This chapter focuses on the idea of rivers as frontier or boundary, beginning with stories from Late Antiquity, and including a discussion of the ways that river frontiers are porous and permeable. A case study of the monastery of Prüm shows the complexity of these dynamics. The main focus of the chapter is on the impact of the Viking raids on medieval monastic writers, and how their stories about this moment created a new view of rivers as sites of danger and disruption. Then, the chapter explores ways that rivers were seen as sites of destruction and oblivion – an alternate to the idea of rivers and memory explored in the previous chapter. Finally, it looks at how monastic communities reinvented their histories in the wake of the Viking attacks, and how this helped them in turn to reimagine and restore their relationships with rivers.