Wednesday, September 3, 2014

Four summer observations from the Elwha River Delta

As usual, the Elwha dam removal and restoration offered up some really interesting action in the nearshore zone during the summer of 2014. Here are four things that, for one reason or another, I found noteworthy.

Gravel Arrives

To be fair, gravel was probably there before this summer, but I didn't see a whole lot of it. But during a set of field surveys in April associated with a class I co-taught at the University of Washington's Friday Harbor Lab, I was struck by the size of gravels on the outermost part of the sediment deposit around the river mouth. Gravel has been of interest, and there have been questions regarding when gravel would arrive on the beach, in what quantities, and how it would distribute along the shoreline. In general gravel has been associated with reduced rates of erosion on shorelines, so at this beach which, before dam removal, was very erosive, it might be that the gravel load from the dam removal may help to stabilize the shoreline over longer time-scales.

So in reference to the map above, here are Photos 1-3:

The dot at Photo 4 in the map above references the location of the video at top, as well as the photo below showing Dr. Andrea Ogston and UW-Tacoma student Julia Dolan digging a pit through a mass of gravel on the outer bar just east of the river mouth:

Bar Evolution

Just to the east of the river mouth multiple bars have developed over the past year or more:

Annotated image of the Elwha River mouth from 6 June 2014 (courtesy of Andy Ritchie, Olympic National Park), with 3 bars labelled along the profile transect shown in the plot below.

And those bars have moved around quite a bit, perhaps most dramatically between July and August of this summer, when bars 2 and 3 effectively welded together:

Intertidal Sand Transport Around The Delta

One of the more interesting observations for me has been how, for the first year and a half after "new" sand first appeared at the river mouth, the beach on the eastern part of the delta has remained relatively coarse. This is shown in profile data from Line 190, at the very tip of the delta:

The sub-plot at lower right above shows a time-series of the position of the Mean High Water (MHW) contour, which shows that even through the arrival of substantial volumes of sediment at the river mouth starting in December 2012, Line 190 continued to erode while remaining relatively coarse. You can also see it in an oblique of the site, taken from about Mean Higher High Water:

Oblique photo along Line 190 taken on June 13, 2014. Here the lower intertidal is sandy, but the coarse upper beach is visible in far field

Between June and August, though, sand moved up on to the beach as seen in the profile plot above, and in the photo below from 12 August 2014:

Line 190 seems to be representative of most of the delta, and in late August I surveyed the entire delta to map sand, finding that there was more or less a continuous band of sand stretching in the intertidal all the way around to the east side of the delta (but not, apparently, extending east of the delta in the intertidal):

New Organisms Move In

Accounts of the ecological response to new sediment in the coastal environment are beginning to come in (for example here and here). In the shallow (less than about 20-25' depth in general) sub-tidal part of the delta east of the river mouth (the darker areas in the acoustic backscatter data in the map above) parts of the sea-floor that used to look like this:

Now look like this:

Our general impression is that species that we didn't see in the "old" type of habitat are being quick about moving in to the "new":

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