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

[Display omitted] •Riparian vegetation cover increased in riverbanks and banks in the post-dam period.•Riparian patches downstream of dams were larger, but with lower spatial complexity.•Riparian cover at the two reservoir rivers was greater than at the run-of-river setting.•Riparian vegetation had...

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Bibliographic Details
Published in:Landscape and urban planning 2016-09, Vol.153, p.83-98
Main Authors: Aguiar, Francisca Constança, Martins, Maria João, Silva, Pedro Cristiano, Fernandes, Maria Rosário
Format: Article
Language:English
Subjects:
abandoned land
agricultural land
case studies
Channels
Dams
dams (hydrology)
edge effects
forests
Historical imagery
hydrology
Imagery
islands
land cover
Land use
land use change
Land-use and land cover change
Landscape metrics
Landscapes
Reservoirs
Riparian cover change
Rivers
Run-of-river dams
Storage reservoirs
Trajectories
washing
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Summary:[Display omitted] •Riparian vegetation cover increased in riverbanks and banks in the post-dam period.•Riparian patches downstream of dams were larger, but with lower spatial complexity.•Riparian cover at the two reservoir rivers was greater than at the run-of-river setting.•Riparian vegetation had different growth trajectories depending on type of dam-induced hydrologic alterations.•Riparian changes were driven by LULC change and hydrology with complex interactions. 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.
ISSN:0169-2046
1872-6062
DOI:10.1016/j.landurbplan.2016.04.009