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The present study deals with reconstruction of the ancient courses of the Tigris and the Euphrates in the Mesopotamian floodplain, which covers most of the central and southern parts of Iraq. The focus is on tracing palaeochannel courses, determining when these palaeochannels were active, and understanding the patterns of avulsion and its impact on human settlements of ancient civilisations. The research was carried out using a combination of geological, geomorphological, remote sensing, historical and archaeological approaches. Fieldwork included “groundtruthing” of the remote sensing work. A total of thirty seven boreholes were dug, sedimentary and geomorphologic documentation has been carried out, and twenty five shell samples were collected, and analysed by radiocarbon dating. This study has reconstructed palaeochannels and archaeological sites within the area of southern Mesopotamia; intensive networks of palaeochannels and archaeological sites within the study area have been identified. More than eight thousand archaeological sites have been plotted during this study, and most of them show a location and alignment consistent with an identified palaeochannel. Eleven major river avulsions and their nodes have been identified, five for the Euphrates and six for the Tigris. It has been found that these avulsions contributed to the shaping, formation and aggradation of both the ancient and present–day landscapes of the floodplain. Two kinds of avulsion have taken place in the floodplain, re-occupational and progradational. In the first of these types of avulsion, the major flow diverted into a previously existing channel. In contrast, the progradational avulsion began by inundating a large section of the floodplain between elevated ridges, producing prograding deposits that filled topographic lows of the floodplain. These avulsions have affected the distribution, flourishing and degradation of human settlements of the southern Mesopotamian civilisations. The present study has demonstrated how human impact played a leading role in distribution of sediments across the floodplain and shaping both the Holocene and the recent landscapes of the Mesopotamian floodplain. By using periods of human occupation of archaeological sites to date associated palaeochannels, we can get acceptable accuracy on their timing and duration, and can give clear indications about the activity of a given channel.