Environmental change and catastrophic flooding in the Voidomatis and Aoos basins, northwest Greece

Catastrophic floods are important events, particularly in the Mediterranean region, where they cause damage to life and property and have major effects on the landscape. Unfortunately, their violence and rarity means that investigating them using conventional hydrological monitoring is often not possible. This study uses palaeoflood hydrological techniques to extend flood records and further our understanding of the occurrence and effects of catastrophic floods in the Mediterranean region. Research has focused on the Voidomatis and Aoos basins; two steepland catchments in northwest Greece. Periods of catastrophic flooding are placed within the context of long-term environmental variability and landscape change, so that the environmental controls on catastrophic flooding, and the effects they have had in the development of these river systems can be fully understood. The late Quaternary behaviour of the Voidomatis River has primarily been controlled by climatic conditions. Uranium-series dating results show that major phases of aggradation took place during the cold stages of the Late Pleistocene. Climatic amelioration in the Lateglacial and the Holocene periods reduced sediment supply and induced progressive incision until the last c. 1000 years or so, when prominent vertical accretion in the contemporary floodplain has occurred. Late Quaternary variations in fine-grained fluvial sediment sources are explored using a quantitative fingerprinting technique. These provenance data are shown to be valuable for reconstructing long-term changes in river activity. Analyses of palaeoflood slackwater deposits provides evidence for the occurrence of two distinct periods of Late Pleistocene catastrophic flooding, at around 21 ka and 17 cal ka. Fine sediment provenance data enable a detailed analysis of catchment conditions at the time of these flood events. Glacial meltwater discharges and general climatic and hydrological instability during the last glacial-interglacial transition were important factors in generating these floods. Palaeoflood information obtained from boulder berms and overbank floodplain sedimentation are combined with the short duration gauged data to develop a record of high magnitude flooding spanning the last 200 years. Catastrophic flooding is shown to reoccur at least every 40 to 50 years. Boulder berms record a decrease in the magnitude of extreme floods over this period, in response to hydrological changes induced by climatic warming following the end of the Little Ice Age (c. 1850 AD). This is one of the first extensive applications of palaeoflood hydrology in the Mediterranean region. High magnitude floods over long timescales are shown to play an important role in the late Quaternary development and contemporary operation of these mountain catchments. This study illustrates the potential value of palaeoflood hydrological techniques to flood prone regions of the Mediterranean, where systematic hydrological data are inadequate for understanding and predicting extreme flood events.