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Title: Short-term geomorphological and riparian vegetation responses to a 40-year flood on a braided, dryland river

Author
item GONZALEZ, EDUARDO - Colorado State University
item SHAFROTH, PATRICK - Us Geological Survey (USGS)
item LEE, STEVEN - Us Geological Survey (USGS)
item LEVERICH, GLEN - Stillwater Sciences
item REAL, RAFAEL - Stillwater Sciences
item SHERRY, REBECCA - University Of Oklahoma
item Ostoja, Steven

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/15/2019
Publication Date: 10/10/2019
Citation: Gonzalez, E., Shafroth, P.I., Lee, S.M., Leverich, G.T., Real, R., Sherry, R.R., Ostoja, S.M. 2019. Short-term geomorphological and riparian vegetation responses to a 40-year flood on a braided, dryland river. Ecohydrology. 12(8). Article e2152. https://doi.org/10.1002/eco.2152.
DOI: https://doi.org/10.1002/eco.2152

Interpretive Summary: Large-scale mass disturbances have the ability to reshape the form and function of ecological systems. Floods are one such example that can exert such a force. We assessed landscape and vegetation response following a 40-year flood event that took place on the Virgin River NV. Using a combination of remote sensing and ground based sampling techniques we found an increase in channel area with as much as a 30% increase in total river corridor area. Two thirds of woody vegetation cover (mainly dominated by Tamarix, with some native shrub Pluchea sericea also known as arrow weed) was lost in areas that eroded (~20% of the river corridor). In the remaining ~80% of stable river corridor (aggrading or no change in elevation) Tamarix remained the dominant species present.

Technical Abstract: In December 2010, a 40-yr flood occurred in the lower Virgin River (SE Nevada, southwestern U.S.), a braided river segment with riparian vegetation largely dominated by invasive shrubs in the genus Tamarix. We assessed geomorphological and vegetation responses to this large magnitude disturbance event by comparing pre- and post-flood remote sensing and field survey data in four river reaches. Analyses of orthophotos and LiDAR-derived topography showed that both the active channel area and channel width increased between ~80 and 258%, representing an increase from 13% to 30% of the total river corridor area. Erosion predominated in the outer bends of the enlarged channel and deposition in the pre-flood channel, causing local avulsions of the low-flow channel. Field-based topographic data recorded before and after the flood in 385 plots also showed that deposition occurred in parts of the floodplain that were not eroded. Two thirds of woody vegetation cover (mainly dominated by Tamarix, with some native shrub Pluchea sericea) was lost in areas that eroded (~20% of the river corridor). In the remaining ~80% of stable river corridor (aggrading or no change in elevation) Tamarix remained dominant. Following erosion, but also where sediment deposition predominated under the Tamarix canopy, the most common colonizing vegetation in the understory was comprised of annual plants, especially Salsola tragus. Our study supported previous studies describing large floods in braided rivers: we documented the first phase of a cycle of channel widening and increase in vegetation heterogeneity that is commonly followed by narrowing and vegetation homogenization.