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Riparian Health - Understanding the Relationship between Vegetation and Soil Moisture Characteristics

Last Updated: January 25, 2012

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Vegetation can be an indicator of a water table

The riparian vegetation evident in the center of the above valley is evidence of the width of the water table, even though the stream itself is quite small. The sagebrush on either side indicate where the water table ends.

The presence of riparian-wetland vegetation can be one way of looking for evidence that the water table level is being maintained or is moving toward its potential extent. The maintenance or recovery of an existing water table is vital to the maintenance or recover of a riparian wetland area.

Categories of Riparian Plants

Riparian-wetland species are divided into categories relative to the likelihood of their occurrence in wetlands or non-wetlands. These categories are:

Obligate Wetland (OBL) -- Occurs almost always (estimated probability 99%) under natural conditions in wetlands.
Facultative Wetland (FACW) -- Usually occurs in wetlands (estimated probability 67%-99%), but occasionally found in non-wetlands.
Facultative (FAC)-- Equally likely to occur in wetlands or non-wetlands (estimated probability 34%-66%).
Obligate Upland (UPL)-- Occurs in wetlands in another region, but occurs almost always (estimated probability 99%) under natural conditions in non-wetlands in the regions specified. If a species does not occur in wetlands in any region, it is not on the National List.
Facultative Upland (FACU)-- Usually occurs in non-wetlands (estimated probability 67%-99%), but occasionally found on wetlands (estimated probability 1%-33%).

Definitions taken from USDA Plants Database

Plants that occur in wetlands are hydrophytes, and they have to be in contact with the water table, which is why they can be used as indicators of soil moisture characteristics.

Warning Signs

Some warning signs that the wrong vegetation is present, which may be indicative of declining health or “unraveling” of riparian areas include:

Domination by FACU or UPL plants - These types of plants don’t have the root mass needed to hold streambanks and dissipate energy, as well as maintain the water table.

Some intermittent systems, depending on duration of flow, could be somewhat different, as their potential may be FAC plants.

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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