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.

<p class="p3"><span class="s2">Several species of Pacific Salmon migrate upstream every year in the Fraser River (British Columbia, Canada) to reach their spawning grounds. A large portion of the fish get delayed in specific sections of the river, where the morphology and the flow create sections with high flow velocity, high turbulence and jumps in bed elevation that constitute hydraulic barriers. Several fish populations migrate along the Fraser River and any barrier for their passage (created by the river morphology, the flow structure or a localized landslide) can severely endanger the survival of such species, while also impacting indigenous communities for which fish is a fundamental element. In the Fraser River those hydraulic barriers have been identified but not yet thoroughly studied. Here we present results from an extensive monitoring campaign conducted in the last 3 years to measure surface flow velocities in previously identified hydraulic barriers. We selected segments of the Fraser Canyon where distributions of surface flow velocities due to specific river morphologies are potentially impacting fish passage. These include areas where series of constriction</span><span class="s2"><span lang="EN-US">-pool-widenings create plunging flows, bedrock step rapids and overfalls. </span>In such areas we collected video of surface flows (with fixed field cameras and drones) at high frequency during the freshet season and obtained surface flow velocity maps using Large Scale Particle Image Velocimetry (LSPIV) for different values of flow discharge.</span> <span class="s2">Using this extensive dataset, we can detect how stable coherent flow structures are for different flow depths and flow discharges and we are also able to identify which areas are most problematic for fish passage (and for which values of flow), helping fish management agencies and public authorities to better protect the survival of vital species in British Columbia.  </span></p>

Bedrock inner gorges, or narrow and deeply-incised canyons set within broader valleys, are common features in post-glacial landscapes and may reflect the interaction of glacial-fluvial processes. Though widespread, the origins of bedrock inner gorges are enigmatic and have been variably attributed to subglacial meltwater during deglaciation, outburst floods, and subaerial fluvial incision as a response to base level change. It is also unclear if their morphology reflects erosion from a single deglacial period or evolution over multiple glacial-interglacial cycles.Given widespread inner gorges, quartz-bearing rocks, and a history of multiple glaciations, the Fraser Canyon – a 375-km stretch of the Fraser River in British Columbia (Canada) characterized by alternating bedrock and non-bedrock reaches – is an ideal area to explore the drivers of inner gorge incision. Using topographic analyses to characterize the morphology of these bedrock gorges, we assess incision rates required to form the canyons since deglaciation (~14 – 11.7 ka). Using the morphology (e.g., slope) of glacio-fluvial terraces and channel long profile analyses (e.g., ksn), we evaluate whether inner gorges likely formed through 1) interaction of glacial and fluvial erosion during glaciation, 2) one or more catastrophic outburst floods, or 3) steady subaerial fluvial erosion due to isostatic uplift/base level fall since deglaciation. We conclude by exploring the relative efficacy of these erosional processes and implications for the longevity of fluvial and glacial landscapes.

Each year a variable portion of adult Pacific salmon in the Fraser River, British Columbia, Canada die trying to retrace and ascend the river network to their natal spawning grounds. A major factor in migration failure is the severe hydraulic conditions experienced in the Fraser Canyon where encounter velocities can exceed upstream swim speeds of adult salmon, creating a migration barrier. Hydraulic barriers are defined as reaches of river where upstream fish migration is delayed due to high water velocity. A few barriers have been identified along the river and have structures in place designed to help facilitate fish passage. We explore other locations in the Fraser River that are apt to be hydraulic barriers to fish migration based on measured centerline velocity. We classify the barriers as either 1) plunging flows in canyons where the channel is deep and the fastest velocities are observed deep in the water column, 2) rapids where flow is fast and shallow over one or more bedrock steps, or 3) overfalls where fast flow occurs over a step with a substantial drop in elevation. We used drone footage at various discharges and Large-Scale Particle Image Velocimetry (LSPIV) to examine flow structure at typical plunging flows, rapids and overfalls. Surface velocities for the discharges when salmon species are known to be migrating upstream were then compared with published salmon swimming capabilities to determine which locations are likely to create the greatest barriers to salmon migration. We find that there are twenty-two sites, sixteen measured and six suspected high velocity locations, that are potential hydraulic barriers. Overfalls present the greatest barrier to salmon migration, creating vertical barriers in addition to high velocity across the entire width of the channel in narrow laterally constricted reaches. Rapids have high velocity in the segments of the water column where salmon typically swim, but often have back eddies along the banks for fish to rest. Plunging flows in canyons have high depth-averaged velocities, and higher bank velocities as a result of turbulent upwelling along the walls, but typically lower surface velocities than the overfalls and rapids. Pacific salmon populations are already threatened by external factors – such as climate change, habitat degradation, fishing, and disease – and cannot afford to have these impacts amplified by additional barriers to migration. Our observations provide important information for salmon conservation and can be used to better understand salmon migration which in turn helps to inform future mitigation efforts to improve salmon survival rates.

The width and depth of rivers are generally inversely related. For a given discharge, wider rivers tend to be shallower than narrower rivers, which are correspondingly deeper. This is particularly true in bedrock and mixed bedrock-alluvial channels, where deep pools occur downstream of lateral constrictions, downstream of which the channel becomes wider and shallower. However, covariation of width and depth in bedrock and mixed bedrock-alluvial rivers has never been explored due to the lack of field measurements. Here we present a 375 km survey of width and depth measurements in the Fraser Canyon, British Columbia, which alternates irregularly among alluvial (no bedrock exposed on either bank), bedrock-constrained (bedrock exposed on one bank), and bedrock-bound (bedrock exposed on both banks) sections. We find that bedrock-bound reaches have the deepest and narrowest sections, followed by bedrock-constrained reaches and alluvial reaches, which feature the shallowest and widest sections of channel. There is an inverse relation between width and depth for all the channels, with alluvial channels having the highest correlation between these two variables, and thus the greatest covariance. We further explore the relation between width and depth and the downstream hydraulic geometry of 42 individual bedrock-bound canyons. There is an inverse relation between canyon width and depth, with substantial variation within individual canyons. The downstream hydraulic geometry for these bedrock-bound canyons does not follow that typical of alluvial channels; depth is the only variable that adjusts substantially as a response to increasing discharge and upstream basin area.

The width and depth of rivers are generally inversely related. For a given discharge, wider rivers are shallower than narrower rivers, which are correspondingly deeper. At a river scale, past research largely supports the claim that the width of a bedrock channel is expected to be similar to that of an alluvial river with the same drainage area but narrower than that of an alluvial channel with the same discharge. However, when examined at the reach scale, river sections confined by bedrock on both banks have been suggested to be consistently narrower and deeper than the surrounding alluvial reaches. Building off of past research by using a more detailed data set to explore the covariation in width and depth values, as well as exploring this covariation on a canyon to canyon basis, I intend to demonstrate that bedrock reaches are consistently narrower and deeper than their alluvial counterparts.