Over the nineteenth century, England and Portugal contested the possession of Delagoa Bay in south eastern Africa. Using documents from a British hydrographical survey mission of the east coast of Africa, and British Parliamentary Papers recording the dispute over possession of the bay in the 1870s, I demonstrate that bordermaking in the region was rooted in interimperial claims and disputes reaching back to the 1820s. The English claim to Delagoa Bay highlights how hydrography and cartography were used as tools of empire in the new imperial expansion and land appropriation of the nineteenth century. The production of the colonial border between Portuguese Mozambique and British South Africa interrupted and disconnected indigenous networks of relation, trade, and movement, while simultaneously entangling imperial projects of white worldmaking through racialized dispossession.