What is an ETM?
Strong tidal forces push salinity upriver beneath the outflowing river water. The turbulence caused by this tidal forcing results in resuspension of sediment and other particulate material present on the river bed. Concurrently, dissolved material in the river water flocculates when it comes into contact with the salt wedge pushing its way upriver. The combination of these two processes results in elevated levels of suspended particulate material: the estuarine turbidity maximum (ETM). Within the region of the ETM, material in the water column, and on the bed of the estuary, is trapped, resuspended and advected (see schematic). ETMs vary in strength and distance they move with the tides depending on the strength of the tide and the flow of the river (see energy and ETMs). We have observed that the Columbia River ETM seems to follow the leading edge of the salt wedge as it makes its way upstream as the tide is flooding, and then as it retreats during an ebb tide.
An animated video shows the progression past a stationary vessel of an ETM during tidal ebbing on 07 May 1997. The nine frames span 2 hours and 15 minutes with approximately 15 minutes between frames. The blue line is the optical backscatter (OBS) turbidity as measured by one of our sensors. A higher value indicates greater backscatter caused by more particulates in the water. Notice how there is a sharp peak in OBS associated with the flushing of salt from the estuary. Salinity is the red line. A measure of phytoplankton (AC-3 chlorophyll) is represented by the green line. View animated video now.
We have also observed that the strength of the ETM varies over a monthly cycle. When the moon is full or new, the tidal range is greater (spring tides) than during first or last quarters (neap tides). The following figure plots the suspended particulate material concentration (colored shading) and salinity (contour lines) over time during a spring tidal cycle (upper panel) and a neap tidal cycle (lower panel). Notice that the salinity is only completely flushed from the river during the greater ebb tide, and there is still saline water at the bottom following the lesser ebb. Also notice how the tidal range affects the strength of the ETM (darker shading indicates greater suspended material). View this figure.
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