Delmarva Peninsula - A century ago, nontidal freshwater wetlands and forests covered most of the Delmarva peninsula’s central regions.
Miles of ditches have altered Delmarva peninsula hydrology
Chesapeake Changes and Challenges / By Dr. Robert Magnien - Bay Journal
A century ago, nontidal freshwater wetlands and forests covered most of the Delmarva peninsula’s central regions. The groundwater in these regions was very close to the surface, saturating soils much of the year and supporting vegetation and animals that were adapted to this environment. As settlers arrived on the Delmarva peninsula, much of this land was drained and cleared of its natural vegetation so that the land could be used for agriculture. In its natural state, many of the soils were too saturated with water to grow crops and support machinery or man-made structures. This widespread drainage of land has led to significant changes, not only in the obvious loss of habitat, but also in the underlying hydrology (surface and subsurface movement of water) which controls such processes as nutrient transport to the Bay.
How was this land drained?
The concept is quite simple. Ditches were dug in a network at intervals that allowed groundwater to drain from the soils down to the depth of the ditches and then to travel via these man-made conduits into the natural stream channels. Even some of the natural stream beds were altered by straightening and/or deepening the channels. Groundwater was thereby lowered several feet, allowing surface soils to dry enough to allow the land to be used for such purposes as growing crops. (Tidal wetlands, which are regularly inundated by tidal waters connected to the Chesapeake Bay, were for the most part not drained by ditching because the land is near sea level.)
These agricultural drainage systems start with a network of ditches in fields that average 2 to 6 feet in width. These field ditches drain into larger Public Drainage Association ditches, which may be several yards wide. PDA ditches are designed to convey the equivalent of four inches of rain in a 24-hour period, and therefore help to convey stormwater from roadways and rural towns. To maintain their function, the ditches are managed to control woody brush growth and to maintain vegetative buffers, which reduce erosion. The ditches are “cleaned out” on an “as needed” basis, to remove sediment accumulations that are impeding the flow of water
Today, the network of ditches covers large areas of the Delmarva’s central region. According to information from the Maryland Department of Agriculture, there are approximately 814 miles of ditches maintained by the PDAs of Queen Anne’s, Caroline, Somerset, Wicomico and Worcester counties on the mid to lower Eastern Shore of Maryland. This mileage does not include field ditches. These ditches help to drain approximately 213,000 acres of land which now supports crops, forests, roads and homes. Most of the drainage originally done in Queen Anne’s, Kent and Dorchester counties has not been maintained and has reverted back to wetlands. Additional miles of ditches and acres of drained land exist in adjacent areas of Delaware.
What has been the significance of the large-scale conversion from a nontidal wetland ecosystem to a drained agricultural ecosystem?
As in any system where the land cover has changed substantially, native species of plants and animals have been altered. This is a common occurrence throughout the Bay watershed as land cover is modified for housing, roads, golf courses, agricultural fields and other uses. What is somewhat unique on the Delmarva, is that not only have the native plant species been replaced (often referred to as land cover change), but the hydrology, or “natural plumbing,” underlying these areas has also been changed in very significant ways. Because such significant changes have occurred both on the surface and below it, these are some of the most altered ecosystems in the Bay watershed.
A number of studies have been conducted in recent years that shed light on the effects of the hydrological changes and the implications for waterbodies into which these areas drain. One of the effects is the change in water flow from groundwater (relatively slow-flowing waters moving through the ground) to surface water (relatively rapidly flowing waters moving above the ground). In its natural state, much of the rainwater falling on the Delmarva percolated into the soils in these areas and then moved slowly through the ground, eventually emerging in natural stream channels. When the land was ditched, much of the rainwater still percolated into the soils but then emerged much sooner because the long flow paths through the ground to natural streams were intercepted by the ditches. Once in the ditches, the flow of this surface water to rivers was much more rapid as it was not impeded by having to move through the ground.
One of the concerns raised by this change in hydrology is that natural processes which occur in the soils, such as the retention and conversion of nutrients, may be short-circuited. For example, natural wetlands are well known for their ability to convert nitrogen from forms dissolved in water, which can damage the Bay, to an inert gas. (Most of our atmosphere is inert nitrogen gas.) Coupled with the high levels of nutrients applied to the land in many of these areas, the change in hydrology makes the challenge of reducing the flow of nutrients to adjacent tributaries and the Bay that much more difficult. Studies have shown that the water in these ditches and adjacent receiving waters contain high nutrient levels.
While it is impractical to consider the wholesale conversion of these drained wetlands back to their natural state, is there anything that can be done to mitigate some of the impacts these systems have in changing the flow of water and increasing the delivery of nutrients to the Chesapeake Bay?
One of the land management techniques that is being more widely implemented by farmers is the careful use of nutrients so that they are not applied in excess of crop needs (i.e. agricultural nutrient management). This has been particularly daunting for the Delmarva peninsula because of the large amounts of animal waste produced by the poultry industry and the excess of phosphorus relative to nitrogen that exists in this source of nutrients when compared to the needs of crops. Until recently, farmers in the Delmarva region applying animal waste to crops were following the recommendations of nutrient management plans that were written to satisfy crop demands for nitrogen only. While it was known that excess phosphorus was being applied under these circumstances, it was thought that the phosphorus would bind to the soil and not be able to run off the land in storms, as long as soil erosion was controlled. We now know that at very high levels, excess phosphorus in the soil can go into solution and move more easily into adjacent waterbodies. To remedy this situation, states in the region are moving toward nutrient management plans that address both phosphorus and nitrogen application rates.
Another way of addressing the excess phosphorus in animal waste is through the addition of an enzyme, known as phytase, to poultry feed. This enzyme allows more of the phosphorus in feed to be absorbed by the chickens, thereby reducing the amount of phosphorus in their waste. Some of the poultry producers on the Delmarva are starting to use this enzyme. Other promising methods of reducing the amounts of animal waste applied to land on the Delmarva include transporting it to areas that are in need of this fertilizer or burning it to generate heat or electricity.
In addition to addressing the issue of excess nutrient application, other programs are being developed to reclaim habitat, reduce erosion and modify water flows in a way that would allow for the continued use of previously drained areas while restoring some of the natural wetland functions that once existed. These programs are being developed with the participation of farmers, government agricultural and natural resource managers, PDA managers and citizen groups. Federal and state funding has recently been made available for these types of projects, thus increasing the opportunities to implement these restoration measures.
As in many areas of the Chesapeake Bay watershed, we are struggling on the Delmarva to mitigate the impacts that our land use practices have on water quality, habitat and ultimately, the Bay’s living resources. Fortunately, the scientific properties and impacts of this altered ecosystem are now being more fully understood. This is allowing the development and implementation of new practices and programs that should reduce the flow of nutrients and sediments off the land and, it is hoped, reclaim some of the lost wetlands and their important ecological functions. The survival of some of the Bay’s most vital habitats, for example the blue crab and speckled trout nurseries in adjacent waterbodies such as the Tangier Sound region, depend upon these efforts being successful.
Dr. Robert Magnien heads the Maryland Department of Natural Resources' Tidewater Ecosystem Assessment Division.
Miles of ditches have altered Delmarva peninsula hydrology
Article from Bay Journal April 1999
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