Recent studies in human geography and political ecology have examined the anticipatory politics of proposed infrastructure and their effects. Less work analyses how speculation and rumour are mobilised to promote or resist such infrastructures. In this paper, I examine how speculative ideas about long-proposed hydropower dams and transbasin water diversions in the Salween River Basin are strategically deployed by differently placed actors working towards project development or demise. I draw on empirical research on the proposed Yuam River water diversion project in Northwest Thailand and near the Thai–Myanmar border. I develop a conceptualisation of proposed infrastructures as amorphous as they constantly change form and enrol a shifting network of actors, and development plans and imaginaries. The Yuam diversion becomes amorphous through the contested ‘facts’ of the case and over time. I illustrate this using two longitudinal examples. First, infrastructure is made amorphous through protracted development processes for the Yuam diversion and earlier iterations. Second, I examine speculative ideas about how the Yuam diversion is intertwined with proposed dams in the Basin including the Hatgyi Dam in Myanmar. In this case, the lack of confirmed project developers and financiers for the Yuam diversion, and rumours of Chinese actors’ involvement under the Belt and Road Initiative, creates strategic space for project promotion and resistance. What is at stake is not just theorising proposed infrastructures as speculative ideas but how the ‘amorphous’ qualities of infrastructure are deployed strategically, by whom, and to what effect.