This thesis is about a number of rural communities resisting flooding and eradication of their ancestry, history and culture by opposing the implementation and imposition of a large dam in rural Mexico that would supply water to two large cities. The importance of this case lies in the unlikely odds not only of resisting a State-led, large-scale infrastructure project for almost two decades, but to actually build up a grassroots movement that grew in extent, scope and scale to claim for a comprehensive water management transformation in Mexico. The scientific analysis of the conflict and this grassroots movement, informally dubbed ‘Temaca’, contributed to several scientific disciplines including water conflicts, science-policy processes and transdisciplinary action research. The analysis of the case study shows how politics influences science by determining a limited decision space that can only superficially address the serious water problems of large cities. As a result, cities continue a development pathway that may deepen their water problems in the long term. Therefore, water conflicts and grassroots movements play a crucial role in opening the decision space. This thesis demonstrates that through transdisciplinary action research scientific knowledge can become actionable and relevant; addressing power asymmetries and finding sound alternative water management solutions that are more equitable and sustainable.