Two contemporary theories of river basin management are compared. One is centralised 'regulatory river basin management' with an apex authority that seeks hydrometric data and nationally agreed standards and procedures in decisions over water quality and allocation. This model is commonplace and can be identified in many water training curricula and derivatives of basin management policy. The other, 'polycentric river basin management', is institutionally, organisationally and geographically more decentralised, emphasising local, collective ownership and reference to locally agreed standards. The polycentric model is constructed from the creation of appropriate managerial subunits within river basins. This model emphasises the deployment of hydrologists, scientists and other service providers as mediating agents of environmental and institutional transformation, tackling issues arising within and between the basin subunits such as water allocation and distribution, productivity improvement and conflict resolution. Significantly, it considers water allocation between subunits rather than between sectors and to do this promulgates an experimental, step-wise pragmatic approach, building on local ideas to make tangible progress in basins where data monitoring is limited, basin office resources are constrained and regulatory planning has stalled. To explore these issues, the paper employs the 'Cathedral and Bazaar' metaphor of Eric Raymond. The discussion is informed by observations from Tanzania, Nigeria and the UK.