The collective letters analyzed in this article are a response to hydroextractivism, which uses hydrological control and extraction to convert cultural rivers into decultured waters, transforming them into a neoextractivist commodity. As dams, mining, agribusiness, and cattle ranching infringe on the Amazon River basin, in territorial defence the Munduruku use resistance strategies including collective letters attacking the epistemic and ontological foundations of neoextractive, government-promoted Amazonian “development”, while presenting a counter-narrative insisting on their own cosmology and historical, cultural occupation of their territory. Using an Environmental Humanities approach, based on Munduruku collective letters authored primarily by Ipereğ Ayũ, I identify 19 repeating rhetorical features, including perspectivist representations of non-humans and ancestral spirits. Instead of underlining biological harms of the Teles Pires and São Manoel dams per a Western ontology, the letters instead emphasise cultural harms, creating narratives that insert cosmological entities from oral traditions into current cosmopolitics and a growing authority of women and shamans. I focus on letters tracing the Munduruku attempt to achieve the return of Itiğ’a (urns) removed during dam construction. I discuss Munduruku authorship-authority, antagonism-persuasion, and translation of the sacred from a perspectivist to non-perspectivist audience. I argue self-published Indigenous letters are part of a literature of territory from peoples confronting deterritorialisation that deserves publication and allyship-scholarship in Literature and other fields.