Buried silt on the Elwha sea floor

This week I was, once more, part of an annual subtidal survey of the marine community around the Elwha River delta during which we visited our site called, 4SP1, just to the east of the Elwha River mouth, roughly here. This is a place that was buried under 2-3 feet of sand in 2013, as the dam removal pumped out tons of sediment into the Strait of Juan de Fuca.  The site IS still very sandy, here is a bit of a view from this year:

and it does still look very different than it did before dam removal started (check it out in 2011 here).  But this year the site was also obviously different from last in that much of the deep mantle of sand deposited during dam removal had been stripped off.  So that is interesting in and of itself...we've been curious about how the Elwha River delta would evolve in the long-term now that dam removal is long over, and this gives us some indication.  But what really struck me was what was UNDER the remaining thin layer of sand on the seafloor...a few inch thick layer of silt and mud, which you can see in this photo below, with my slate for scale:

And get a sense for the composition in this video.  So what is the big deal?  This is a high current site, where mud and silt generally doesn't stick around.  I think this layer of mud and silt tells us something about how rapid and how massive the delivery of sediment was during the dam removals.  I envision a pulse of very fine material blanketing the sea floor, presumably followed almost immediately (perhaps within a tidal cycle?) by a pulse of sand, both pulses massive enough to blanket the seafloor with many inches of material...