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Restoration of Riparian Zone Conditions

Cropland Remediation

The United States Department of Agriculture (USDA) Forest Service (FS) and the USDA Natural Resource Conservation Service (NRCS) have developed guidelines for riparian forest buffers (Welsch 1991). These buffers have three distinct zones. Zone 1 is a 5 m wide strip of undisturbed mature trees that begins at the edge of the streambank and provides the final buffer for materials moving through the buffer strip and directly influences the in-stream ecosystem by providing shade and large and small organic matter inputs. Zone 2 is a 20 m wide zone of trees managed to provide maximum infiltration of surface runoff, and nutrient uptake and storage while also providing organic matter for microbial processing of agrichemicals. Zone 3 is a 6 m wide zone of grazed or ungrazed grass which filters sediment from sheet flow generated in the uplands and causes large quantities of water and agrichemicals to infiltrate into the biologically active rooting zone where nutrient uptake and microbial processing occur. The FS and NRCS guidelines were developed after extensive reviews of forested riparian zones in the eastern United States. However, the guidelines and model may not be well suited to the agroecosystems of the Midwest and Great Plains where many smaller order streams drain highly modified (prairie) agricultural and grazed landscapes with few trees.

Figure 1: The Leopold Center for Sustainable Agriculture, Agroecology Issue Team, Multispecies Riparian Buffer Strip (MSRBS) Model. This model can be varied depending on site conditions, land-use practices and owner objectives.

Multi-Species Riparian Buffer Strip

The Agroecology Issue Team (AIT) of the Leopold Center for Sustainable Agriculture located in Ames, Iowa and the Iowa State University Agroforestry Research Team (IStART) have developed multi-species riparian buffer strip (MSRBS) system for application in the Midwestern and Great Plains agroecosystem (Schultz et al. 1993, 1995). The MSRBS contains three zones similar to those of the FS and NRCS riparian forest buffer strip model. However, the widths and plant species compositions of the zones in the MSRBS model can be varied depending on landowner objectives, the upland land use patterns and the characteristics of the riparian zone. The MSRBS system is an integrated management system which also includes willow-post soil bioengineering features to stabilize streambanks and small, constructed wetlands, within the buffer strip. The wetlands are placed at the outlet of field drainage tiles to process agrichemicals contained in tile flow before it enters the stream. Figure 1 illustrates the three zone MSRBS model while Figure 2 illustrates the whole MSRBS system.

Beginning at the streambank edge, the first zone of the MSRBS is 10 m wide and contains 4-5 rows of rapidly growing trees, the second zone is 4 m wide and contains 1-2 rows of shrubs, and the third zone is a 7 m wide zone of native, warm-season grasses. This zonation is important because the trees and shrubs provide perennial root systems and long-term nutrient storage close to the stream, while the shrubs add more woody stems near the ground to slow flood flows and provide a more diversified wildlife habitat. The native grasses provide the high density of stems needed to dissipate the energy of surface runoff and the deep and dense annual root systems needed to increase soil infiltration capacities and provide organic matter for large microbial populations.

Fast-growing trees are needed to develop a functioning MSRBS in the shortest possible time. It is especially important that rows 1-3 (the first row is the closest to the streambank edge) in the tree zone (zone 1) include fast-growing, riparian species such as willow (Salix spp) and cottonwood (Populus spp) species. If, throughout the year, the rooting zone along the streambank is more than 1.2 m above normal stream flow and soils are well drained, then upland deciduous and coniferous trees and shrub species can be planted in rows 4 and 5. Although these slower growing species will not begin to function as nutrient sinks as quickly as faster growing species, they will provide a higher quality product to the landowner at harvest. Shrubs are included in the design because their permanent roots help maintain soil stability, their multiple stems help slow flood flows and their presence adds biodiversity and wildlife habitat. Many native shrubs can be used and are often selected because of their desirable wildlife and aesthetic values.

Figure 2: The Leopold Center for Sustainable Agriculture, Agroecology Issue Team, Multispecies Riparian Buffer Strip Model System which integrates the willow-post soil bioengineering system for streambank stabilization and constructed wetlands for reducing non-point source pollution in agricultural drainage tile flow.

