THE IMPORTANCE OF ALLUVIAL VALLEY HISTORY ON MODERN CHANNEL INSTABILITY AND REACH-SPECIFIC RESTORATION

Many drainage networks in small watersheds in the mountains of central Nevada are inherently unstable and have recently experienced rapid incision. A large number of these unstable channels flow through wet meadow complexes that provide regionally unique habitat for neotropical migrants and a variety of endemic species that rely on these oasis-like ecosystems. Channel incision can de-water complexes and lead to a shift from riparian to up land vegetation. Meadow complexes with elevated water tables are preferentially located immediately up-valley of mid-late Holocene age side-valley alluvial fans that have prograded into or across the main valleys. The fans are hinge points for modern channel incision because fan progradation has resulted in the development of oversteepened reaches where the mainstem channels flow across the fans. Channel instability often starts where the main channel gradients steepen across side-valley fans. Once channel erosion breaches the baselevel established by the side-valley fan, a wave of incision migrates upstream through the meadows. Channel erosion in the meadows lowers the baselevel for groundwater discharge in the hyporheic zone along the channel margin. The baselevel drop increases the groundwater gradient to the channel and facilitates groundwater sapping of the channel banks. Efforts to maintain un-incised meadow complexes and restore degraded meadows must focus on channel morphology and dynamics, both of which are inherited from events that transpired during the Post-neoglacial drought (a period of aridity when the side-valley fans formed approximately 2,500-2,000 years B.P.). We have begun detailed characterization of an actively degrading meadow in the Toiyabe Range as a precursor to restoration. We plan to install a series of grade control structures beginning at the side-valley fan hinge-point and extending upstream into the meadow to facilitate sediment deposition, reconstruction of the channel profile, and regrading of the water table to a higher baselevel. Sapping zones will be replanted with naturally occurring rushes and sedges. The channel’s response to restoration, as well as the changes in adjacent groundwater levels and vegetation patterns will be monitored in future years.