Abstract: Session B 10:30 am (Back to Session B)
Stream Restoration and Re-Establishment of Bankfull Floodplain Connection Toward Restoration of Tamarisk Beetle Impacted Riparian Corridors
Lee W. Forbes, PE, DWRE
SWCA Environmental Consultants, Inc.
Restoration of tamarisk beetle impacted riparian corridors in the arid southwest will require a comprehensive approach that addresses not only the riparian vegetation and terrestrial and aquatic habitat directly, but also the hydrology, hydraulics, geomorphology, and sediment transport mechanisms associated with the river itself. Natural alluvial streams and their adjacent riparian floodplains are formed and maintained by the water and sediment delivered to them by their catchments and adjust over long periods of time to changes to these loadings to achieve a new dynamic equilibrium. However, in the past 100 years, streams and their watershed in the arid southwest have been forced to rapidly respond to multiple consecutive anthropogenic watershed disturbances such as channel rectification, surface and groundwater extraction, installation of large reservoirs, and of recent note, the introduction and invasive growth of the non-native tamarisk and, subsequently, the eradication of the tamarisk monoculture via the introduction of the tamarisk beetle, resulting in a barren riparian corridor. Affected streams have not had enough time to achieve a new equilibrium between disturbances and are fixed in a repeating cycle of incomplete channel evolution involving channel incision and over-widening. Stream restoration and stabilization of such systems involves re-establishing the previous point of equilibrium (Priority 1 restoration) or establishing a new equilibrium at a lower elevation (Priority 2 restoration). The result is a stream that has a stable form that does not suffer from degradation or aggradation and, most importantly to riparian restoration, re-establishes a proper hydraulic connection of the stream to its riparian floodplain at bankfull discharge and raises the water table in the riparian zone, both of which are critical to vegetative restoration of the denuded riparian corridor. Stream restoration should therefore be a major component of efforts to restore tamarisk beetle impacted riparian corridors in the arid southwest.