This three-year-old
wetland was constructed at a field tile outlet along Bear Creek in central Iowa.Restoration of a 66- to 100-ft. wide riparian buffer strip on land that presently is cropped or heavily grazed can provide many of the same benefits provided by a natural riparian zone. A restored, or managed, riparian buffer strip system may be done in three ways, depending on the region and its location within a watershed. Landowners can use any or a combination of these three components to create a riparian buffer: 1) a multi-species buffer strip (the tree-shrub-grass component); 2) a streambank stabilization area (the vegetative bioengineering component), and 3) a small, constructed wetland at field tile outlets (the wetland component).
Multi-species buffer strip component
A minimum vegetative buffer strip of 66 ft. wide on both sides of the stream
is needed where row crops are grown. This width is needed to trap sediment and
provide enough time for water moving across the area and into the soil to interact
with the "living filter." Ideally, the strip should consist of four or five
rows of trees planted parallel and adjacent to the stream, then one or two rows
of shrubs, and a 20- to 24-ft. strip of native warm-season prairie grasses.
Cool-season grasses used in lawns and most pastures do not provide sufficient stems and roots to be effective. In grazed pastures where streambank stabilization is the major concern, the buffer strip can be 20 to 30 ft. wide and livestock should be kept out of the area. The strip can accommodate rotational grazing and controlled water access.
Streambank stabilization component
Live and dead plant material (trees, shrubs, and grasses), fiber matting, and
rock can protect bare streambanks that are easily eroded by the current. Collapsing
banks produce up to 50 percent of the sediment in streams. The goal of streambank
stabilization is to re-shape streambanks with permanent vegetation. It involves
placing live plant material, typically willow, into the stream bed and bank,
and anchoring bundles of dead trees, such as Eastern red cedar or small hardwoods,
as "revetments" at the toe of the bank in the water. A narrow band of large
rocks also can be used in place of the dead plant material. The bundled trees
absorb fast-flowing water, physically trap debris and soil, and allow willows
to be placed among the bundles to help anchor both into the bank.
Only the inside bends of severely eroded banks should be stabilized. Vertical banks more than six ft. high should be re-shaped with equipment, such as a backhoe, to produce a 2:1 slope before installing plant material.
Wetland component
If the adjacent field crop is drained by tile, a small wetland can be constructed
within the 66- to 100-ft. buffer strip where tiles enter the stream. The idea
is to use the natural water "purification" processes of a wetland. Basically,
the wetland works by providing annual organic matter from cattails for millions
of microbes to live. These microbes consume nitrogen and immobilize many of
the herbicides that enter the wetland with the tile water.
Based on wetland research at Iowa State University, the general rule for sizing a constructed wetland is one acre of wetland for every 100 acres of row crop drained by the tile line. To construct a wetland, excavate a small depression near the tile outlet and install a water outflow structure to control water retention time and level. Cattails and bulrushes are planted in the depression, surrounded by a berm that can be planted to prairie grasses and forbes.
Financial installation assistance
Restoration of riparian zones often qualify for government and non-government
cost-share assistance. In addition, many of these areas can qualify for reduced
property taxes. See your district forester or soil conservationist for further
information.
For more information on ways to restore riparian buffers, see Pm-1626b Stewards of Our Streams: Multi-Species Buffer Strip Design, Establishment and Maintenance