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Flow in bedrock canyons



Bedrock erosion in rivers sets the pace of landscape evolution, influences the evolution of orogens and determines the size, shape and relief of mountains1,2. A variety of models link fluid flow and sediment transport processes to bedrock incision in canyons. The model components that represent sediment transport processes are increasingly well developed3,4,5. In contrast, the model components being used to represent fluid flow are largely untested because there are no observations of the flow structure in bedrock canyons. Here we present a 524-kilometre, continuous centreline, acoustic Doppler current profiler survey of the Fraser Canyon in western Canada, which includes 42 individual bedrock canyons. Our observations of three-dimensional flow structure reveal that, as water enters the canyons, a high-velocity core follows the bed surface, causing a velocity inversion (high velocities near the bed and low velocities at the surface). The plunging water then upwells along the canyon walls, resulting in counter-rotating, along-stream coherent flow structures that diverge near the bed. The resulting flow structure promotes deep scour in the bedrock channel floor and undercutting of the canyon walls. This provides a mechanism for channel widening and ensures that the base of the walls is swept clear of the debris that is often deposited there, keeping the walls nearly vertical. These observations reveal that the flow structure in bedrock canyons is more complex than assumed in the models presently used. Fluid flow models that capture the essence of the three-dimensional flow field, using simple phenomenological rules that are computationally tractable, are required to capture the dynamic coupling between flow, bedrock erosion and solid-Earth dynamics.

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Figure 1: Centreline transects of flow through a narrow bedrock canyon of the Fraser River.
Figure 2: Cross-sections of horizontal velocity magnitude and vertical velocity in Black Canyon downstream of a constriction.
Figure 3: Conceptual model of flow in a bedrock canyon.


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This study was supported by Natural Science and Engineering Research Council grants to M.C., J.G.V. and C.D.R. We thank D. Baerg and his crew at Fraser River Raft Expeditions for undertaking the logistics of the river traverse, R. DiBiase for reviewing an early draft of the manuscript, and M. Lin and C. Adderley for assistance with data processing.

Author information

Authors and Affiliations



M.C. planned and organized the field campaign and provided guidance through the analysis. J.G.V., C.D.R. and M.C. performed the survey and supervised data processing and analysis by J.B., R.W.B and M.L. J.G.V. analysed the data and wrote the manuscript with input from C.D.R. and M.C.

Corresponding author

Correspondence to Jeremy G. Venditti.

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Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Bedrock canyons of the Fraser River, British Columbia.

From Quesnel to Yale, the river crosses the Interior Plateau where the surficial rock is dominantly volcanic, with sedimentary rock along the river. From Lillooet Rapid to Chilliwack, the river flows along a fault between the Coast Mountains to the west and the Cascade Mountains to the east.

Extended Data Figure 2 ADCP measurement survey through the entrance to Black Canyon.

a, Satellite image of Black Canyon with black lines across the channel marking the entrance and exit of the canyon. b, Velocity profile measurement locations (red dots) obtained using the 1,200-kHz ADCP. The surveyed length was 700 m. c, Bed topography measurement locations (blue dots) from the ADCP. The image is from the Ikonos satellite on 11 July 2007, processed by GeoEye/DigitalGlobe and accessed via ESRI ArcGIS software (

Extended Data Figure 3 Interpolated bed topography at the entrance to Black Canyon.

Interpolated using kriging onto a regular 2 m grid. Canyon walls were assumed to be vertical where there are no other data available. The channel margins are defined by the water level in aerial photos, where discharge is approximately 6,440 m3 s−1 at Hope.

Source data

Extended Data Figure 4 Example of an interpolated velocity magnitude grid in Black Canyon.

Velocity magnitude is defined as (Un2 + Ue2 + Uw2)0.5, where Un is the Northing, Ue is the Easting and Uw is the vertical velocity. The interpolation is done in Tecplot ( software using kriging on a three-dimensional grid generated using the prism grid function with a vertical resolution of 0.25 m, and a horizontal resolution of 2 m. Interpolated velocities are shown with a 30% transparency so that all data in the three-dimensional grid can be seen.

Source data

Supplementary information

Video 1

Animation of cross-stream slices through a grid of the horizontal velocity magnitude field in Black Canyon. (MP4 10172 kb)

Video 2

Animation of cross-stream slices through a grid of the vertical velocity field in Black Canyon (MP4 9864 kb)

PowerPoint slides

Source data

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Venditti, J., Rennie, C., Bomhof, J. et al. Flow in bedrock canyons. Nature 513, 534–537 (2014).

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