Many of the wetland areas in the United States
have suffered some form of alteration from their natural state. The drainage
of wetland areas for agriculture, forestry, development, and other uses
was a common practice for well over 100 years. It has been estimated that
of the 89 million ha (221 million acres) of wetlands originally in the
conterminous United States, only 53 percent remained by the 1980's (Dahl,
1990). Wetland losses continue despite Federal legislation aimed at achieving
"no net loss" of wetland areas in the conterminous United States. It is
further estimated that between the years 1985 and 1995, despite wetland
creation and restoration activities, a net loss of 46,800 ha (117,000 acres)
of wetlands occurred in the United States (USFWS, 1997). However,
this rate is 60 percent less than the rate of loss experienced during the
1970's and 1980's (USFWS, 1997).
Large wetland losses have also been documented
in North Carolina. Wilson (1962) estimated that the coastal plain counties
of North Carolina contained approximately 95% of the state's 2.4 million
ha of wetlands. Cashin et al. (1992) estimated that approximately 51 percent
of the original wetlands in the North Carolina coastal plain had been altered
in some way, based on a study of 27 randomly selected study sites. The
primary causes of wetland alteration on the North Carolina coastal plain
are given in Table 1. In Beaufort and Craven counties, where the two sites
for this research are located, Moorhead (1992) compared the total area
of hydric soils with the total area of wetlands delineated by the National
Wetlands Inventory and estimated that 70 percent and 43 percent, respectively,
of the original wetland areas had been altered. In these counties, alteration
included drainage and conversion to agriculture or silviculture.
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Typical Land Drainage Practices
Typical practices for conversion of wetlands
to agriculture included extensive drainage and site preparation (Lilly,
1981). Wetland alteration consisted of first providing sufficient drainage
to allow for trafficable conditions for native timber harvest. Narrow tracts
of timber were harvested and special machines were used to install drainage
canals, typically to a depth of 1 m or deeper and spaced 500 to 1000 meters
apart. These canals created an outlet for water ponded on the soil surface
to be discharged and flow off-site. After allowing the areas to dry and
trafficable conditions to develop, the remaining native timber was harvested.
For silvicultural practices, debris left behind from harvesting practices
was left on the soil surface and typically the land was bedded and replanted.
For agricultural conversion, all debris was
removed from the soil surface. If additional drainage was necessary, parallel
open drainage ditches and/or subsurface drain tiles were installed from
60 to 120 cm deep at regular intervals across the field and connected to
the perimeter network of drainage canals. Often the land was then graded
to produce a "crown" in the center of the fields between adjacent drainage
ditches so that the center of the field was slightly higher (approximately
10 - 15 cm) than the edges. Rainwater ponded on the surface would flow
down-slope to the drainage ditches which minimized ponding on the surface.
The fields could then be used for agricultural production.
Description
of the Beaufort County Site
Two prior converted wetland sites located in the
Tidewater Region of eastern North Carolina were chosen for restoration.
One site is located on the Pamlico Plain within Beaufort County, North
Carolina near the town of Aurora. The site is owned by the PCS Phosphate
Company and was chosen for restoration to fulfill wetland mitigation requirements.
Although the site is not part of the WRP, it was selected for this research
because it is typical of the type lands that are included in the WRP. The
10 ha site contains four field ditches that were used to drain the site
for agriculture. The ditches are spaced on an interval of approximately
68 m, at an average depth of 1 m. The site was cropped with corn and soybeans
for a period of years prior to restoration, and is surrounded on three
sides by forested land, with the fourth side adjacent to fields currently
in agricultural production (Fig. 1). The predominant soil series on the
site, determined from inspection of the soil profile, is Roanoke (clayey,
mixed, thermic Typic Ochraquult), characterized by sandy loam textured
sediments in the upper 40 cm. Over much of the site, a shallow plow pan
exists at a depth of approximately 20 cm. The intermediate layers of the
soil profile, approximately 40 to 80 cm, consist of sandy clay loam textured
sediments. From 80 to 230 centimeters, the site is underlain by sandy loam
sediments atop a marine clay layer, which is assumed to be the aquitard
for the shallow water table at the site. The Roanoke series is classified
as a hydric soil (USDA, 1987). According to the county soil survey, these
soils form on stream terraces, and high water table conditions (i.e. water
table less than 30 cm deep) are apparent from December through March.
Description of the Craven County Site
The second site is located on the Talbot Plain within Craven County, North Carolina near the town of Vanceboro, approximately 50 km southwest of the Beaufort County site. Like the Beaufort site, the 18 ha Craven County site contains four field ditches which were installed in the mid 1950's for the purpose of agricultural production. Ditches at the Craven site are spaced on an interval of approximately 85 m, at an average depth of 0.75 m. The site was cropped primarily with a corn and soybean rotation until 1993 when it was put into the WRP by the landowner. The site is surrounded on all sides by forested land (Fig. 2). The predominant soil series on the site, determined from inspection of the soil profile, is Leaf (clayey, mixed, thermic Typic Albaquults). These soils are characterized by a higher clay content throughout the soil profile than the Beaufort County site. The top soil layer, to approximately 25 cm, is comprised of silty loam textured sediments. Below 25 cm, there is a significant textural change to a clay with strong to moderate prismatic structure to a depth of approximately 120 cm. From approximately 120 cm to 250 cm, texture changes to a sandy loam, increasing in sand content with depth. At a depth of approximately 250 cm, clay loam textured sediments are reached which are assumed to be the aquitard for the shallow water table aquifer at the site. These hydric soils (USDA, 1987) form on uplands, and high water table conditions (i.e. water table depth of 15 to 45 cm deep) are apparent from January through April, according to the county soil survey.