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Vegetation along stream banks has the ability to change channel morphology characteristics by altering velocity patterns and changing deposition and erosion. Recent research has focused on quantifying the effects of woody, rigid species (Petryk and Bosmajian 1975, Pasche and Rouve 1985, Musleh and Cruise 2006), rather than flexible vegetation (Kouwen and Unny 1973, Kouwen, Unny, and Hill 1969, Carollo, Ferro, and Termini 2005) or a combination of the two (Freeman, Rahmeyer, and Copeland 2000, Järvelä 2004). However, in low-gradient streams, dominant flexible vegetation can affect deposition when little to no rigid species are present. Here we examine the impact of the invasive and flexible Phalaris arundinacea L. (reed canarygrass) on the channel hydraulics of the low-energy, meandering Sprague River in eastern Oregon by modeling its effect on velocity, water depth, and bed shear stress within the 2-D model Nays2DH (McDonald, Nelson, and Bennett 2005). In addition, we compare its geomorphic effect to that of native Eleocharis palustris (creeping spikerush) and Carex vesicaria (inflated sedge).
Investigating the effect of invasive riparian vegetation is critical because invasive species can cause major shifts in ecosystem dynamics, outcompete native species, and produce many other changes of which we are still not yet aware. This is of particular importance when the species has the potential to become an ecosystem engineer, causing changes in biotic or abiotic materials (Jones, Lawton, and Shachak 1997, Jones, Lawton, and Shackak 1994, Corenblit et al. 2008). Within fluvial systems, vegetation can be an ecosystem engineer when it passively modifies the environment by providing roughness within the channel bed or along the banks, decreasing flow velocity and increasing sediment deposition (Corenblit et al. 2009).
Roughness can manifest itself and alter the river system in multiple ways with regard to flexible vegetation. Macrophyte shoot density and shape influence flow velocity and therefore, deposition (Schulz et al. 2003, Clarke, Lake, and O'Dowd 2004, Asaeda, Rajapakse, and Kanoh 2010). For example, submerged Sparganium erectum shoots were found to increase fine sediment deposition within the vegetation stand. Following increased growth in the spring, the shoots emerged from the water column which raised the water level and subsequently reduced water velocities (Asaeda, Rajapakse, and Kanoh 2010). Gurnell et al. (2010) examined multiple macrophytes across 467 British rivers and found they have the ability to trap fine sediment and cause channel form changes. Within Callitriche cophacarpa stands, Sand-Jensen and Mebus (1996) found an eleven-fold decrease in stream velocity due to the establishment of dense stands and shoots.
Regarding bank and island vegetation, on the Northern Plains of Australia, various grasses along channel banks and floodplains were found to provide higher levels of stability and erosion protection than trees and shrubs in the area due to their substantial biomass and expansive root networks (Tooth and Nanson 1999). Carex nudata in California creates substrate upon which other species can colonize as well. Although Carex occupied only 33.05% of the channel area on the South Fork Eel River, California, the 13 most common species had an average of 84% of their individuals on Carex tussocks (Levine 2000).