Vegetation-Erosion Dynamics - River Dynamics and Integrated River Management - page 108

and facilitated by, the establishment of dense woody vegetation (Hupp and Simon, 1991; Hupp, 1992).
The woody vegetation on recently stabilized banks and floodplains increases soil strength through root
development and reduces flow velocity by increasing surface roughness (Williams and Wolman, 1984;
Hupp and Simon, 1991; Shields et al., 1993 and 1995). This vegetated depositional area expands from
low on the bank slope, and depending on the magnitude of prior degradation, may ultimately extend to
the former floodplain elevation.
Many rivers are now channelized and the riparian vegetation experiences devastation and recovery.
The amount of vegetation cover, age of the riparian plants, and species richness varies with the stage of
channel recovery after channelization (Fig. 2.53; Simon and Hupp, 1987). Vegetation cover is high during
stage I and stage III where it occurs above the limit of channelization; here the mature riparian vegetation
has not been affected by channel incision. For channelized reaches, cover is lowest in stage II, the
construction stage, during which woody vegetation typically is removed, and in stage IV where numerous
bank failures remove woody plants and preclude the establishment of new vegetation (Fig. 2.53). Cover
and number of species increase from late stage IV through stage VI. The age of the woody plants through
the course of channel evolution closely matches the trends in cover for obvious reasons. Trends in the
number of species (or species richness) also match those of cover and age. The greater site stability of
riparian areas in stages I and III may promote greater species richness, while severe instabilities in stage
IV and early stage V preclude all but the most vigorous, ruderal species.
Fig. 2.53 Age, cover, and numbers of species as functions of the stage of channel degradation and recovery (after
Simon and Hupp, 1987)
2.4.2.3 Large woody debris (LWD)
Wallerstein et al. (1997) found LWD induced sedimentation exceeded LWD induced scour, thus the overall
effect of debris jams is grade control and accelerated sedimentation, promoting stable channel features
that may trigger the onset of recovery. They also offer a classification of debris jams based on debris
length L and channel width (Fig. 2.54) that may be used as a conceptual model to evaluate types of LWD
along incised channels. The use of artificially placed LWD has been shown to increase channel stability
along incised channels, whereas removal of LWD increased degradation (Shields and Gippel, 1995).
Vegetation patterns seemingly develop largely in response to surface-stability conditions, accretion
tolerance, inundation tolerance, and, for some species, light availability. For example, three distinct suites
of vegetation were identified as recovery vegetation along incised channels in west Tennessee (Hupp and
Simon, 1991; Hupp, 1992), which develop in succession beginning near the end of stage IV through stage
VI (Fig. 2.53) of the Simon and Hupp (1987) model of channel evolution, matching the ameliorating
conditions during the recovery period. The initial pioneer suite (Suite l, Table 2.4) of riparian plants
establish late in stage IV or early in stage V and are hardy, fast-growing plants dispersed in late spring,