Full text can be accessed at http://www.sciencedirect.com/science/article/pii/S2212608213000028How does water governance change over time? What are the outcomes for smallholders and the ecosystems that support them? We review the development of water resources management over the past 40 years in the Kimani catchment of the Usangu plains, in southwestern Tanzania. Our analysis is based on a conceptual framework for water governance comprising a system of resources, arrangements for access, and outcomes for people and ecosystems. We discuss how the resources for water governance have changed over time, we consider the changing arrangements for water allocation, particularly relating to water rights, water management organisations and physical infrastructure, and we analyse the outcomes in terms of access to water for people and of maintenance of environmental flows. Development of water resources in Kimani has been successful on many counts, resulting in assured supplies for many users, with consequent improvements in livelihoods, including, in particular, increases in land-holding size. However, these improvements are accompanied by some negative outcomes, as they reduce water access for other users and threaten downstream flows. The experiences from Kimani highlight the need to map the whole institutional landscape and to ensure that physical infrastructure reflects institutional arrangements when designing interventions to enhance water security. Such interventions may well have significant outcomes for equity and power relations amongst water users.