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This article is partially excerpted from "Streambank Stabilization / Riparian Restoration: Action Plan for the Buffalo National River," an August 1994 report for the National Park Service and US Department of the Interior.
Clearing of streamside riparian forests for farming was a common practice along the Buffalo River prior to the establishment of the National Park in 1972. Many of the river's banks consist of fine alluvial sediment and become highly unstable when vegetation is removed. Even with cessation of farming activities, many streambank erosion sites continue to experience soil loss at alarming rates (estimated as high as 40,000 tons per year at one site). As a result of this erosion, stream channels become shallower and wider, the aesthetic appeal of the stream is degraded, aquatic habitats are impaired, and water quality is diminished by increased sediment loads and turbidity. Forested riparian corridors also serve as important foraging habitat for the resident endangered gray bat.
The river's free-flowing status and the quality of its waters are the National Park Service's primary concern. The Buffalo National River's enabling legislation and planning documents all rank water quality and stream corridor protection as our highest priorities. Not all eroding banks are regarded as a problem, but where streambanks and stream channels have been altered by past land use, restoration efforts are being considered.
A myriad of land use activities, mostly related to agriculture, occur in the watershed, including wilderness, logging, beef, dairy, swine, and poultry operations. It is common for the alluvial flood plains to be farmed up to the present channel banks with little or no buffer strips between cleared ground and the river. This type of farming practice occurs throughout the region and has undoubtedly increased the vulnerability of the river banks to erosion. It is also probable that past and present land use practices in the watershed have increased the delivery of sediment to the river and further encouraged the tendency for lateral channel shifts, especially where riparian forests are removed.
Resource Management staff have been monitoring 25 erosion sites over the past ten years. These eroding banks have been categorized according to length (250 - 2300 feet), height (5 - 32 feet), rate of erosion, and other factors. The average site erodes one to three feet annually, but some sites lose as much as 20 feet per year. Erosion mitigation alternatives have been formulated with stream management consultants, through interagency workshops, and with representatives from the Water Resources Division.
Several courses of remedial action have been attempted, including rip rap, stacked rocks, and in-channel modifications. The most successful and cost-effective method employed thus far uses natural tree revetments and restoration of the riparian forest. Tree revetments employ large cedar trees positioned horizontally at the base of the bank and anchored with cable in a continuous shingle-like fashion. Depending on the height of the bank, the angle of the bank may be cut back to a 45 degree slope. Seedlings of native riparian trees such as cottonwood, sycamore, oak, ash, and sweet gum are then planted on the cut bank and in a 100 foot riparian buffer behind the bank.
The cedar tree revetment slows the force of the flood waters and promotes deposition of sediment within the branches. The newly deposited sediment serves as a seed bed for the growth of additional vegetation which further stabilizes the bank. Behind the revetment, the planted seedlings produce root systems which bind the soil. The revetment protects the bank long enough for the trees to gain sufficient size and root mass to restore the inherent stability of a forested streambank.
The National Park Service has recently initiated an extensive three-year streambank stabilization and riparian restoration project through the Natural Resources Preservation Program (NRPP). Fourteen sites, with a total of 5,763 feet of eroded stream bank, have been identified for stabilization using cedar revetments and other bioengineering techniques. Working with the U.S. Army Corps of Engineers and the State Historic Preservation Officer, the staff has complied with the Clean Water Act and National Historic Preservation Act in obtaining a "404" permit and required archeological clearances. In addition to the revetment sites, 18,881 feet of stream corridor will be restored with 100-foot wide forested buffers. To document the success of the restoration efforts and interpret responses in geomorphic and habitat features, a comprehensive monitoring plan has been developed and implemented. Because tree revetments have never been used to stabilize eroding banks of such magnitude, this is in many ways an experimental program.
In our first year, a seasonal crew of 5 with the assistance of a heavy equipment operator, completed 4 revetments. Approximately 450 cedar trees and a mile of cable were used to revet banks ranging from 300 to 500 feet in length and from 15 to 22 feet high. In addition, a minimum of 40,000 seedlings will be planted during this period.
Shortly after completion of the fourth revetment, heavy rains caused the river to rise at this site almost 33 feet in just 36 hours. Flood waters covered the bank and most of the adjacent field. After the waters receded, our concern turned to optimism - all the revetments had remained intact. Upon closer inspection, we also found sediment deposits within the branches. It appears the experiment has passed the first of many tests.
David N. Mott is a hydrologist and Michael Naranjo is a biological technician at the National Park Service's Buffalo National River headquarters based in Harrison.