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In instances where vegetation plays a dominant role in the riparian landscape, the type and characteristics of species, particularly a dominant invasive, can alter water velocity at high flows when vegetation is inundated. However, quantifying this resistance in terms of riparian vegetation has largely been ignored or listed as a secondary characteristic on roughness reference tables. We calculated vegetation roughness based on measurements of plant stem stiffness, plant frontal area, stem density, and stem area of three dominant herbaceous plants along the Sprague River, Oregon: the invasive reed canarygrass, native creeping spikerush, and native inflated sedge. Results show slightly lower roughness values than those predicted for vegetation using reference tables. In addition, native creeping spikerush and invasive reed canarygrass exhibit higher roughness values than native inflated sedge, which exhibits values lower than the other two species. These findings are of particular importance where the invasive reed canarygrass is outcompeting native inflated sedge, because with invasive colonization, roughness is increasing in channel zones and therefore is likely changing channel processes. Direct depositional measurements show similar results.

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