The Afon Tywi is one of the most dynamic rivers in Wales and sediments stored in its valley floor record a history of erosion, deposition and flooding dating back to the end of the last glaciation around 16000 years ago. Analysis of floodplain sediments has provided information about how climate and land cover change have affected river behaviour during past millennia. Migration of the Afon Tywi across its floodplain, punctuated by shorter episodes of river down-cutting, has formed a staircase of terraces, the oldest of which probably formed more than 12000 years ago and the most recent in the last 200 years or so. Radiocarbon dating of organic material found in sediments within former river channels indicates that since c. 3000 BC the River Tywi has down-cut by nearly 4 metres and has eroded, swept and sedimented almost its entire valley floor. Periods of rapid river down-cutting and channel straightening at shortly after AD 1220, 1940 BC and 3020 BC would appear to be associated with major floods and climate change. The historical period (c. 1750 to present) has been characterised by renewed channel bed incision and lateral channel migration leading to the development of multiple meander cut-offs. Major changes in river behaviour have influenced human activity and settlement patterns in the Tywi Valley. The history of Late Quaternary valley floor development has significantly influenced the locations where both prehistoric and historical archaeological finds and sites have been preserved. The destruction and replacement of older valley floor sediments as a result of lateral channel migration has undoubtedly led to the loss of archaeological resource in the middle Tywi valley. However, vertical incision and the burial of sites and artefacts on valley floors through the deposition of overbank sediment, although not destroying the archaeological resource has probably significantly reduced the likelihood of discovery of features and artefacts. In spite of this, some of the older terraces preserve remains of occupation dating from as far back as the Bronze Age.