Bed-material mobility in a large river below dams

The mobility of river-bed surface particles is examined in the lower Ebro River downstream from dams that were closed several decades ago. Mobility is analysed by comparing photographs of painted bed areas taken before and after the 2003-2004 flood season, and also by comparing lines of painted particles after the floods. The peak of the maximum flood had a recurrence interval of 2 years. Medium and coarse gravels were widely exported from all the study reach, reflecting a disequilibrium between the competence of the flow to entrain and transport them and the rate of supply from upstream. By contrast, very coarse gravels and larger particles remained immobile, since floods were not competent enough to mobilise them. The partial entrainment of surface particles caused a coarsening of the river-bed surface, even in areas where sediments are loose and the river-bed is highly active during small floods.