In nature, deep-sea sediment flows triggered by earthquakes or extreme storms can be important for creating oil reserves and other geologic deposits, as a component of the global carbon cycle, and responsible for burying communication cables.
Computer models and the geologic record suggest that when the sediment is in high-enough concentrations, it goes directly to the ocean floor. Instead of the fresh river water floating on top of the seawater, the river water becomes denser than the sea, and the sediment-laden river water plunges below the ocean water.
For the Elwha, that path would take much of the sediment away from the coastline and deep into the Strait of Juan de Fuca.
"A surface plume is very much at the whim of the winds and tides, whereas these underflows are just going down the steepest gradient," Ogston said. "These are two very different mechanisms that would create very different impacts to the seabed."
The dams initially powered a pulp mill and were built unusually close to the ocean the upper dam is just 13 miles from the river mouth. Their removal provides a unique opportunity to study large river discharges.
"There is an understanding of the general type of flow, and people have predicted that it occurs in rivers, but no one has seen the smoking gun yet," Nittrouer said. "This is a chance to document a 100-year storm. It's really somewhat new territory."
So far there have been dramatic changes to the seabed in the shallows, but few changes below about 20 feet, Ogston said.
Where the sediment ends up is of practical interest. Sediment can make the water murky, creating conditions that make it
|Contact: Hannah Hickey|
University of Washington