forced lateral constriction at the canyon entrance. Our experiment shows that the forced constriction can cause a primary CPW to form allogenically because the backwater upstream of the forced constriction causes sediment deposition that creates an elevation drop, promoting flow and sediment to plunge toward the bed and carve a primary pool. Channel widening occurs at the primary pool exit because sediment deposit forms that deflects sediment into the banks, causing lateral erosion. Downstream of the primary widening, channel width declines and a new lateral constriction forms, which causes the formation of pools and widening downstream, resulting in downstream CPW propagation. In our experiment, the bedrock channel evolved until a persistent alluvial cover formed, reaching a steady state morphology without further vertical erosion until perturbed by higher discharge. Our experiment shows that discharge variation is necessary for a channel to evolve in the absence of uplift. Mountain rivers often alternate between wide, shallow channels that are not constrained by bedrock and deep, narrow canyons that are laterally constrained by bedrock. Canyons often exhibit repeating sequences of channel narrowing that are accompanied by deep pools and channel widening near the pool exit. Here, we conducted a laboratory experiment to better understand how these morphological sequences are formed. Our experimental bed and banks were made of a type of foam board commonly used for insulation because the foam board erodes like rock, but at a much faster rate. We setup our experimental channel so that there was an initial channel narrowing, like at the entrance to a canyon, and then studied the erosion patterns created by gravel transport through the simulated canyon. We found that sediment deposition upstream of the channel narrowing creates a steep water surface slope entering the canyon, which drives sediment into the bed carving a slot at the canyon entrance. The slot gradually widens forming a pool that is elongated downstream. At the pool exit, flow slows down and sediment deposition occurs, which then deflects sediment into the channel banks, creating a channel widening. This initial constriction-pool-widening is propagated downstream, creating new sequences of this morphology. Lateral constriction of bedrock canyons can cause the formation of pools through vertical erosion and the widening through lateral erosion The constriction, pool and widening morphology can self-propagate downstream Bedrock channel morphology can reach a steady state through a balance between uplift and lateral and vertical erosion rates

Abstract Riverbeds often fine downstream, with a gravel‐bedded reach, a relatively abrupt gravel‐sand transition (GST), and a sand‐bedded reach. Underlying this behavior, bed grain size distributions are often bimodal, with a relative paucity (gap) around the range 1–5 mm. There is no general morphodynamic model capable of producing the grain size gap and gravel‐sand transition autogenically from a unimodal sediment supply. Here we use a one‐dimensional morphodynamic model including size‐specific bedload and suspended load transport, to show that bimodality readily evolves autogenically even under unimodal sediment feed. A GST forms when we include a floodplain width that abruptly increases at some point. Upstream of the transition, non‐gap gravel ceases to move and gap sediment is preferentially transported. At the transition, non‐gap sand rapidly deposits from suspension, enhancing gap sediment mobility and diluting its presence on the bed.

In deltas and estuaries throughout the world, a fluvial-to-tidal transition zone (FTTZ) exists where both the river discharge and the tidal motion drive the flow. It is unclear how dune characteristics are impacted by changes in tidal flow strength, and how this is reflected in the hydraulic roughness. To understand dune geometry and variability in the FTTZ and possible impacts on hydraulic roughness, we assess dune variability from multibeam bathymetric surveys, and we use a calibrated 2D hydrodynamic model (Delft3D-FM) of a sand-bedded lowland river (Fraser River, Canada). We focus on a period of low river discharge during which tidal impact is strong. We find that the fluvial-tidal to tidal regime change is not directly reflected in dune height, but local patterns of increasing and decreasing dune height are present. The fluvial-to-tidal regime change is reflected in dune shape, where dunes have lower leeside angles and are more symmetrical in the tidal regime. The calibrated model allows to estimate local patterns of dune heights using tidally averaged values of bed shear stress. However, the spatially variable dune morphology hampers local dune height estimation. Changes in dune shape do not significantly impact the reach-scale roughness, and estimated dune roughness using dune height and length is similar to the dune roughness inferred from model calibration. Hydraulic model performance with a calibrated, constant roughness is not improved by implementing dune-derived bed roughness. Instead, the data analysis revealed that large-scale river morphology may explain differences in model roughness and corresponding estimates from dune predictors. Where rivers meet the sea, the flow will often be driven by tides from the sea and by river flow, resulting in a fluvial-to-tidal transition zone. The transition can be abrupt or gradual, which might influence the bed of the river, which is covered by dunes. Dune geometry is important in understanding the degree of friction in the river, which in turn determines water levels. It is unclear how bedform characteristics and the related friction are impacted by change in tidal flow strength. This study of the Fraser River in Canada used survey data of the river bed and a computer model of the river flow to study the geometry of dunes and the corresponding friction in this transitional region. We find that dune height and length vary considerably, but they are is unrelated to the fluvial-tidal regime change. Instead, only the dune leeside, that is, the downstream facing side, was impacted. The difference in leeside angle before and after the regime change, did not result in a different friction produced by the dunes. Using the friction produced by dunes in the model, instead of a constant friction, does not improve model performance. Instead, large-scale river morphology determines roughness variations. Hydraulic roughness in the fluvial-to-tidal transition zone estimated from dune geometry agrees with calibrated model roughness Variation in dune asymmetry and leeside angle across a fluvial-to-tidal transition zone has little impact on reach-scale hydraulic roughness Estimated spatial bedform patterns from modeled shear stress match measured bedform patterns, but absolute dune heights do not

