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Riparian Health - Understanding a Channels Ability to Dissipate Energy

Last Updated: January 26, 2012

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Dissipating Energy - a Channel's Job

A floodplain being used in a high flow event. Both the spreading out of the water and the vegetation are slowing stream energies and allowing sediment depostion.

While the primary function of riparian channels is to convey water and sediment, they also dissipate stream energies. Energies can be dissipated within the channel through boulders or large wood, or by sinuosity. If flows and energies are high enough, then floodplains help dissipate the energies, especially in wide valleys. Floodplains act as overflow buffers and serve a critical function in mitigating the downstream impacts of floods. Floodplains comprise the area adjacent to channels over which out-of-bank flows are diffused. These areas reduce the energy by spreading out the high flows. If floodplains are vegetated with riparian plants, then the vegetation will further absorb energies and slow the flows.

Different Channel Types Mean Different Ways to Dissipate Energy

Channels such as the one pictured above dissipate energy through boulders within the stream as well as step-pools.

Channel characteristics vary by channel type which must be identified in order to evaluate streams correctly, and understand the ways in which energies are dissipated. (For help in determining your channel type, see [Rosgen Classification Method]). For example, a B2 channel type is moderately entrenched, typically located in or on coarse alluvium, and has a limited floodplain. This channel type has channel characteristics of boulders and small cobbles as well as some floodplain for expansion and often woody riparian vegetation to dissipate stream energy. On the other hand, a C4 channel type is a slightly entrenched and gravel-dominated stream, which has to have access to a floodplain and channel characteristics such as backwater areas, oxbows, and overflow channels along with riparian vegetation to dissipate stream energy.

Channels which have lost those characteristics that help them to dissipate energy are quickly degraded.

Warning Signs

Some floodplain and channel characteristics warning signs to look for that may indicate declining health or “unraveling” of riparian areas include:

Low sinuosity – In those channel types which use sinuosity to dissipate energy, a loss or lack of sinuosity indicates that the channel is not dissipating energy. The lack of sinuosity often manifests itself in other ways, such as downcutting and excess bank shearing.

Lack of large woody debris – In those channel types where large woody debris is necessary for disippating energy, a lack of woody debris would cause an increase of energy which may lead to downcutting or widening of the stream. Woody debris also creates pools and dams that capture bedload sediment to maintain bed elevation.

One way in which a stream accounts for extra energy is to erode away the channel bottom, causing a downcut and incised channel.

Downcutting – when a disturbance causes the stream to increase its velocity, one way it accounts for the extra energy is to erode away the channel bottom. An incised channel is one in which the average flood--of the size and intensity which occurs roughly every one to three years--is unable to "access its floodplain" by overflowing its streambanks. As the channel cuts downward, the groundwater table is lowered. (For more information on downcutting and vertical stability see Riparian Health - Understanding Vertical Stability in Riparian Areas)

Channel Widening – by spreading the water out in a wider channel, energy can be diffused. Unfortunately, this widening raises temperatures above thresholds for fish and reduces available shading. Overwide channels do not transport their bedload sediment adequately and aggradation can cause the channel to move.

References

Surber, Gene and Bob Ehrhart. 1998. Stream and Riparian Area Management. Montana State University Cooperative 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

Zaimes, G.N. (editor). Understanding Arizona’s Riparian Areas. Univeristy of Arizona Cooperative Extension, Publication #az1432. Available online at: http://ag.arizona.edu/extension/riparian/

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