As in the FS and NRCS forest buffer strip model, the native grasses function to intercept and dissipate the energy of surface runoff, trap sediment and agricultural chemicals in the surface runoff, and improve soil quality by increasing infiltration capacity and microbial activity as a result of their annually high turnover of roots. Native tall-prairie grasses are better suited to the MSRBS than the introduced cool season grasses that are usually used for grassed waterways because of their taller and stiffer stems and their more deeply distributed roots. The native grasses have 9 times greater root mass extending more than three times as deep as cool season grasses (Schultz et al. 1994, 1995). A minimum grass zone width of 7 m is recommended to dissipate the surface runoff, trap sediment, and promote significant infiltration.

The three zone MSRBS model of trees, shrubs, and prairie grasses is well suited to the agro-ecosystems of the Midwest and eastern Great Plains. Although these species combinations provide a very effective riparian buffer strip plant community, there are other combinations that can be effective. These might include combinations with more trees or shrubs or without any trees or shrubs, except for those used for streambank stabilization. The grass zone is the most critical of the three zones in the MSRBS. Site conditions, major buffer strip biological and physical functions, owner objectives, and cost-share program requirements should be considered in specifying species combinations.

Figure 3: The Bear Creek MSRBS site near Roland, Iowa in March 1990. The land on the right hand side of the stream had been in cultivation and the land on the left hand side had been grazed. Notice the condition of the streambanks.

Figures 3 and 4 show the dramatic changes that can take place in as little as four growing seasons after establishment of a MSRBS system located on the Risdal farm, along Bear Creek, near Roland, Iowa. This buffer strip has trapped 80-90 percent of the sediment carried in surface runoff and has reduced nitrate and atrazine agrichemical pollutants moving through the soil solution of the rooting zone or in the shallow ground water by over 90 percent, with resulting concentrations well below the maximum contaminant levels allowed by the US Environmental Protection Agency.

It costs about $875 per ha to install the three zone MSRBS. This includes plant purchases, site preparation, planting, labor, and maintenance costs in the first year. About $50 per ha should be planned for annual maintenance for the first 3-4 years.

Figure 4:The Bear Creek MSRBS site near Roland, Iowa in June 1994. Notice the rapid growth of riparian vrgetation and the dramatic improvement in the condition of the streambanks after only five seasons since establishment of the MSRBS.

Although the MSRBS model was developed to be 21 m wide on each side of a stream, different widths may be needed to fit specific sites and land ownerships. The total width of the buffer strip depends in large part on its major functions and the slope and use of the adjacent land. If the major purpose of the buffer strip is sediment removal from surface runoff, a width of 15 m may be sufficient on slopes of 0-5%. If excess nutrient removal also is an important function, a width of 15-30 m might be necessary depending on the kind and quantity of agricultural chemicals applied and the soil and cultivation system used.

As the slope, intensity of land use, or total area of the land producing NPS pollutants increases, or as soil permeability decreases, a wider MSRBS is required. Castelle et al. (1994) recommend buffer strips 10-60 m wide for sediment removal, 5-90 m wide for nutrient removal, 5-100 m wide for species diversity and 15-30 m wide for stream water temperature moderation. Welsch (1991) summarizes the work of others and suggests that buffer strip widths could be 20% of the total NPS pollutant area, or widths of land capability classes I, II, V = 29 m; III & IV = 36 m; VI & VII = 52 m. The FS riparian forest buffer model has a width of at least 29 m.

MSRBS Streambank Bioengineering and Tile Wetland Options

Streambanks that have been heavily grazed or that have had row crops planted to the edge of the bank are often very unstable and need extra protection beyond that provided by the MSRBS. In these situations soil bioengineering techniques, such as the willow post method, can be employed (Frazee and Roseboom 1993). On vertical or actively cutting streambanks, combinations of dormant willow 'posts' are planted along with anchored dead tree revetments to protect streambanks. These plant materials provide a frictional surface for absorbing stream energy, trapping sediment, and provide shade and organic matter for in-stream biota. Dormant willow posts (> 7.6 cm diameter and 2.1 m long), willow stakes (2.5-7.6 cm diameter and 0.5-1.8 m long) and willow cuttings (0.5-2.5 cm diameter and 30-45 cm long) are collected during the winter or very early spring. Rows of posts are driven into the streambank beginning at the waters' edge with spacing between posts of 90-120 cm. Up to 4-5 more rows of posts, stakes, or cuttings are planted in parallel rows up the bank from the base row using 60-120 cm spacing within and between rows.

