Rock Control of River Geometry: The Fraser Canyons

Abstract

AbstractA 524‐km reach of Fraser River, British Columbia, was traversed, where the channel alternates between gravel‐bedded reaches that are incised into semiconsolidated glacial deposits and bedrock‐bound reaches, including Fraser Canyon. Continuous centerline acoustic Doppler current profiler survey and observations of bedrock confinement, supplemented with measurements on air photos, reveal the influence of bedrock on channel properties. The surveyed reach was subdivided into 10 morphological reaches, ranging from dominance by alluvial gravel‐bed channels to dominantly rock‐walled canyons. Measured data were grouped within each reach based on the presence of bedrock on both banks, one bank, or neither bank. From alluvial to mixed to canyon‐dominated reaches, river widths decreased while water depths, flow velocities, water surface slopes, and bed shear stresses increased. Within reaches, subreaches with bedrock on both banks were narrower and deeper with higher bed shear stresses but, at the observed stage, had lower depth‐averaged velocity and water surface slope than partly confined and alluvial (unconfined) subreaches. A conceptual model is presented representing a bedrock river as an alternating series of relatively low gradient, deep, rock‐walled pools and steep semialluvial or alluvial sections. Downstream hydraulic geometry relations have scaling exponents expected for alluvial channels provided that the alluvial and rock‐influenced sections are analyzed separately. In Fraser River, canyons become relatively narrower downstream, apparently in response to increasing rock uplift during the Quaternary period. The observations are compared against prior semitheoretical frameworks established to explain variations in width, slope, and channel shape along rockbound channels.