Riparian Vegetation and Sediment Supply Regulate the Morphodynamic Response of an Experimental Stream to Floods

Abstract

Feedbacks between woody plants and fluvial morphodynamics result in co-development of riparian vegetation communities and channel form. To advance mechanistic knowledge regarding these interactions, we measured the response of topography and flow to the presence of riparian tree seedlings with contrasting morphologies in an experimental, field-scale, meandering stream channel with a mobile sand bed. On a convex point bar, we installed seedlings of Tamarix spp. (tamarisk) and Populus fremontii (cottonwood) with intact roots and simulated a bankfull flood, with each of eight runs varying sediment supply, plant density, and plant species. Vegetation reduced turbulence and velocities on the bar relative to bare-bed conditions, inducing sediment deposition when vegetation was present, regardless of vegetation density or species. Sediment supply also played a dominant role, and eliminating sediment supply reduced deposition regardless of the presence of vegetation. Unexpectedly, plant density and species architecture (shrubby tamarisk vs. single-stemmed cottonwood) had only a secondary influence on hydraulics and sediment transport. In the absence of plants, mobile bedforms were prominent across the bar, but vegetation of all types decreased the height and lateral extent of bedforms migrating across the bar, suggesting a mechanism by which vegetation modulates feedbacks among sediment transport, topography, and hydraulics. Our measurements and resulting insights bridge the gap between laboratory conditions and real dryland sand-bed rivers and motivate further morphodynamic modeling.

Influence of Rigid Emerged Vegetation in a Channel Bend on Bed Topography and Flow Velocity Field: Laboratory Experiments

“…However, velocity in the downstream direction decreased in the area close to the bed for both protected and non-protected bed conditions. The results are in good agreement with the findings of Bywater-Reyes et al [58] and Lightbody et al [59], who reported that dense configurations of vegetation alter flow velocities to a significantly greater degree than sparse or no vegetation conditions.…”

An Ecogeomorphic Framework Coupling Sediment Modeling With Invasive Riparian Vegetation Dynamics

“…The analysis presented here of how sediment balance (as expressed by CSR) relates to channel and floodplain geomorphic change at different time scales complements laboratory studies investigating how sediment balance mediates ecogeomorphic feedbacks. For example, Lightbody et al (2019) found that topographic change on bars was insensitive to vegetation density and morphology under sediment-deficit conditions (high CSR), in contrast to the strong influence of vegetation on deposition observed under equilibrium supply. Diehl, Merritt, et al (2017) and also documented differences in how vegetation size and morphology influenced bar-surface topography for sediment deficit versus equilibrium conditions.…”

“…For example, Lightbody et al. (2019) found that topographic change on bars was insensitive to vegetation density and morphology under sediment‐deficit conditions (high CSR), in contrast to the strong influence of vegetation on deposition observed under equilibrium supply. Diehl, Merritt, et al.…”

Deriving Planform Morphology and Vegetation Coverage From Remote Sensing to Support River Management Applications

“…This signifies a loss of structural complexity, a key characteristic of less anthropized systems. Vegetation has a recognized feedback on fluvial morphodynamics, influencing the patterns of sediment deposition and modifying the channel pattern toward a single-thread (Tal and Paola, 2007;Bertoldi et al, 2014;Lightbody et al, 2019). The planform changes for the studied reach of the Po River are typical of anthropogenically impacted large rivers, which usually have an oversimplified planform morphology, banks mostly covered by stable vegetation and a relatively deep main channel as a consequence of the low variability of the hydrological regime (Pettit et al, 2001;Camporeale and Ridolfi, 2006;.…”