TIDE-INDUCED GRAVITY FLOWS ON THE FRASER RIVER DELTA, BC, CANADA

Real-time physical and chemical oceanic properties of the upper Fraser River delta front exhibit episodic, rapid decreases in salinity accompanied by an increase in water temperature and turbidity. In 2008, three of these events were recorded on the delta front and are ascribed to tide-supported gravity flows. These flows coincide with the Fraser River’s freshet (i.e., snowmelt induced high flow conditions) and spring ebb tides, and are characterized by deposition of fine-grained sand and silt beds. Observations show that the suspended load in the river settles in the vicinity of the river mouth during ebb tides and forms a bed with high water content. During slack tides, these sediments produce very high near bed suspended sediment concentrations that reach levels of excess density to generate offshore-directed hyper-concentrated flows. The effects of these flows can last for up to 10 hours in the Fraser River Delta and are confined to the upper (< 100 m) delta front. Such gravity flows are important because they can initiate slope failure, transport significant amount of coarse-grained material into the basin and impart significant environmental stresses on burrowing infauna (e.g., high sedimentation rate, decreased salinity, and increased temperature). They are likely to occur on the delta fronts of other large tide-dominated and tide-influenced deltaic systems. In shallow-water deltaic systems (< 100 m water depth), these tide-supported gravity flows can affect sediment deposition on both the delta front and prodelta.