Channel change and sediment movement after a major level drawdown at Kremasta reservoir, Western Greece

A major level drawdown of about 20 m, due to low inflows and reservoir operation, was experienced at the time interval between the hydrographic survey of the Kremasta reservoir (July 1999) and the collection of two sedimentary cores from the reservoir's invert (September 2001). The minimum reservoir level was recorded in December 2000. The sedimentary cores were taken at the Acheloos River mouth within a meandering reservoir section of about 2.3 km distance from each other. At the latter time, a significant channel change was observed at the upstream core cross section with respect to its configuration in July 1999. At the maximum level drawdown, this cross section was actually upstream of the reservoir and the surface of the deposited sediment was exposed while the downstream one was only marginally submerged. This change of the channel profile is attributed to a significant erosion of loose, fine grained, deposited sediment during the first winter intense floods around the time of the reservoirSs minimum level. The channel geometry changed completely and the vertical scour of the deposited sediments was at least 2 m. The eroded sediment was transported downstream to the reservoir's interior and re-deposited according to flow velocity regime. The downstream core is investigated for possible signs of deposition of the eroded sediment. Apart from the analysis of the recent reservoir drawdown, cores were also described and correlated using lithology and sedimentary structures. The sediment deposits are composed of poorly graded sands to low plasticity clays and the horizontal stratification implies major flood events, possibly associated with the historical drawdowns of the reservoir (e.g. silty sand on top of low plasticity clay). The above observations illustrate the dynamic behaviour of the reservoir siltation as a result of reservoir operation, intense flood events and incoming sediment load. The analyses of the sedimentary cores' lithological sections, can serve as a useful tool for reconstructing the siltation history of the Kremasta reservoir.