Greater understanding of the role of woody vegetation on fluvial processes would enable land managers to better utilize vegetation on stream banks to provide for well integrated river system management. The importance of riparian vegetation for fish habitat is well documented (Elmore and Beschta, 1988.) The estuary of the Mattole River was identified by fish biologists as a critical link in the recovery of wild salmonids, and recovery of floodplain vegetation is key is the achievement of cool pools, root overhang pools and increased food sources for fish species.
New and innovative methods for recovery of streambank vegetation on persistently bare gravel bars include "bioengineering" techniques, which use both live and dead woody material to stabilize sediments. What is called in English "bioengineering"or "soil bioengineering" is a collection of technologies mainly derived from European research and application over the recent 200 years.
Bioengeering methods exist for stream bank, floodplain, hillslope and gully environments. Properties held in common by these methods include use of the flexible resistence of live woody stems to interact with fluvial processes, creating flow zones of slowed velocity and reduced boundary shear stresses, and reducing entrainment of fine sediment particles. One such technique is the live siltation baffle, a design by the Swiss bioengineer Hugo Schiechtl, modified by Evan Engber of Laytonville, California.
Live siltation baffles were designed and installed in November, 1993. The baffle structure consists of a series of trenches perpendicular to the channel planted thickly with willow and cottonwood cuttings (see appendix 1). Using a backhoe, perpendicular trenches were placed on the upstream side of existing willows. (Schiechtl, Hugo, 1980.)
We placed six to eight foot long cuttings, 2+ inches in diameter in the trenches approximately 3.5 ft. deep. Stems were packed tightly together (>6 stems per foot) in the trenches, as evenly as possible. The backhoe replaced the soil matrix, and the hydraulic force of a fire hose watered the stems into place. Large rocks (>D85) are placed on the upstream side of each trench, to reduce erosion of disturbed soil material.
In January, 1995, a ten-year strom event occurred. The baffles withstood a discharge of 60,000 cfs. On the fall limb of this storm, five baffles on the upstream end washed out. Recovered specimens showed root development of 0.5-2.0 ft. in length, with few branching roots With little supplemental irrigation over dry summer months, root development was not substantial, indicating that supplemental water may be important for young plants in a dry cobble/gravel matrix to increase root strength and resistance.
This storm was followed by another 2 year storm event in March 1995. During the second high flow, the remaining baffles were eroded by lateral bank migration, an impact for which they were not designed. We learned from these two events that meander analysis may be an important component in the placement of revegetaion structures. Early analysis indicates that meander migration was predictable for the site chosen for planting, but these data were not available to us at the time of planning the project.
In highly disturbed watersheds, the natural hydrologic regime is significantly altered when soil moisture retention functions are lost. The ability of a system to absorb rainfall impacts and deliver moisture to the channel slowly over time is absent in clearcut watersheds such as the Mattole. The cumulative effects of these processes are propagated in the downstream direction, rendering the lower mainstem areas vulnerable to large scale channel and floodplain changes during relatively small storm events.
Many coastal estuaries are in need of more riparian forest cover, both for biological and physical reasons. It is appears that our revegetation efforts should be directed to upstream channels and hillslopes to reduce the fluvial impacts of sharply peaked storm hydrographs However, further experimention with bioengineering of lower mainstem areas should be combined with meander analysis. We may be able to better integrate revegetation structures with stable floodplain features where these exist, to assist the survival and recovery of wild salmonids.
Dingman, S. Lawrence. 1984. Fluvial Hydrology. W. H. Freeman Co. New York.
Dunaway, Donette, Sherman Swanson, Jeanne Wendel and Warren Clary. 1994. The effects of herbaceous plant communities and soil textures on particle erosion of alluvial steambanks. Geomorphology, 9 (1994) 47-56.
Dunne, Thomas and Luna B. Leopold. 1978. Water in Environmental Planning. W.H. Freeman Co. New York.
Elmore, Wayne and Robert L. Beschta. 1988. The Fallacy of Structures and the Fortitude of Vegetation. in Proc.of Calif. Riparian Sytems Conference, Davis, Calif.
Gray, Donald and Andrew Leiser. 1982. Biotechnical Slope Protection and Erosion Control. Van Nostrand Reinhold Co.
Lu, J.Y., E.A. Cassol, G.R. Foster and W.H. Neibling. 1988. Selective Transport and Deposition of Sediment Particles in Shallow Flow. Trans. of the Amer. Soc. Ag. Engineers Vol. 31(4): July-Aug, 1988.
Schiechtl, Hugo. 1980. Bioengineering for land reclamation and conservation. University of Alberta Press.
