Over the past 40+ years, evolution of water institutions responsible for allocation and distribution of water has been enormous. This paper analyzes the evolution of both formal and informal water management institutions and how they affect today's processes of allocating and distributing water in farmer-managed irrigation schemes (FMISs). It also assesses how farmers translate and modify introduced formal institutions in the rights of the existing informal institution while trying to solve water management challenges based on the local conditions. In-depth and focus group data for the study were collected using a checklist of questions administered to 40 informants, one group discussion per village in Itunundu, Mboliboli, Mkula and Magombera villages, Tanzania. The basic argument of this paper is that water institutions have changed over time. Drawing from study findings, the evolution process of water institutions might be understood as an outcome of the government efforts to address emerging challenges with respect to increasing water demand and multi-use. However, institutional evolution is accompanied by some negatives outcomes, as they weaken social norms and threaten sound water management. The experiences from the irrigation schemes highlight the need to include locally evolved institutions while re-crafting formal institutions. Such interventions may well have significant outcomes for efficient, equity and power relations among water users.