Where there is a concern for active undercutting of the bank, bundles of eastern red cedar or small hardwoods (3-4.5 m long silver maples, willows, etc.) can be tied together into 2-4 tree bundles. A row of these bundles is laid along the bottom most row of willow posts with the lower trunks pointed upstream and the bundles anchored to the willow posts or streambank.

In areas of artificial drainage, small wetlands can be constructed at the end of field tiles to interrupt and process NPS pollutants before they enter water bodies. A 0.5-1 m deep depression is constructed at the ratio of 1:100 (1 ha of wetland for 100 ha drainage). A berm should be constructed along the stream. It can be stabilized on the stream side with willow cuttings and seeded with a mixture of prairie grasses and forbs. If a coarse textured soil is encountered, the bottom of the wetland can be sealed with clay and topped with original soil. A gated control structure for controlling water level should be installed at the outflow into the stream. In designing the wetland it is important to remember that most of the chemical transformation and retention occurs at or near substrates (sediments or plant litter). Wetlands containing large amounts of vegetation and decaying plant litter will thus have a much greater capacity for pollutant removal. Any management technique which accelerates vegetation establishment (active regeneration) of litter buildup (addition of organic substrate) will improve chemical retention.

The willow-post soil bioengineering technique and the small field tile wetland are integral components of a complete riparian zone management system that effectively intercepts and treats NPS pollution. However, a MSRBS system cannot replace upland conservation practices. An agricultural landscape will be more sustainable if both upland conservation practices and a MSRBS system are in place.

Applying the MSRBS system at the landscape level becomes a real challenge because of ownership patterns and government set aside programs. Critical riparian areas in a watershed must be protected with riparian buffer strips. Farm boundaries typically are not based on watershed topography, and set-aside programs such as the Conservation Reserve Program encourage farmers to set aside whole fields rather than setting aside the same area of land as riparian buffer zones. Both voluntary or mandatory measures are needed to motivate landowners to install riparian buffer strips at the field level. At the landscape or watershed level, new or highly modified agricultural policies may be required to allow consumers and producers in areas without riparian zones to compensate producers who establish buffer strips and protect riparian zones for the loss of land necessary to meet watershed-wide soil conservation and water quality goals (National Research Council 1993).

Grazing Land Remediation

The semi-arid and arid western rangelands cover a wide latitudinal and elevational range with many potential plant communities so that prescription of the ideal grazing program for the riparian zone is difficult. In developing a grazing program for a given riparian zone several principles should be remembered (Chaney et al. 1993). First, grazing access to the riparian zone should be limited during those times when streambank soils are moist and most susceptible to compaction and collapse. This condition frequently exists during the early spring following snow melt and early spring rains. Second, enough plants and stubble or plant heights should be left on the streambank to ensure protection of the banks (Clary and Webster 1990). Stubble heights of 1.5-2.5 cm are often recommended. Third, grazing pressure should be controlled enough to allow desirable plants enough time to regrow and store enough carbohydrates for overwinter dormancy and competition with other undesirable species. Various seasonal strategies are available and will be discussed below.

Within any rangeland ecosystem the riparian zone will be most heavily used because of favorable forage, water, and microclimatic conditions. Excluding livestock from the riparian zone is the simplest method of management. However, exclusion is often not necessary if intensity, duration, and season of grazing are controlled (Chaney et al. 1993; Elmore 1992). Using riparian pastures that are separate from upland pastures can control the grazing of the riparian vegetation but increase the complexity of management. The most complicated strategy is to attempt control of grazing intensity and timing through herding (Chaney et al. 1993).

Chaney et al. (1993), Elmore (1992) and Clary and Webster (1990) provide the following summary of grazing strategies for western riparian zones. Season-long or continuous grazing is the most detrimental unless it can be strictly controlled according to recommended stubble heights. With this scheme plants receive no rest for regrowth and carbohydrate storage for the dormant season. Woody plants are heavily impacted because constant brow.