Monday, March 8, 2010

Landforms of the Chesapeake - Freshwater Salt Marsh Ponds

Existing historic freshwater pond in the salt marsh near Ocean City, Maryland
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So I've been looking forward to writing about a very special natural resource on the coast that's suffered all types of indignities in the past, and is currently receiving some attention (and some band-aids) from those of us in the habitat business. Salt marshes exist where brackish estuaries, dry (eroding) land, and very salty water come together. Salt marshes can stretch for hundreds of miles, get flooded regularly, and are only inhabitated by the most salt-tolerant plants and animals on earth. In the salt marsh, salt is not only in the water, but in the soil, in the air, and covering everything above the ground, living or dead. Within the salt marsh, water is not a limiting resource, but fresh water certainly is.
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Scattered throughout salt marshes are features called "fresh ponds." These fresh ponds are "perched" wetlands, and unlike the rest of the marsh, whose hydrology is driven by the rising and falling tides (and salt water), these depressions (rarely more than 14" deep) are filled with water that is brackish to fresh - primarily rainwater. These shallow ponds are very important to wildlife, particularly insects, birds, and bats, as a source of fresh water in an otherwise extremely salty environment.
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You'll note that I mentioned "insects." Yet, 90% of the mosquito species that bite humans do not live anywhere in the salt marsh. In a healthy marsh, mosquitos are a favorite food item of birds and fish. A healthy salt marsh is not a great place to be a mosquito! However, starting in the early 1800s, a misconception grew among Americans that salt marshes - particularly "fresh ponds" in salt marshes, harbored disease and mosquitos (200 years later, we all know that the most dangerous place to encounter a mosquito is in a tire dump or your back yard....not a marsh).
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Efforts were undertaken at different scales to "drain" the marshes of the Northeastern United States, under the guise of public health. The most significant period of marsh draining (primarily through ditching) was under the Civilian Conservation Corps from 1933 to 1942.


CCC ditch digging crew, courtesy of the US Bureau of Land Management
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Arguably, ridding the Northeast of mosquitos, typhus, malaria, and the Plague was a great reason to put unemployed Americans back to work. Unfortunately, they were looking in the wrong place - the mosquitos happily continued to lay their eggs in the ditches, perhaps in even greater numbers than they had in the fresh ponds!
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And it's easy for us to judge - 60 years later. The CCC kept thousands of Americans employed during the Great Depression, and that's nothing to scoff at. And back to the story - ultimately, the CCC ditched hundreds of thousands of acres of marsh in the Northeastern United States. Here's what a salt marsh, fresh pond, and beach system in Delaware looks like 60 years after ditching:

Fresh ponds were ditched - allowing the fresh water to drain out, and the salt water to fill in during high tides
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As the decades have continued, biologists have started to understand the value of fresh ponds in the salt marsh, and the relative ease with which the fresh ponds can be restored. Engineers have been hesitant, since these ditched marshes (see below) show absolutely no sign of grade change - it's hard to understand what would be flooded or drained by taking these ditches "out of service."

Salt marsh ditch on Maryland's eastern shore - wide enough to run two boats through, side by side
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A debate has been underway for more than 30 years about how to remedy this damaged habitat. Regulatory agencies like the Army Corps of Engineers absolutely forbade, for many years, any type of "filling" in the salt marsh for any reason (fearing to set a precedent that could be used by developers elsewhere). Bringing in soil from somewhere else doesn't solve that problem, and would be expensive. Mosquito experts developed a system called "Open Marsh Water Management (OMWM)" that provides for mud to be moved around into a system of small ponds that hold fish that theoretically eat mosquito larvae. Experts agree that this very expensive habitat enhancement technique removes mosquitos (and provides some habitat value it provides for other wildlife, like waterfowl), OMWM's system of mimicking the historic fresh pond and tidal pond habitats has found favor in most states in the Northeast.
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But if we can all agree that it makes sense to create new tidal ponds and fresh ponds (where there were none before) to control mosquitos, wouldn't it make sense to also restore the fresh ponds that used to exist, but now have ditches running through them?
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That's what several agencies in Maryland are now pursuing in earnest.

Vinyl, timber, and mud "crib" installed to stop the ditch from draining freshwater, or bringing in saltwater. Eastern shore of Maryland - project by Maryland DNR and Ducks Unlimited.


Vinyl and mud "ditch plug" by Maryland DNR and Maryland State Highway Administration. Outside Ocean City, Maryland.

Several different approaches are being tried. The key goals are:
  1. allow freshwater to stay in the fresh pond so that plants and wildlife respond
  2. construction method should be a long-term fix
  3. construction method should be as cheap as possible, while achieving #2
  4. construction method should be as minimally invasive as possible, while achieving #2

    There are now a half-dozen or more agencies and non-profit organizations now involved in this work in Maryland, and each of them have additional goals that they'd "like" to see these restored fresh ponds achieve. Data has been collected for the last 2 years, and hopefully the results will show that we've found a few ways to restore this important part of the Chesapeake's natural heritage.
Restored fresh pond near Chincoteague, Virginia; project completed by Maryland DNR.

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