The case study describes large-scale environmental change related to, and recent responses associated with, growing water scarcity in the Usangu Plains, a catchment of the Great Ruaha River in south-west Tanzania. The analysis uses outputs from two recent projects to critically examine various theories of environmental change and the ‘fit’ of new river basin management strategies to the problems found, arguing that various perspectives are worryingly at odds with each other. We find that the investigators of the two projects presented a reasonable and sufficient case of the causes of water scarcity. Yet despite efforts to disseminate scientific findings, different stakeholder groups did not agree with this case. This, we believe, was due to three combined factors; firstly highly entrenched views existed that were also based on quasi-scientific reasoning; secondly, the projects’ deliberations to date, in acknowledging their own uncertainty, were not assertive enough in ascribing causation to the various processes of change; thirdly, policy-uptake was not sufficiently managed by the scientists involved. We conclude that this complexity of the science–policy interface is a feature of integrated water resources management (IWRM) and that the norms of scientific uncertainty in the face of competing theories (held by their protagonists with greater certainty) obliges scientists to take a more active role in sensitively managing the advice-to-policy process in order to improve management of water within river basins. Thus, the paper argues, the nature of integrated water resources management is one of ‘action research’ to move towards an improved understanding of change, and of ‘action policy-advising’ to draw policy-makers into a cycle of considered decision-making.