Impacts of Vegetation on Dryland River Morphology: Insights from
Spring-Fed Channel Reaches, Henry Mountains, Utah
Abstract
A better understanding of how vegetation influences alluvial channels
could improve (a) assessments of channel stability and flood risks, (b)
applications of vegetation as a river management tool, and (c)
predictions of channel responses to climate change and other human
impacts. We take advantage of a natural field experiment in the
semi-arid to arid Henry Mountains, Utah, USA: Large spatial differences
in bed and bank vegetation are found along some alluvial channels due to
localized perennial springs caused by aquicludes in the underlying
bedrock. Airborne LiDAR topography and flood modeling are used to
constrain channel morphology, vegetation density, and flow velocity at
different flood discharges for three spring-fed reaches along
intermittently-flowing streams. The spatial distribution of vegetation
quantitatively influences both the magnitude and direction of channel
adjustment. Reaches with abundant bed vegetation are
significantly wider (by an average of ≈ 50%), with shallower flows and
lower velocities, than reaches with little bed vegetation. Reaches with
dense channel bank vegetation are ≈ 25% narrower and ≈ 25%
deeper than sparsely-vegetated reaches. We interpret that sediment grain
size influences the spatial distribution of vegetation within spring
reaches, but that bank vegetation may be more important than grain size
for “threshold” width adjustments. Widths, depths and velocities are
fairly insensitive to whether local hydraulic roughness is parameterized
in terms of local vegetation density or is assumed spatially constant,
suggesting that the underlying “bare earth” topography of the channel
bed, banks and floodplain exerts more control on local flow than does
local vegetation density.