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: http://www.nwi.fws.gov/bha/list96.html or http://www.charttiff.com/WetlandMaps/WetlandPlants/plantlists.html 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.
Steve Nelle, NRCS