In Arequipa, Peru’s second largest city, engineers work hard to control water flows and provide different sectors with clean and sufficient water. In 2011, only 10 percent of the totality of water used daily by Arequipa’s then close to 1 million people—in households, tourism, industry, and mining—was treated before it was returned to the river where it continues its flow downstream towards cultivated fields and, finally, into the Pacific Ocean. It takes specialized knowledge and manifold technologies to manage water and (...) sustain life in Arequipa, and engineers are central actors for making water flow. Examining the ecology of water management, this article asks to what extent we can talk of a way of knowing and enacting water that is particular to engineers. Through engineering practices, a technical domain emerges as separate from and superior to political and social domains. This production of categories can be understood as practices of purification. However, a purely technical grip on water is never possible. Unruly elements, like weather, contamination, urban dwellers, and competing interests, interfere and make processes of intervention unstable. Water is never completely cleaned, and, equally, the continuous processes of purification of categories and domains take place while other processes work to blur their boundaries. (shrink)