Streambank erosion is a physical process that occurs along virtually all natural channels. Not only is it a normal part of channel evolution and meander migration, but it is also essential for creating and maintaining a variety of aquatic and riparian habitats. When bank erosion becomes excessive, it can destroy significant channel and floodplain habitats. This occurs as these areas are excavated and sometimes buried under massive amounts of sediment. The best protection against excessive erosion is the preservation of adequate, desirable vegetative cover to dissipate the erosive forces acting upon the channel banks during periods of high stream flow.
Bank erosion occurs when the eroding force (shear stress) of water moving along the bank exceeds those forces in the bank that are resisting the shear force. Shear force on the bank is directly proportional to the rate at which velocity increases when moving away from the bank. Due to friction, the highest water velocity in a stream is about 1/3 of the way down from the water’s surface. This puts more of the energy against the streambank at about 1/3 of the depth. Thus, if velocity increases very rapidly in the near-bank region, the velocity gradient is steep and shear stress is high. Conversely, if velocity increases slowly or not at all in the near bank region, shear stress of the bank will be minimal or negligible. With either situation, the energies associated with the stream’s velocity are being exerted under the surface against the streambanks. Adequate vegetation will ensure that the riparian vegetation root masses are present and capable of withstanding the energies. Root masses are a key factor due to the fact that they increase the tensile strength of the bank. Particularly in noncohesive soils and sediments, the presence of vegetation may greatly increase binding forces in bank material. Tensile strength provided by root masses of riparian vegetation may be the primary source of resistance in the alluvial sediments of many Western streams. Tensile strength will be dependent upon both the kind of vegetation present and the extent and density of root masses in the sediments. Determination of root-mass adequacy is site specific, as less cohesive sediments will require greater root mass to achieve the same level of stability as more cohesive sediments elsewhere.
Vegetation also has the potential to influence the balance of energy by the occurrence of living or dead vegetation (or any other cover, for that matter) that extends into the flow of the stream/river. It has the potential to reduce near-bank velocities, thus reducing erosive shear forces acting upon the bank. In an ideal situation, vegetation along the bank is sufficient to produce a zone of near-zero velocities near the bank, effectively moving the velocity profile away from the bank so that shear stress is dissipated in turbulent eddies in the flow. A similar process occurs in the over-bank region when density of vegetation is sufficient to produce near-zero velocities in overbank flow during flood events.
In order to determine if the vegetation in a channel is adequate, the following items should be assessed regarding riparian species:
If each of these items is evaluated and found to be healthy and doing well, then the vegetation present is adequate. If any of these items are absent or minimal, then there is some question as to whether or not the vegetation is adequate.
Some warning signs to look for that may be indicative of inadequate vegetation include:
Bare ground--soil that is not covered by vegetation, litter or duff, downed woody material, or rocks--is highly susceptible to erosion. It may contribute both to overland sediment flow and to the erosion of streambanks. In both cases, it can affect water quality as well as contribute to the loss of valuable soil and acreage. Bare stream banks are also prime areas for invasion of noxious weeds or other undesirable plant species. Bare ground increases the possibility of compaction or bank shearing by hoofed animals, vehicles, or people. This reduces the water-holding capacity of the soil.
Riparian Species present, but not adequate - If riparian plant species appear to be low in vigor, losing diversity and species, or not producing the root masses needed, there might not be adequate vegetation to sustain the riparian area and protect it during high flows.
Upland plants in the riparian area – Upland plants in a riparian area are indicative of an unhealthy riparian area. Upland species do not have the root masses capable of holding a streambank together and resisting the energies within the stream.
Riparian Health - Evaluating the Health of Riparian Areas - An Overview
Riparian Health - Understanding the Function of Floodplains
Riparian Health - Understanding the Role of Beavers in Riparian Areas
Riparian Health - Understanding if the Channel is in Balance with the Landscape
Riparian Health - Riparian Areas and Water Storage
Riparian Health - Understanding How Uplands Contribute to Riparian Health
Riparian Health - Understanding Riparian Vegetation Age-Class and its Role in Health
Riparian Health - Understanding Species Diversity
Riparian Health - Understanding the Relationship between Vegetation and Soil Moisture Characteristics
Riparian Health - Understanding Root Masses and Bank Stability
Riparian Health - Understanding Plant Vigor
Riparian Health - Understanding the Role of Large Woody Material in Riparian Areas
Riparian Health - Understanding a Channels Ability to Dissipate Energy
Riparian Health - Point Bars
Riparian Health - Understanding Lateral Stability in Riparian Areas
Riparian Health - Understanding Vertical Stability in Riparian Areas
Riparian Health - Understanding if the Channel is in Balance with the Soil and Water Being Supplied
Surber, G., B. Ehrhart. 1998. Stream and Riparian Areas Management: A Home Study Course for Managers. Montana State Extension Service. Information also available at http://www.animalrangeextension.montana.edu/riparianmgt/index.htm
USDI Bureau of Land Management. 1998. Riparian Area Management: A User Guide to Assessing Proper Functioning Condition and the Supporting Science for Lotic Areas. Technical Reference TR 1737-15. 124 pp. More Information available at: http://www.blm.gov/or/programs/nrst/index.php