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

Abstract

In December 2010, a 40-year flood occurred in the lower Virgin River (SE Nevada, southwestern United States), 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 light detection and ranging-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.

How dynamic is the Brahmaputra? Understanding the process–form–vegetation interactions for hierarchies of energy dissipation

“…Similarly, riparian vegetation has a significant impact on river morphodynamics since it interacts with the flow-sediment discharge at different spatio-temporal scales, T A B L E 1 The bar frequency statistics and confluence-bifurcation geometries in the last 20 years (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019) for two large braided reaches of the Brahmaputra River Gurnell, 2014;Gurnell & Petts, 2002;Henriques et al, 2021). In addition, aquatic vegetation considerably adjusts the flow and turbulent structure and affect the riverine habitat (Bornette & Puijalon, 2011;Boruah et al, 2008), water quality (Dosskey et al, 2010), pollutant-nutrient distributions (Perucca et al, 2009;Shucksmith et al, 2010) and sediment transport process (Baptist, 2003;Bertoldi, Gurnell, et al, 2009;Gonzalez et al, 2019;Jordanova & James, 2003;Kothyari et al, 2009;L opez & García, 1998). Vegetation is also related to the morphodynamics by governing the braiding intensity (Belletti et al, 2015;Gran & Paola, 2001;Kui et al, 2017;Tal & Paola, 2007), promoting bar accretion and bank stabilization (Bertoldi et al, 2011;Gurnell, 2014).…”