Adjustments in channel form, sediment calibre and vegetation around check-dams in the headwater reaches of mountain torrents, Calabria, Italy

The structure and dynamics of vegetation in valley bottoms are both strongly associated with fluvial processes and landform dynamics. All of these associations are disrupted by the installation of engineering control works. We use survey and analysis methods developed previously to investigate the impact of the installation of check-dams within the confined headwaters of steep seasonally-flowing streams (fiumaras) in Calabria, southern Italy, on active channel form, sediment calibre, and the richness, cover and development of riparian vegetation. Based on detailed field measurements along transects across the active channel, estimates of indices of vegetation extent (GCC), development (WCH) and their cross-sectional variability (coefficients of variation of both indices at each survey site CVGCC, CVWCH), the number of species present (Ns), channel shape (w/d – the width/depth ratio), cross-sectional area (CSA), downstream gradient (slope), surface bed sediment calibre (D50) and subsurface fine sediment content (percentage less than 250 µm by weight) were obtained for 60 transects located immediately upstream (U), downstream (D) and at intermediate sites (I) around 20 check-dams located in four different headwater catchments. Analysis of this data set suggests that statistically significant changes in channel form and sediment calibre upstream of check-dams are associated with more consistent vegetation development across the active channel, including an increase in species richness relative to other transects, but notable increases in vegetation cover and development only arise where the physical characteristics of the channel are notably different from intermediate and downstream channels. Because of the naturally steep profile of the study torrents, intermediate sections between check-dams tend to be more similar in form to channels located immediately downstream of check-dams than those located upstream, leading to similar structural properties in the riparian vegetation. The intermediate transects support considerably more species than downstream reaches, but the conditions upstream of the check-dams appear to be so favourable for riparian vegetation development that species richness exceeds that found in intermediate reaches. Despite the confined headwater locations, these contrasts in form, sediment and vegetation development around check-dams are strong and consistent across the study catchments, over-riding more subtle contrasts in species richness and sediment calibre between catchments.