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Riparian Health - Understanding Root Masses and Bank Stability

Last Updated: August 16, 2011

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The Importance of Having Vegetation with the Right Roots

Riparian vegetation has extensive root systems that has the ability to stabilize banks.

Vegetation is important in slowing flow velocity, stabilizing streambanks, and reducing erosion. Streambanks dominated by vegetation without extensive root masses are undercut during high flow events and collapse. This collapse results in a change in the active channel’s width/depth ratio, gradient, and sinuosity, which reduces a riparian-wetland area’s ability to dissipate energy. The best soil stabilizers and streambank holders are woody species such as willows and cottonwoods, and herbaceous species such as sedges, and rushes. The extensive root systems of these species are especially effective in the development of overhanging banks, which provide habitat for fish and other aquatic organisms. These types of plants are known as Obligate Wetland of Facultative Wetland plants.

Annual herbaceous species and those species that indicate uplands generally lack sufficiently dense, deep root systems to provide much protection. These species are known as Facultative Upland or Upland plants.

When Vegetation Doesn't Play a Role in Stability

There are exceptions where riparian vegetation with root masses capable of withstanding high flow events is not required. These include high gradient, bedrock, or boulder/cobble stream types. In these systems, vegetation contributes little, if any, to bank stability.

Warning Signs

When vegetation with shallow root systems dominate an area, such as the Kentucky Bluegrass dominated system above, the riparian area is incapable of withstanding high flow events and maintaining the stability of the banks.

Some warning signs that the vegetation present lacks root masses capable of holding banks, which may be indicative of declining health or “unraveling” of riparian areas include:

Undercut Banks – Streambanks that are continually undercutting and shearing off indicate the plants present don’t have the root masses needed.

Presence of upland plants in the riparian area - Species such as Kentucky bluegrass, redtop, blue grama, (most grasses) and sagebrush, do not have the root masses capable of withstanding high flow events. If these plants dominate plant communities along the streambank, it is an indicator that the stream is in need of better vegetation.

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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