Landslides are major drivers of landscape evolution. Mass-wasting events connect hillslope and channel processes through the downslope transferal of sediment, which can impact fluvial systems in a multitude of ways. Coarse sediment delivered into the channel can impact flow dynamics, alter river incision rates through the tools and cover effect, deflect reach-scale river alignment, and disrupt riverine ecosystems. The feedbacks between landslides, fluvial processes, and landscape evolution remain largely unexplored despite increasingly detailed landslide inventories, enabled by the availability of airborne lidar mapping and high-resolution topographic data. To better understand the nature of these feedbacks across varying lithologies, tectonic conditions and valley morphologies we explore post-glacial landslides (~14 ka to present) in the Fraser River valley in southwest British Columbia, Canada. We created a landslide inventory using existing literature and 2,560 km2 of new airborne lidar along the Fraser Canyon corridor, a 375-km stretch of the Fraser River, which flows through multiple regions with distinct climate, morphology, and geology. We documented ~300 landslides with areas between 2 x 103 and 2 x 106 m2. Failure types include translational bedrock slides, earthflows, and rock avalanches, which vary systematically with bedrock geology and valley morphometrics. We find more translational bedrock and earthflow failures in the broader, U-shaped valley of the northern Fraser River canyons, where sedimentary, metasedimentary, and volcanic bedrock are more common. Rock avalanches are more common in the southern Fraser River canyon, where valley walls are composed of plutonic and metamorphic rocks, however the southern region has fewer landslide features overall. These findings suggest individual failures and their combined impact on the post-glacial evolution of the Fraser River are sensitive to both the geologic and glacial history of a particular stretch of river.

Riverbed sediments often lack fine gravel between 1 and 5 mm, a phenomenon referred to as the ‘grain size gap’. The gap corresponds to the rapid reduction in grain size associated with the gravel-sand transition, where median bed material grain size reduces from ~10 mm gravel to ~1 mm sand. Fine gravel grain sizes are often present in hillslope sediment, so it is not clear why they are absent on riverbed surfaces. We present a phenomenological laboratory experiment examining changes in sediment dynamics across a gravel-sand transition to explore the fate of grain size gap material. Our observations indicate that where sand falls out of washload, forming persistent surficial deposits at the gravel-sand transition, grain size gap material experiences enhanced mobility. This is due to hydraulic smoothing by sand that occurs because of a geometric effect, where medium sand bridges interstitial pockets in fine gravel bed surfaces. Our experiments show that fine gravel flux is enhanced by sand deposition making gravel beds at the threshold of motion, mobile. We are unable to maintain an immobile fine gravel bed when sand is fed, which explains why gravel beds composed of 1 to 5 mm particles are so rare on Earth. Our experiment shows that fine gravel particles mobilized by sand deposition are transported out of the flume. We hypothesize that in natural systems, fine gravel particles are either buried in the diffuse extension of gravel-sand transitions or transported into coastal and marine environments where they are more commonly observed.

&lt;p&gt;On November 1&lt;sup&gt;st&lt;/sup&gt;, 2018 the Big Bar Landslide temporarily blocked Fraser River, the most productive salmon-bearing watershed in Canada, presenting a barrier to upstream salmon migration in 2019 and 2020. &amp;#160;The landslide is an example of an ecohazard, similar to a natural hazard, but with immediate and direct impacts on the biosphere, rather than on people and infrastructure. &amp;#160;Like natural hazards, ecohazards have cascading effects, where a geophysical process triggers additional events, often with dramatic consequences. &amp;#160;The Big Bar Landslide originated from collapse of a steep bedrock wall and deposited 89,000 m&lt;sup&gt;3&lt;/sup&gt; of rock into one of the narrowest sections of the Fraser River, damming the channel for over 7 hours, and impounding 650,000 m&lt;sup&gt;3&lt;/sup&gt; of water. &amp;#160;The rockfall debris formed a bank to bank step with an &amp;#8216;overfall&amp;#8217; that was ~4 m at low flow and ~7 m at high flow.&amp;#160; The overfall resembled a waterfall, but without a freefall into a plunge pool. &amp;#160;A backwater formed upstream that extends ~750 m upstream at low flow, but several kilometers at high flow trapping incoming sediment. &amp;#160;The overfall generated a hydraulic barrier to upstream salmon passage, and significantly impeded salmon migration to the Upper Fraser Basin in 2019 and 2020, but rock work has partially ameliorated the impact. &amp;#160;Fish passage monitoring indicates success in passing the landslide in 2019 was species and discharge dependent with population-specific estimates ranging from &lt;1% to over 80% success. &amp;#160;Passage success was particularly low for early timed populations exposed to the highest flow in 2019; so few fish successfully migrated to the spawning grounds that there was a risk of functional extinction of those runs. &amp;#160;There is an ongoing risk to all salmon populations above the landslide until hydraulic conditions at the slide stabilize.&amp;#160; The event will have cascading effects on upstream ecosystems, Indigenous peoples who rely on the salmon fishery throughout the Fraser River Basin, and commercial ocean fisheries, but the extent of the cascading effects is not yet known. &amp;#160;Collapse of bedrock canyon walls, like the one that started the ecohazard cascade in the Fraser River, are geologically commonplace, but the risk they pose to migratory fish populations in mountainous river systems is largely unknown.&lt;/p&gt;