Spatial variability of channel changes in a Mediterranean ephemeral stream in the last six decades (1946-2006)

Abstract

An analysis of morphological changes during the last six decades is presented for the Rambla de Cervera, a Mediterranean ephemeral stream located in the eastern sector of the Iberian Mountain Range. The studied channel (16.5 km) has two contrasted sectors: a horst confined sector (5.5 km) and a graben sector (11 km). Channel changes were analysed through the analysis of aerial photographs, with geographical information systems (GIS) and comparison of topographic surveys made with a GPS-RTK. Between 1946 and 2006 the Rambla de Cervera, underwent channel adjustments comparable to those observed in other Mediterranean rivers. The gravel channel markedly narrowed, losing width (68.5%) and channel surface area (45.7%), and incision processes occurred along the entire study reach at an average depth of 3.5 m. The progressive reduction of discharge and sediment supply due to land use changes, gravel mining and hydroclimatic variability has been considered responsible for these changes. However, these morphological changes exhibit an interesting temporal variability, with a maximum decrease of the gravel channel in the period 1946-1956 and another clear narrowing stage between 1977 and 1991, whereas incision mainly progressed after 1977. In addition, a contrasted behavior between the graben and horst sectors has been observed and analyzed. The horst sector underwent a constant and continuous trend, whereas the graben sector had more intense changes and two stages with significant reverse trajectories. The differential behavior of the graben and horst sectors was caused by the different responses generated by the overlapping of the lack of sediment supply, the timing and intensity of flood events and the particular hydraulic conditions of both sectors. The hydraulic geometry of the different river sections conditioned the effectiveness of flood events, influencing on the existence or absence of floodplain destructive trajectories and on the stability of fluvial forms.