Construction of Small Wetlands
Many areas in Missouri could potentially be developed into productive wetland communities. If the site is suitable, with minimal construction a landowner can own a piece of this valuable but disappearing habitat without affecting existing farm operations. For technical assistance, check with the Conservation Department or the Natural Resource Conservation Service.
The most important considerations are the objective for the wetland and having an adequate water supply.
An adequate water supply is a very important consideration. The amount of water needed to flood the wetland will depend on the objective of the wetland, topography of site, preferred water depths and substrate soil types. The timing of water availability is also an important consideration in achieving the wetland objective. For example, to be successful for duck hunting, the site must be flooded during the duck season. To be successful as water treatment areas, wetlands need water available during the summer to allow for growth of wetland vegetation.
The water holding ability of the soil is another consideration for a wetland site. Most bottomland soils are suitable for wetland development, but if there is any doubt, consult a soil scientist or take soil tests Sandy soils or soils with sandy inclusions are sometimes found in floodplains. These soil types may prevent wetland management if the permeability of the soil is extremely high. Sites with sandy inclusions may require specific engineering or construction techniques to ensure their water holding capability.
The topography of the site should be as flat as possible to get the most surface area from the water used. A 1 percent slope (a 1 foot rise for each 100 feet in horizontal distance) or less is preferred.
The objective for the wetland will dictate the desired water depth. For example most species of wetland wildlife prefer water depths 10 inches or less for feeding. This is particularly important for wading and shorebirds, If the primary interest is habitat for dabbling ducks such as mallards, pintail and teal, water depths should be 18 inches or less over 75 percent of the pool area. Diving ducks such as scaup, ring-necked ducks and buffleheads, prefer deeper water. Wetlands developed for water quality improvement should be shallow enough, generally less than 12 inches, to allow growth of thick stands of wetland vegetation.
Wetland sites along streams and rivers can be managed to provide spawning and fish rearing areas. Water control structures should be installed that will allow fish movement, and water levels should be maintained throughout the summer months to allow time for young fish to grow. This type wetland should provide some deeper water to moderate high water temperatures that can be prevalent in the summer months. The wetland can then be drained in the fall to allow fish back into the adjacent stream. These wetlands also provide excellent mud-flat habitat in the fall for shorebirds and waders, such as herons and egrets.
Also consider the accessibility of the area, not only for people, but for construction and maintenance needs. Access to the site is necessary for earth moving equipment during construction. And later, farm equipment will be necessary for managing and maintaining the developed wetland.
Development
Water for a wetland can be pumped from a stream or well.
Once an appropriate site is selected, the next step is proper wetland development. Most marsh sites need only low-profile levees for flooding of shallow water areas. Generally, levees should have 4-to-1 slopes (or flatter) on the front and back slopes, depending on the marsh's location. Levees that are subjected to occasional flood water should be less steep with a minimum of a 6 to 1 slope. Flatter slopes will also help minimize muskrat damage to levees if the wetland is flooded throughout the summer months Levee heights will depend on the depth of water and the topography of the site, but at least 18 inches of freeboard (height of levee above water level) is necessary so the levee is protected from wave action and water-logging. The minimum top width of the levee should be four feet - 12 feet if used as a road. For vehicular access, the levee should be compacted sufficiently to prevent rutting, which will create a future maintenance expense. Emergency spillways built into the levee are important if periodic flooding occurs or if there is a large drainage area above the wetland. To prevent erosion of levee slopes, levees should be seeded immediately after construction is completed. Switchgrass is an excellent grass for this purpose. When construction is finished, maintain levees by mowing or burning to prevent growth of woody plants. Levees should also be checked annually for muskrat, groundhog or beaver damage.
The area from which soil is taken to build a levee, called a borrow area, can be located inside or outside the flooded area, depending on your intended use and the soil type. Locating the borrow area inside the levee can deepen the wetland, provide a diversity of water depths and vegetation and provide interior drainage. However, borrow areas require a lot of water to re-flood so the water supply should be sized accordingly. Borrow areas outside the wetland area will mean less pumping and provide more uniform water depths within the wetland. These borrow areas outside the levee can also be important for amphibians and other wetland wildlife as breeding and nesting areas.
A variety of water control structures are available for managing water levels in wetlands. The objective for the wetland is also an important consideration in determining the type of control structure. Regardless of the type used, it should have the capability to hold water and also have the ability to provide drainage of the wetland. Wetlands are dependent on fluctuating water levels to maintain their productivity. Wetlands constructed without the ability to draw the water down become unproductive and are often difficult to manage. Listed below are several types of control structures along with a few examples for developing small wetlands.
To avoid a violation of wetland protection laws, you may need a permit from the Corps of Engineers and permission from the Natural Resource Conservation Service before beginning construction. Your local Corps of Engineers and NRCS offices should be contacted to determine if these provisions apply to the site you have selected for wetland development. For further information see Wetlands and the Law.
Wetland Restoration in Floodplains
Most wetland sites occur in floodplains of Missouri streams and rivers. Flat land capable of holding water can generally be developed easily into a manageable wetland. These wetlands can provide a number of uses, from crawfish production to waterfowl hunting. Following are considerations for development of wetlands in a floodplain:
- Water for the wetland can usually be provided by overflow from adjacent streams, runoff from adjacent uplands or by pumping from the stream or well.
- The levee should be constructed to restrict water from draining quickly.
- A water control structure should be installed to allow efficient drainage and flooding. This is very important in flood plain wetlands to allow proper vegetation management.
- The levee slope should be a minimum of 4:1 up to 10:1, if the levee will be subject to overtopping by flood waters. The levee top width should be wide enough to allow equipment travel for levee maintenance purposes.
- The land used to make this type wetland need not be taken out of crop production. Fields flooded after completion of harvest provide attractive habitats, and waste grain can be an important food source for migratory waterfowl during the winter.
A Wetland Below a Pond
A stop-log gate and screwate valve can be used to control water flow for a wetland situated below a pond. The Natural Resource conservation Service and the Conservation Department can provide help in building these structures.
A low-profile levee can be used to hold water for a pond-fed wetland.
Ponds or lakes make excellent water sources for wetlands located in flat areas below the dam. However, the following considerations should be taken into account:
- A drain pipe should be installed in the pond dam to provide gravity-flow water for the wetland. A simple screw-gate structure should be installed on the drain pipe to allow opening and closing. The size of the drain pipe depends on the size of the wetland and the length of time available to complete flooding of the wetland.
- An existing pond without a drain pipe can provide wetland flooding by use of siphon pipes or pumping.
- Water in ponds should be maintained at least 8 feet deep throughout the winter to insure fish survival.
- The emergency spillway of the pond should bypass the wetland. If not, the wetland should have an emergency spillway of its own, capable of discharging the pond's overflow.
- The pond need not provide all the water necessary to flood the wetland if other water sources are available. If the pond is the only water source, the wetland should not be larger than the amount of water the pond can supply. Gravity flowed water from the pond or lake can supplement other water sources to reduce pumping costs or to irrigate wetland vegetation during the summer.
- The wetland levee should have 4:1 slopes and have a minimum top width of 4 feet. A simple stop-log type water control structure will provide effective water level control and allow drainage of the wetland.
