This paper presents an analysis of the poetry of Guto'r Glyn, one of the most prominent Welsh poets of the medieval period, in order to gain an insight into medieval Welsh society's perception of the fluvial environment and ways in which it appropriated rivers and floods for cultural purposes. Rivers and watercourses were used by poets for a wide variety of different purposes and in many different genres. It is shown that their use in elegies was spearheaded by Guto'r Glyn, who named specific rivers in the context of grief and sorrow and furthered his use of this elegiac topos in his poems of compassion. Rivers were an integral part of a complex cultural landscape of topos and genre and were linked both to Biblical deluges and to myths of submersion. Floodplains were important sites of creativity as well as sites of settlement, cultivation and travel. Such responses point to a cultural heterogeneity that has previously been identified as an important factor in determining the relationship between society and nature. In a future in which the operation of societies on floodplains is likely to be impacted by climate change, such subtle cultural differences may influence the way in which they adapt. It is argued that analysis of historical cultural attitudes to the fluvial landscape, in which extreme events sometimes occurred, may provide evidence of such adaptation and resilience by societies of the past.