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Riparian Health - Understanding the Role of Large Woody Material in Riparian Areas

Last Updated: January 26, 2012

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Large Woody Material - A Specific Attribute in Specific Places

Some riparian wetland systems, mainly those in the Pacific Northwest, require large woody material (LWM) that falls into the stream to capture bedload, aid floodplain development, and dissipate energy. In these systems, it is important to determine if the streamside and upland plant communities are producing the size of woody material over time that can fulfill this need. Without coarse and/or large wood to dissipate energy, these systems cannot handle the normal high-flow events that occur. The LWM in the system needs to be large enough to stay for a period of time that allows it to operate as a hydrologic modifier.

Large Woody Material Plays a Specific Part in Very Complex Systems

The complexity of forest riparian environments has led researchers to study the hydrology, sediment delivery, vegetation, and biology of these systems to determine how each component, including LWM affect specific products, such as water quality and fish. To visualize forest riparian/stream processes, it is necessary to consider each point of interest as interrelated to the whole stream continuum. The location of interest may be anywhere from the headwaters to the ocean. The way each part of the system functions changes as the streams merge and grow larger, and the enormous variety of stream slope, geology, hydrologies, vegetation types, etc., adds to the difficulty of describing how the whole system functions.

The Role of Large Woody Material

Large Woody Material is required in specific riparian areas in order to dissipate energies.

In order to understand the role of LWM, a knowledge of stream hydrology, forest ecology, fisheries, and climate is also required. To cover all of these aspects would be too lengthy and in depth to write here. The following are some key points that are important to remember regarding the role of Large Woody Material (LWM) in a stream:

LWM and living trees are essential to the development and maintenance of some forested riparian stream ecosystems from their headwaters to the downstream end of the forest stream continuum.

The riparian/stream continuum is in a state of dynamic stability when it is functioning properly and the movement of LWM down the stream system is normal and necessary. The function of LWM in the stream and on the floodplain changes from the headwater to the wider downstream valleys.

Floods, fires, windthrow, torrents, landslides, and normal tree mortality are essential delivery mechanisms needed to maintain and restore the riparian stream system’s functionality.

The temporal processes of the forest riparian /stream system must be measured in decades and centuries.

The spatial location of LWM is continually shifting during annual and episodic events. This spatial movement replenishes materials that are broken down or flushed out of the system.

Warning Signs

Some warning signs that not enough LWM is present for the stream, which may be indicative of declining health or “unraveling” of riparian areas include:

An absence of large wood near the stream – Without living mature trees present that will access the stream in the future the stream is without a source of LWM.

Only isolated pockets of trees near the stream – The stream needs to have adequate trees as a source of LWM. Isolated pockets of trees would not fulfill this need.

Many riparian and wetland areas throughout the western U.S. do not require LWM for streams to function and dissipate energy. For more information on LWM and its role in streams, please see the reference listed at the end of the page.

References

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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