Wednesday, March 13, 2013

Elwha Delta

With the lower dam completely gone and the upper dam nibbled down until only a much shorter version remains, the Elwha is releasing the sediment stored in the reservoirs for many decades.  Gravel from Lake Mills is moving its way down the river between the dams as a large wave, filling pools and braiding channels across the valley bottom.  Sand is moving through the system more quickly and is now appearing as large shoals at the delta – low tide and subtidal bars that are encroaching at the base of the cobble beaches on both sides of the river’s mouth.

A bulk of the material appears to be shifted eastward, a predictable consequence of the westerly swell.  Which is fascinating all by itself.  The waves wrap around the delta from the west, breaking along the shore at incredibly high angles, with big implications for both sediment and surfers.

Wood has also been flushed out of the reservoirs.  Wood of all sizes from small chunks of bark to large branches has been piled into broad debris fields at the river mouth, stacked in ridges by successive high tides, and then more recently eroded or washed away, spreading out in a thin band along the beach to the east.

The new sand has had little noticeable effect on the ongoing erosion of the gravel berms on the east side of the delta, which have been retreating rapidly for decades.  Maybe the new sediment will begin to slow this process, but will it do it by accreting new sand and gravel directly to the beach face, by dissipating wave energy on the broad sandy bars, or will the current beach face simply be abandoned as a new series of spits and lagoons forms farther offshore?

The removal of the dams is unleashing a complex chain of events, much as the construction of the dams did a century earlier. Some things will change rapidly – particularly those things that are moved readily by frequent processes, like sand moving through the lower river.  Some things will take much longer, particularly those that involve difficult to move material moved by infrequent processes – like new cobble berms forming on the delta.

The Elwha isn't just about emptying the reservoir and watching the exposed soils revegetate.  It isn't just about restoring a natural grade to a river that had been ponded behind two large dams.  The removal of the dams restores an incredibly complex system that extends far beyond the reservoirs.  Fish that spend most of their time in the open ocean will now be able to swim up into Olympic National Park, for the first time in a century, to lay their eggs in the upper watershed and give rise to new generations of Elwha River salmon.  Sand and gravel that has been trapped in reservoirs will now move downstream, rebuilding the delta and influencing the beaches all the way to Ediz Hook in Port Angeles.

I’m just an excited spectator out at the Elwha.  There are many folks much more actively involved and much more knowledgeable about the river and its beaches. Here are a few of them.

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