Exploring the role of trees in the evolution of meander bends: The Tagliamento River, Italy

Abstract

To date, the role of riparian trees in the formation of scroll bars, ridges, and swales during the evolution of meandering channels has been inferred largely from field observations with support from air photographs. In situ field observations are usually limited to relatively short periods of time, whereas the evolution of these morphological features may take decades. By combining field observations of inner bank morphology and overlying riparian woodland structure with a detailed historical analysis of airborne LiDAR data, panchromatic, and color images, we reconstruct the spatial and temporal evolution of the morphology and vegetation across four meander bends of the Tagliamento River, Italy. Specifically we reveal (i) the appearance of deposited trees and elongated vegetated patches on the inner bank of meander bends following flood events; (ii) temporal progression from deposited trees, through small to larger elongated vegetated patches (pioneer islands), to their coalescence into long, linear vegetated features that eventually become absorbed into the continuous vegetation cover of the riparian forest; and (iii) a spatial correspondence between the resulting scrolls and ridge and swale topography, and tree cover development and persistence. We provide a conceptual model of the mechanisms by which vegetation can contribute to the formation of sequence of ridges and swales on the convex bank of meander bends. We discuss how these insights into the biomorphological processes that control meander bends advance can inform modeling activities that aim to describe the lateral and vertical accretion of the floodplain during the evolution of vegetated river meanders.

Remotely-sensed planform morphologies reveal fluvial and tidal nature of meandering channels

“…In tidal environments, most of the sedimentary products are related to tidal inundation and organic production 10,46 , and a more homogeneous soil texture is expected, owing to the continual sediment redistribution operated by tidal currents and to the quite homogeneous grainsize of the www.nature.com/scientificreports www.nature.com/scientificreports/ transported sediments 47 . In contrast, fluvial floodplains display a higher variability in terms of riparian flora and fauna [48][49][50] , pedogenic processes [51][52][53] , and both floodplain lithology and stratigraphy 5,19,54 . Moreover, self-formed lithological heterogeneities (i.e., generated by meander migration itself) of the plains hosting meandering channels are more pronounced in fluvial than in tidal landscapes, due to the greater capacity that meandering rivers have to autogenically modify their on floodplains (e.g., crevasse splays and oxbow lakes) 5,55,56 .…”

Extending the conceptual model of river island development to incorporate different tree species and environmental conditions

“…In the lower reaches, Pathway (2) may also be active in the tail of scroll bars (elongated pioneer islands that develop around lines of sprouting deposited trees-Pathway (3)) on the inside of meander bends (Zen, Gurnell, Zolezzi, & Surian, 2017;Zen, Zolezzi, Toffolon, & Gurnell, 2016). Gurnell and Petts (2006) extended the island development model by considering the opposing effects of unit stream power and groundwater depth in widening and narrowing reaches of the Tagliamento.…”

Trees and wood: working with natural river processes

“…van de Lageweg et al , ). However, along wooded rivers, large wood is often incorporated into the scrolls (Nanson, ), helping to reinforce and accentuate them, and where living wood is present, tree growth traps further sediment and wood to rapidly produce root‐reinforced ridges (Zen et al ., ). The contribution of living large wood incorporated into developing point bars (Fig.…”