Ecological dynamics

The stream channel on this site is sinuous, has a very low width to depth ratio, is slightly entrenched, and the dominant substrate size is gravel (E4 Rosgen channel). The valley bottom is moderately wide for the area. This site typically occurs further downstream in the watershed. The stream channel in this site appears to move from a low width to depth ratio stream channel (“E”) under reference conditions to a “C” channel under the impacts of continuous season-long livestock grazing. Grazing impacts vegetation throughout the riparian area, although some vegetation is more sensitive to grazing impacts, such as Northwest Territory sedge. Grazing can also shift the vegetation composition or create open space for different plants to establish, such as Kentucky bluegrass or reed canary grass. Kentucky blue grass does not have the same root characteristics as Nebraska sedge or Northwest Territory sedge. Riparian meadow communities dominated by sedges and rushes have a much greater root densities than grasses (Manning et al. 1989). So when these communities are degraded and loses the sedges and Kentucky bluegrass moves in, it decreases the stream bank stability. As stream bank stability decreases, fine sediment on the stream bank is eroded, thus creating different channel morphology.

This ecological site has been found in one location in Rich County Utah. It is found in a valley bottom with a perennial stream channel with a slope less than 2%. The channel in the valley bottom is highly sinuous and can be ponded through beaver activity. High water typically occurs in May with a mean annual flow recorded of 16 cubic feet per second. The channel is typically a Rosgen “E” channel when not under the influence of beaver and upstream of beaver dams, the channel can take on “DA” morphology (Rosgen 1996). Vegetation in a low width to depth ratio channel is essential in holding the stream banks together. Typical stream bank vegetation on this site is Nebraska sedge (Carex nebrascensis) and Northwest Territory sedge (Carex utriculata). These herbaceous roots growing on the stream banks in this site have an important effect in reducing erosion rates of channel banks (Smith 1976), developing new stream banks, and in providing stability to mature stream banks (Kleinfelder et al. 1992). The root structure of these rhizomatous sedges makes them particularly effective at bank stabilization in fine sediment soils (Steed & DeWald 2003) by binding the soil and adding extra cohesion to the stream bank (Thorne 1990).

Plant communities on this site are arranged according to proximity to channel and depth to water. Willows, sedges and rushes are typically found adjacent to the channel (plant community component 1). They have characteristics that allow them to tolerate long periods of inundation and flooding. Graminoids, including grasses, sedges and rushes, can be found growing on the flood plain (plant community component 2) and willows and graminoids can be found on the flood-plain step (plant community component 3). This arrangement is typical without the presence of beaver. The vegetation composition and arrangement on the valley bottom will change with beaver influence. Beaver dams can temporarily change a lotic to a lentic environment (Cooper et al. 2006). Dam building and foraging activities of beaver can alter the riparian ecosystem by ponding and diverting water and selectively foraging on certain trees or shrubs (Rosell et al. 2005; Gurnell 1998). Ponding and diverting water can then affect stream bank hydrology, impact of flood events, local velocity above and below a dam, and scour and deposition sediment patterns (Gurnell 1998). The stream channel above a beaver dam can take on a stable multithread channel (Rosgen “DA”) (Gurnell 1998). Beaver dams will also change vegetation by creating wetter habitat above a dam, increasing the area that can be colonized by obligate wetland species. Vegetation dynamics are also altered from the foraging preferences of beaver (Rosell et al. 2005). They preferentially use willow and aspen if available (Rosell et al. 2005). This foraging behavior can create a patchy landscape and may eventually lead to food source scarcity, forcing the beaver to leave the area, temporarily. Site abandonment by beavers can cause failure of beaver dams, allowing the impounded water to be released and fine sediment to be carried downstream. Bare sediment left behind the beaver dam can then be colonized by willows that would have been unlikely to establish with the presence of an active beaver dam because of the saturated/anaerobic soil (Cooper et al. 2006). This site will have more wetland obligate species present upstream of a beaver dam and there will be more open water than downstream of a dam. Plant community component 1 represents the wettest habitat and this suite of species is generally found in more abundance with the influence of beaver.