During rising and high tide, the sea pushes back against the river, reducing its flow at the mouth. As the tide falls, approaching low tide, the flow at the river mouth increases, and the sediment laden river water is “released/floods” into the Strait of Georgia. The river water, especially during periods of faster moving flow, is laden with silt and fine sands. As the flow spreads out into SoG, slows, and even mixes with the salt water, the larger/heavier sediments are released and sink, mostly likely by flocculation and differential settling. Shown here are data from three different instruments mounted on the Delta Dynamics Laboratory (DDL), capturing the cascade of sediment laden water as it sinks through the water column. The top panel is the 200 kHz echo-sounder record from the ZAP (S/N 1009). The second panel is the vertical velocity as measured by the co-located 300 kHz ADCP, capturing the downward movement of water and sediment (blue region). The bottom panel is the pressure record from the co-located CTD, showing that the sediment waterfall occurs during the low tide. This signal has been occurring for the last few weeks, and may continue throughout the spring freshet. Sediment accumulation during the freshet can be as much as 1 m in locations, and after tidal resuspension and redistribution throughout the year, has an average of 10-20 cm over many square kilometers.
Download High-Resolution Image (PDF): Sediment fall at the mouth of the Fraser River
