Riverscapes downstream of hydropower dams: Effects of altered flows and historical land-use change

Abstract

Dams strongly impair the fluvial environment by altering downstream flows. We analysed riverscapes downstream of three dams and hypothesized that different dam types in rivers with diverse history of land-use and land cover (LULC) change have significant riparian cover differences at diverse biogeomorphic units (banks, riverbanks, islands). We performed a temporal comparison using pre-dam (1965) and post-dam (2013) high-resolution airborne imagery. A new approach was devised to correct the spatial offset between historical and contemporary imagery. Riparian vegetation and LULC (200m-buffer) were mapped in three rivers of Portugal regulated by the dams Touvedo (run-of-river), Vilarinho das Furnas and Fronhas (storage reservoirs). Five landscape metrics, measures of shape complexity, area and edge effect of riparian patches were computed, including the Weighted Class Area, a metric developed to better interpret the landscape variation. Our findings provide support for the hypothesis of highly altered riverscapes in the post-dam period. For all case studies riparian patches are presently larger, but with less complex shapes and smaller edges. In the present study riparian patches encroach into the river channel, occupy more area and are larger at the two reservoir rivers than at the run-of-river setting. Riparian growth trajectory at the latter is mainly outwards from the active channel and non-vegetated areas in riverbanks and banks are significantly larger; likely due to the washing flows. Redundancy analyses indicated that riparian change was driven by both LULC (agricultural land abandonment and unmanaged forests) and hydrological alterations that jointly determine the structure and spatial trajectories of riparian expansion.