by Steve Nell, NRCS, San Angelo, Texas

Originally Published in Riparian Notes. Note Number 10, February 2005.
Reprinted with permission from the author. Website links have been updated from the original version.
One of the key attributes of a properly functioning riparian area is the relative stability of banks and channels. That stability will only be provided when banks are covered by the right kinds of riparian vegetation. Riparian species, in general, are extremely well rooted and are able to withstand the extreme erosive forces of turbulent floodwaters.
The right kinds of vegetation for riparian stability are those native plant communities that are well suited to frequently flooded bottomland situations and will normally consist of a variety of grasses, sedges, forbs, shrubs and trees. These riparian plant communities will be distinctively different from adjacent upland vegetation. One way that riparian species can be determined is by looking at the “National List of Plant Species That Occur in Wetlands” and the Indicator Categories assigned to each plant species. This list can be found at: . Using this system of classification, all plants are assigned into one of five categories as described below:

  • OBL Obligate Wetland species almost always occur in wetlands (greater than 99% probability).
  • FACW Facultative Wetland species usually occur in wetlands (67% – 99% probability), but are occasionally found in non wetlands.
  • FAC Facultative species are equally likely to occur in wetlands and non wetlands (34% – 66% probability).
  • FACU Facultative Upland species usually occur in non wetlands (67% – 99% probability), but are occasionally found in wetlands.
  • UPL Upland species almost always occur in non wetlands (greater than 99% probability) and almost never occur in wetlands.

On perennial creeks, OBL and FACW and some FAC species are considered to be riparian species. These are usually the plants that have the necessary dense root masses capable of withstanding high flow events. On seasonal creeks, some OBL or FACW species should be present, but the dominant riparian plants may be FAC and some FACU species. Riparian areas dominated by FACU and UPL species will very likely not be functioning properly.
Another rating system has been developed to estimate the ability of plants to resist erosion and stabilize creekbanks. A rating of 10 would provide the maximum stability and is equivalent to the strength of anchored rock. A rating of 1 is equivalent to bare ground. Generally, if riparian areas are dominated by combinations of plants rated 6 – 9, stability would be considered adequate.
Draft Stability Ratings have been proposed for some of the key species in west and central Texas and this list is available by contacting . The abbreviated list below shows the Wetland Indicator Category and Proposed Stability Ratings for some of the more common plants found in riparian areas. By looking at these classifications, the riparian manager can begin to more clearly understand what the right kinds of vegetation are:

  • Emory sedge OBL 9
  • Sawgrass OBL 9
  • Bulrushes (most ) OBL 9
  • Spikerushes FACW 5/6
  • Knotgrass FACW 6
  • Bushy bluestem FACW 6
  • Rabbitsfoot grass FACW 3
  • Switchgrass FAC 9
  • Eastern gamma FAC 9
  • Deergrass muhly FAC 9
  • Big sacaton FAC 9
  • Bermudagrass FACU 5
  • Texas wintergrass FACU 4
  • King Ranch bluestem UPL 5
  • Water willow OBL 8
  • Scouring rush OBL 7
  • Spiny aster FACW 8
  • Frogfruit FAC 4
  • Tall goldenrod FACU 7
  • Maximilian SF FACU 6
  • Common ragweed FACU 2
  • Juniper UPL 5
  • Buttonbush OBL 7
  • Indigobush amorpha OBL 7
  • Black willow FACW 6
  • Baccharis, seepwillow FACW 6
  • Cottonwood, Fremont FACW 6
  • Sycamore FAC 6
  • Little walnut FAC 6
  • Roughleaf dogwood FAC 6
  • American elm FAC 6
  • Hackberry FACU 5
  • Mesquite FACU 5
Colorado Riparian Association