Sediment size variation in torrents with check dams: Effects on riparian vegetation

Abstract

Previously, we investigated the response of riparian vegetation to the installation of check dams along ephemeral Mediterranean rivers and we demonstrated significant changes in the form and structure of riparian vegetation around check dams that appeared to reflect changes in flow hydraulics and related fluvial sediment structures. In this paper we investigate the average properties of both bed sediment and riparian vegetation along transects immediately upstream, downstream and between three check dams along a single torrent headwater reach. Whilst there was overall sediment fining downstream through the study reach, the installation of check dams induced distinct local deviations from this downstream trend in both surface and subsurface sediments as well as in three synthetic parameters of the riparian vegetation (global canopy cover, weighted canopy height, species richness). No simple association was found between surface and subsurface bed sediment characteristics. The potential importance of fine sediment for water retention capacity in the subsurface was demonstrated through the application of a simple pedotransfer function. When transect properties of bed surface sediment (D 50) and subsurface sediment (percentage finer than 0.25 mm by weight) were jointly analysed with the three synthetic vegetation parameters using principal components analysis, the first two principal components accounted for 95% of the variation in the data, all of the variables apart from D 50 were heavily positively loaded on the first principal component (74% of the variation in the data set), whereas D 50 alone was heavily loaded on the second principal component (22% of the variation). Upstream transects were all strongly positively associated with the first principal component illustrating that a more diverse riparian vegetation cover with a better-developed and more complex canopy in this upstream location was strongly associated with higher proportions of fine subsurface sediments and had no association with the calibre of surface sediment. There were also highly significant positive simple correlations between all three synthetic vegetation parameters as well as both the percentage of fine sediment and estimated water retention capacity in the subsurface sediment across all transects. A description of typical changes in channel morphology around check dams places these significant associations between riparian vegetation and bed sediment into a broader environmental context.