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Riparian Health - Understanding Lateral Stability in Riparian Areas

Last Updated: August 16, 2011

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What is Lateral Stream Movement?

Streams located within non-confining landforms are constantly in the process of moving back and forth across the valley floor. This lateral stream movement is a natural process and enables the stream to slow its velocity. It has been defined as meandering or snaking through the landscape, patterned after how a snake moves over land. When this movement is excessive, it can have serious impact on the overall function of a riparian wetland area, limiting its ability to dissipate energies.

Lateral Stream Movement is a Natural Process

Lateral movement of stream channels is a natural process in wide gently sloping valleys. Because sinuosity and lateral stream movement are a function of landscape setting, lateral movement is strongly related to how the stream maintains balance with its landform. Lateral movement occurs through bank erosion, and its rate is influenced by many factors, especially stream type, nature of bank material, and kinds and amounts of vegetation on the streambank, as well as how point bars are functioning within the system.

Bank erosion must be evaluated relative to stream type, particularly as a stream type manifests itself in the bank material sizes present at a site. For example, a meandering riffle-pool stream channel likely will exhibit higher bank erosion rates in areas of sandy material than in areas where silts and clays provide some cohesiveness to the bank. Thus, “natural” rates of channel migration will vary by stream type and material. Streams or rivers with very stable banks and well rooted vegetation become deep and narrow, highly sinuous, and they flood often. If streams lose bank stability and become wider as they move faster, their length shortens as they become straighter. This can also happen if sediment supply is increased. Some streams naturally occur in areas with sandy or gravelly sediment or higher sediment loads and are like this naturally. When meandering streams lose too much bank vegetation and stability or receive too much sediment, they can become braided. Braided channels move rapidly in each high flow. Their multiple channels split apart and come back together. In some settings, a braided channel is natural and in others it represents unnatural conditions.

Warning Signs

A stream that relocates its channel with each event is laterally unstable.

Some warning signs that indicate that sediment is not being captured on point bars, and that the system may be declining or “unraveling” include:

Widening of the Stream Channel - On a naturally meandering channel, new point bars will be extending into the channel on the inside of curves at about the same rate the banks on the outside of curves are eroding away. Thus, the width of the channel remains constant over time. In contrast, a destabilizing laterally cutting channel becomes wider over time.

An Active Channel that Relocates with each Event - An active channel within a riparian area relocates itself with every high-flow event – this is a sign that the stream isn’t laterally stable.

High Amounts of Bank Erosion, especially on the outside of curves – this can be remedied some by the establishment of vegetation with the root masses capable of holding the banks during high-flow events.

High banks and infrequent flooding - If the banks are unstable and do not revegetate because they have become dehydrated, the banks may be eroding because of channel incision, which could be due to too much straightening or lack of piontbar vegetation or sediment.

Some channel types are limited to lateral movement by existing landforms such as bedrock, and should be considered laterally stable.

References

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

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