The topographic signature of vegetation development along a braided river: Results of a combined analysis of airborne lidar, color air photographs, and ground measurements

Abstract

This paper combines archived remotely sensed data (airborne lidar and digital color air photographs) with nonsynchronous ground observations (including observations of topographic form and vegetation cover and growth) to test the hypothesis that colonization of exposed river sediments by riparian trees has an impact on channel form and to quantify any impact that is identified. This is achieved along a 21 km reach of the braided, gravel bed Tagliamento River, in northeast Italy, where the width of the braided corridor typically exceeds 800 m. Lidar data are analyzed to extract a 2 m resolution digital evolution model (DEM) and determine riparian vegetation extent, height, and structure within the active corridor. Aerial photographs are used to map the topography of the submerged parts of the corridor. These data are divided into 1 km length subreaches, which possess strong contrasts in vegetation height and extent. Joint analysis of vegetation and morphological properties of these subreaches reveals significant associations between vegetation properties and reach morphology. Residuals from a gamma function fitted to the topographic data for each subreach show a good fit with poorly vegetated reaches, but a weakening fit with increasing vegetation cover, largely as a result of the appearance of secondary peaks in the elevation frequency distribution associated with the heavily vegetated areas. Furthermore, the overall skewness and kurtosis of the elevation frequency distribution within each of the subreaches are both significantly correlated with vegetation extent, height, median elevation, and growth rate, indicating a clear topographic signature of vegetation development along this braided river that reflects sediment accumulation within and around the vegetated patches.