Chesapeake Quarterly
Not All Sediment Is Created Equal

THE WORD "SEDIMENT" conjures up different things, especially in a muddy estuary like the Chesapeake. There's sand, silt, clay. There's organic matter. According to sediment expert Larry Sanford, most geologists focus primarily on "what's on the bottom," but more and more he's turning his attention toward suspended sediment — sediment so fine or so light that it drifts through the Bay and settles slowly.

The simplest way to categorize sediment is by grain size. Sand is biggest, silt next, then clay. But these grains can also have different properties. Clay is stickier and more plastic than silt or sand. Even finer than clay are so-called colloidal particles, which are less than one micrometer — that's one millionth of a meter, the size of a speck of dust. Colloidal particles are so small they tend to stay suspended in a liquid.

Here is a quick primer on different kinds of sediment in the Chesapeake.

sand - photo by Sandy Rodgers

Sand. Credit: Sandy Rodgers.

silt - photo by University of Southern California

Silt. Credit: University of Southern California.

clay - photo by Siim Sepp

Clay. Credit: Siim Sepp.

Sand: Most of the sediment coming into the Bay is sand, and most sand is from the sea. Sand from the sea enters at the Bay mouth and washes up the estuary. Scientists using colorful pellets to track sand movement have followed them as far up the Bay as Tangier Island. The next biggest pulse of sand is from the head of the Bay, as sand particles wash down the Susquehanna. Much of that sand ends up behind Conowingo Dam or on the Susquehanna Flats, a kind of sand delta. Sand is good habitat for many of the underwater grasses that populate the Bay. Because of its relatively large grain size, sand sinks quickly and doesn't stay suspended for long in the water.

Silt: Washing down rivers and forming plumes during rainstorms, silt is finer than sand and will stay suspended longer. The upper and middle Bay see a lot of silt, which is good for marshes — building them up and keeping them one step ahead of sea level rise — but bad for water clarity.

Clay: Layers of clay, exposed by construction for example, also erode and wash downstream. Generally smaller than silt, clay particles can stay suspended for long periods. If the grains are small enough, they qualify as "colloidal." Clay particles are sticky, so they tend to attach to each other to form flocs and cohesive bottom sediments. Combinations of clay, silt, and organic matter are commonly referred to as mud.

Of course sand, silt, and clay not only wash downstream, they also collect in streambeds, until flushed out by storms. And they can crumble off eroding riverbanks. This erosion is especially important in the Bay, where sea level rise pushes tidal waters farther into the fields and forests of the coastal plain — a process underway since the glaciers began melting some 15,000 years ago, and one that appears to be speeding up with global warming.

And then there's organic matter. In lakes and streams scientists often study organic matter, including the leftovers of leaf litter and other woody debris.

In the Chesapeake, we may be taking organic matter to new levels. In addition to all the plant material (detritus) that normally flows into the Bay, a steady flow of nutrients like nitrogen and phosphorus have led to large algal blooms and to other kinds of productivity, from microbes to macroalgae. All this productivity has led to a rich organic soup, with the remains of broken cells, pieces of jellyfish, and all manner of organic material.

Research by Charles Gallegos and others (see Shadow on the Chesapeake) suggests that all this organic material may be combining with very fine sediment to create a worsening turbidity in the Bay.

September 2008
vol. 7, no. 3
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