Purpose
Some lentic riparian-wetland areas have been altered through the addition of
structures designed to capture more runoff, thus creating a more permanent or larger wetland. The prairie pothole region in eastern Montana and North Dakota provides many examples. However, when structures are placed to alter a riparian-wetland area, it is very important that the structure is designed and maintained to accommo- date safe passage of flows. The purpose of item 7 is to determine if these structures are accommodating safe passage of flows.
Examples
If the ID team determines the structure is stable and accommodating flows, the answer to item 7 would be “yes.” If there is erosion, leakage, or a headcut affecting the integrity of the dam or spillway, the answer to item 7 would be “no.” An
example is provided in Appendix C. For the functional—at risk prairie pothole example in Montana (Appendix C), item 7 would be answered “no” because a headcut has developed in the spillway and is threatening the integrity of the dam. If no structures are present, item 7 is answered “N/A.”
Supporting Science/Quantitative Methodologies
Gully erosion occurs when headcuts form and an area downcuts. As the gully bottom downcuts, it deepens and then widens, thereby endangering the structure. Headcuts extend upstream or upslope into ungullied headwater areas, increasing the number of drainage channels (i.e., by developing additional tributaries) (Heede 1976). If there is a headcut or nickpoint present downstream of a dam or other structure, there is the possibility that the headcut will advance upstream, removing part of the structure and causing the wetland area to be eroded and drained. If left unchecked, a headcut will proceed upstream until some hard point, such as a
bedrock outcrop, is encountered or a smooth transition between upstream and down- stream gradient is attained (Heede 1980).
When assessing item 7, the presence or absence of any headcuts below a structure should be noted.
B. Vegetation
Items 8-15 address vegetation attributes and processes that should be in working order for a lentic riparian-wetland system to function properly. Landform can play a major role in defining the riparian-wetland setting. Wide, flat valley bottoms result in mosaics of vegetation, or complexes. The riparian-wetland complex as described in the Intermountain Region Integrated Riparian Evaluation Guide (USDA FS 1992) is influenced by the valley bottom setting. The complex is comprised of patches of community types, in various amounts and locations. In assessing functionality, the whole complex should be considered in order to understand such items as age class distribution and species diversity.
The site potential should be evaluated when determining vegetation types that can be found in lentic riparian-wetland areas. Many lentic riparian-wetland areas do not have the soil and hydrology conditions needed to support tree or shrub species. Many of these wetland types do not need tree or shrub species for physical stability. These species may be a component for desired condition, but are not required to des- ignate a lentic riparian-wetland area as functioning properly.
Factors such as the kind, proportion, and amount (cover or density) of vegetation in the riparian wetland community contribute to the assessment of shoreline-
forming/soil surface vegetation and the encroachment of upland vegetation. The linear distribution of vegetation is the primary factor affecting the extent of eroded shorelines/soil surfaces, assuming that the right kinds and proportions of species are in the community (or simply the inverse relationship—the amount of shorelines or soil surfaces lacking the right kind and amount of vegetation). Lateral distribution of vegetation determines the riparian-wetland area’s ability to accommodate periods of wind action, wave action, and overland flows and drought. In order to persist or improve, the plant species or communities of interest must be both healthy (vig- orous) and replacing or increasing their numbers or extent through recruitment into the community.
The level III riparian area evaluation from the USDA FS (1992) provides measure- ment techniques for cross-section composition of the riparian-wetland complex, veg- etation composition within a complex and along the greenline, and woody species regeneration. Each item, except for items 12, 14, and 15, can be quantified or inter- preted from quantified information using these techniques.
Riparian-wetland plants are classified into five types based on the likelihood of their occurrence in wetlands or nonwetlands (Reed 1988). These classes are: obligate wetland (OBL), facultative wetland (FACW), facultative (FAC), facultative upland (FACU), and obligate upland (UPL). OBL species are likely to occur in wetlands >99 percent of the time, whereas FACW species occur in wetlands between >67-99 percent of the time. The FAC species are likely to occur in wetlands 33-67 percent of the time; FACU species are likely to occur 1-<33 percent of the time. UPL species almost never (<1 percent) occur in wetlands. The FACW, FAC and FACU categories are subdivided by “+” and “-” modifiers. An FACW- would be closer to an FAC, or would reflect slightly drier conditions based on probability of occurrence (U.S. Army COE 1987).
Plant lists have been compiled by the FWS for each region in the United States. Some species may be listed as a different wetland indicator type, depending on the state and region in which it occurs. Some states have developed localized plant lists as a subset of the national list.
Item 8: There is diverse age-class distribution of riparian-