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TitleSedimentation across the tidal-fluvial transition in the Lower Fraser River, Canada
AuthorDashtgard, S E; Venditti, J G; Hill, P R; Sisulak, C F; Johnson, S M; La Croix, A
SourceThe Sedimentary Record vol. 10, no. 4, 2012 p. 4-9,
Alt SeriesEarth Sciences Sector, Contribution Series 20120301
File formatpdf
ProvinceBritish Columbia
NTS92G/01; 92G/02; 92G/03; 92H/04; 92H/05
AreaFraser River; Fraser River Delta
Lat/Long WENS-123.5000 -121.5000 49.5000 49.0000
Subjectshydrogeology; sedimentology; deltas; deltaic sediments; deltaic deposits; tidal deltas; fluvial systems; fluvial transport; fluvial studies; sedimentation; sedimentation dynamics
Illustrationslocation maps; profiles; stratigraphic columns; plots
ProgramMarine Geohazards, Public Safety Geoscience
AbstractThe Fraser River is the largest undammed river on the west coast of North America. In its lower reaches, a saltwater wedge intrudes up to 30 km inland during mixed semi-diurnal tidal cycles that range up to 5.3 m in height. Sediments deposited in the lower reaches of the Fraser River show distinctive characteristics that reflect the relative control of river versus tidal processes, as well as the persistence of saline water at each point along the channel. Grain-size trends along the river are controlled by the hydrodynamics in each distributary. Mud deposition is concentrated in the zone of saltwater-freshwater mixing. Coarse-grained sand and mud/fine-grained sand deposition is largely seasonally controlled, wherein bed material (diameter > 0.177 mm) is deposited during the waning freshet, and washload transported mud and fine-grained sand (< 0.177 mm) is deposited during the late-stage waning freshet flow and during base flow. The diversity and density of bioturbation changes according to the volume and residence time of brackish water at the bed. Higher salinity water and greater durations that saline water is sustained at any locale, supports a more diverse and uniformly distributed trace assemblage. With decreasing salinity, the trace assemblage decreases in diversity and bioturbation becomes more sporadically distributed. This results in a reduction of infaunal diversity from 100% on the nearly fully marine tidal flats in the abandoned part of the lower delta plain, to 14% in intertidal sediments of the brackish-water reach. The character of the sediments deposited across the tidal-fluvial transition provide criteria for differentiating sediments deposited within freshwater-tidal reaches, brackish-water-tidal reaches, and mixed tidal-fluvial distributaries. These data are presented as a process-based analog for tidal-fluvial sediments preserved in the rock record. The results can be used to predict changes in facies character across the tidal-fluvial transition of similar tide-influenced, river-dominated systems or their rock-record equivalents.