In this article, we explore a contrast between terrestrial and amphibious ways of imagining and intervening in deltas, which have given rise to contrasting delta ontologies. Whereas the former originated in Europe and focused on removing water for agriculture, the latter conceived of deltas as extending water flows. In Thailand’s Chao Phraya Delta these incongruent approaches have inspired very different forms of infrastructural development over the last century. Examining the entwined histories of agency of people—engineers, scientists, traders, and kingdoms—and non-humans, such as canals, dikes, and landscapes, we trace how the delta’s ontology was transformed by the gradual layering of partly incompatible infrastructures. In light of increasing floods, the continued sustainability of Bangkok may now depend on amphibious infrastructures lying half-forgotten within this ontological palimpsest.