Vegetation-Erosion Dynamics - River Dynamics and Integrated River Management - page 107

banks that would otherwise fail (Thorne, 1990). Roots, growing below the bed of ephemeral channels, may
act as natural grade-control structures in degrading systems and limit headcut migration (Germanoski and
Ritter, 1988).
Once degradation increases bank heights past a critical threshold (shear stresses in excess of material
strength), however, mass wasting of the banks occurs regardless of riparian vegetation, toppling vegetation
down the bank slope on failure blocks (Simon and Hupp, 1987 and 1992). This failed woody vegetation
sometimes re-establishes on the lower bank slope (Hupp, 1992) or may contribute to the LWD load of the
stream. In both cases this vegetation may ultimately ameliorate degrading conditions by increasing channel
roughness, promoting flow deflection, and adding coarse material (LWD), thus decreasing flow velocity
and subsequent erosion (Gregory and Gurnell, 1988; Shields et al., 1994; Fetherston et al., 1995).
If the channel gradient is near equilibrium, riparian vegetation and large woody debris may facilitate
sediment trapping and initiate aggrading conditions and the recovery process (Simon and Hupp, 1992).
Drift or LWD may be generated along incised channels in greater quantities than along non-degraded
streams. Diehl (1997) notes that channelized and subsequently incised streams generate abundant LWD
because increased bank heights promote bank failure in forested riparian surfaces
The effects of degrading channels on riparian vegetation, particularly below dams, has been summarized
by Williams and Wolman (1984). They note that the extent of riparian vegetation below the floodplain
elevation increases after dam closure, most likely due to regulation of peak flows. The reduction of peak
flows and sediment trapping behind dams, however, limits the production of coarse sediment and the
creation of mid-channel bars and islands necessary for some riparian species to establish (Scott et al.,
1996). Thus, species diversity may decrease and community composition may change after dam closure
(Baker, 1989; Stromberg and Patten, 1992; Nilsson and Jansson, 1995). Comprehensive establishment of
riparian vegetation on bars and islands along the North Platte River, Nebraska, occurred after several dam
closures early last century; this vegetation establishment promotes the coalescence of islands and bars
and threatens critical habitat for sandhill cranes. Collier et al. (1996) observed that the steady reduction
of both springtime flows and total annual flow have allowed the encroachment of cottonwood (Populus),
elm (Ulmus), and willow (Salix) on bare sand bars and islands.
Vegetation on former floodplains along incised channels may be adversely affected by degradation as
a result of water stress from lowered water tables. Johnson et al. (1976) attributed a post-dam decrease in
several floodplain species along the Missouri River, in part, to a reduction in high flows that formerly
delivered nutrients and maintained a higher water table. Along these same reaches, Reilly and Johnson
(1982) correlated a substantial decrease in the growth rate of many surviving floodplain species with the
near elimination of over-bank flooding and lowered floodplain water tables following dam closure.
2.4.2.2 Aggradation
Channel evolution is a complex response punctuated by geomorphic thresholds (Schumm, 1973). One of
these thresholds, the regime shift from bed degradation to bed aggradation, signals the beginning of the
recovery cycle following channel incision. The shift from general degradation to general aggradation
reflects a regime shift from a regime dominated by vertical processes (erosion) associated with non-
equilibrium incision to a regime dominated by lateral processes (point-bar development and meander
initiation and extension) consistent with equilibrated conditions (Schumm et al., 1984; Harvey and Watson,
1986; Simon, 1989; Hupp and Simon, 1991).
Channel banks may not achieve equilibrium conditions coincident with the establishment of channel-bed
equilibrium. If the channel bed has degraded past a critical bank-height threshold, bank failure and
subsequent bank widening may continue until bank angles are reduced to a stable form (Simon and Hupp,
1987; Hupp and Simon, 1991). Sustained accretion on the low parts of bank surfaces is coincident with,