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Channel bar radar architecture and evolution in the wandering gravel-bed Fraser and Squamish Rivers, British Columbia, Canada
Academic Work
The three-dimensional (3D) subsurface alluvial architecture and evolution of four kilometer-scale gravel bars in the wandering gravel-bed Fraser and Squamish Rivers are described. Ground-penetrating radar (GPR) surveys, historic bathymetry soundings, and time-sequential aerial photographs are used to correlate the internal architecture (from basal scour surfaces to bartop deposits) to the evolution of unit bars, point bars, mid-channel bars, and bank-attached bars in the previous ~50 years.
Decimeter resolution and 3D reflection configurations were obtained from 15km of GPR profiles shot with 50, 100, and 200 MHz antenna frequencies in ~200m x ~200m grids. GPR-imaged bounding surfaces matched bathymetric soundings and former bar positions mapped from historic photographs. Likewise, GPR-imaged sedimentary structures were coincident with patterns of bar translation and morphology evident in the photographic record.
The subsurface architecture of small (<1 km long>50 years) channel-scale macroforms is complex and consists of assemblages of configurations of long-lived (>50 years) channel-scale macroforms is complex and consists of assemblages of configurations that are not genetically related. Unlike the case of unit bars, sediment is not uniformly transported, eroded, or deposited across macroforms.
Low-angle subhorizontal reflections are prominent throughout the alluvial architecture and record the migration of low-amplitude gravelly bedload sheets (2 to 3 grains thick) onto bar surfaces and in-channel. This style of stratified sheet-like sedimentation records the vertical, lateral, downstream, and upstream accretion of sediment (dependent on the dip of the strata). Of the five alluvial radar facies and two radar elements identified, the prevalence of small- to medium-scale (0.5 to 3m), steeply inclined reflections (interpreted as slipface accretion) distinguishes wandering deposits from gravelly meandering or braiding successions. Two-dimensional, concave-up basal reflections identify channel and chute elements, which signify the multiple channeled character of wandering rivers. In contrast, channel scour elements produce 3D, scallop-shaped basal reflections, whose fill over-deepens alluvial successions.
Alluvial sediments (8 to 24 m thick) likely deposited in the last few hundred years overlie large-scale (>6.5 m), steeply dipping reflections, interpreted to be delta foresets deposited ~10,000 years ago during deglaciation. The style of fluvial sedimentation does not appear to have changed in this period, and the preservation of large-scale scour hollows indicates that wandering systems are vertically stable. Together, the unsteady episodic evolution of gravel barforms and their internal architecture confirm the transitional nature of the wandering type of river planform between meandering and braiding river types.
Decimeter resolution and 3D reflection configurations were obtained from 15km of GPR profiles shot with 50, 100, and 200 MHz antenna frequencies in ~200m x ~200m grids. GPR-imaged bounding surfaces matched bathymetric soundings and former bar positions mapped from historic photographs. Likewise, GPR-imaged sedimentary structures were coincident with patterns of bar translation and morphology evident in the photographic record.
The subsurface architecture of small (<1 km long>50 years) channel-scale macroforms is complex and consists of assemblages of configurations of long-lived (>50 years) channel-scale macroforms is complex and consists of assemblages of configurations that are not genetically related. Unlike the case of unit bars, sediment is not uniformly transported, eroded, or deposited across macroforms.
Low-angle subhorizontal reflections are prominent throughout the alluvial architecture and record the migration of low-amplitude gravelly bedload sheets (2 to 3 grains thick) onto bar surfaces and in-channel. This style of stratified sheet-like sedimentation records the vertical, lateral, downstream, and upstream accretion of sediment (dependent on the dip of the strata). Of the five alluvial radar facies and two radar elements identified, the prevalence of small- to medium-scale (0.5 to 3m), steeply inclined reflections (interpreted as slipface accretion) distinguishes wandering deposits from gravelly meandering or braiding successions. Two-dimensional, concave-up basal reflections identify channel and chute elements, which signify the multiple channeled character of wandering rivers. In contrast, channel scour elements produce 3D, scallop-shaped basal reflections, whose fill over-deepens alluvial successions.
Alluvial sediments (8 to 24 m thick) likely deposited in the last few hundred years overlie large-scale (>6.5 m), steeply dipping reflections, interpreted to be delta foresets deposited ~10,000 years ago during deglaciation. The style of fluvial sedimentation does not appear to have changed in this period, and the preservation of large-scale scour hollows indicates that wandering systems are vertically stable. Together, the unsteady episodic evolution of gravel barforms and their internal architecture confirm the transitional nature of the wandering type of river planform between meandering and braiding river types.
AW.00169
2002
Geography
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