George Pess, river geomorphology

George Pess, NOAA

The size and timing of the sediment behind the reservoirs released influences where sediment accumulates in the Elwha.

How will that alter the fish habitat in the river system?

In short term – lots of suspended of sediment and deposits of sand and gravel. We may see the gravel beds increase over time.

Long term – transported coarse sediment and wood

How long will it take to affect the Elwha River habitat? Today, there are daily changes with suspended sediment. In months and years will see sand deposits in pools and slower water habitats. In the long term, over decades, coarser sediment deposits are expected.

Measurements include how much sediment is concentrated in the water as well as how murky the water is.

Graphs show sediment levels are lowering quickly.

500,000 tons of sediment have exited the project, 130,000 tons of new sediment have moved into Lake Mills.

Fine sediment in the Lower Elwha River:

silt and sand along the channels have been observed. Anyone who walks the river knows its a cobblebed river, but the sand and silt has been settling in between the cobblestones.

Silt and sand are also settling in the river’s estuaries at the mouth.

A meter of fine sediment has been accumulating in the floodplain channels. Sediment isn’t just accumulating in channel but also in gravel bars but not as much in the floodplains.

Sand is also depositing on gravel bars.

What’s going on in the places where we can’t see such as the river where its too turbid? NPS has been doing side sonar, where they can find small dunes being created in the river.

What’s going on in middle Elwha – between Elwha and Glines Canyon. Turbidty levels are less than what’s going on down in the lower river but still seeing sediment moving through river system. They are looking at sediment and where it’s going in the floodplain pools. No major changes in middle river area.

Silt and clays are being transported to the Strait. Sand is being deposited at a slower rate. Haven’t had any major floods yet though, so things will change if we get those this winter.



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