Transboundary risks, like all risks, are socially constructed, which means that the orthodox management approach, based on the clear separation of facts and values, cannot be followed. What the transboundary risks really are, and what people variously perceive them to be, can never be fully and unambiguously distinguished, and this means that we need an approach that treats them for what they are: an entanglement of facts and values. In this chapter we try to develop this approach (drawing on cultural theory) by focusing on the dynamics of trust and mistrust that are generated by the interactions between the physical flows and the human inhabitants of the southern slopes of the Himalaya. We begin by discussing the interplay of water and social life and then focus on two specific sets of risks within that interplay: high dams and unwelcome silt.