The persistence of the Zanj Rebellion against the ?Abbasid caliphate has frequently been attributed to the inaccessibility of the marshlands of southern Iraq. The historiography has focused on other issues, however: to what extent it was a “slave revolt”; the rebels’ ethnic origins; class, religion, and ideology; its importance for the disintegration of the ?Abbasid empire. Sources for the revolt are both limited—al-?abar??s history, some remarks in al-Mas?udi’s Meadows of Gold, two coins, and references in later writers which add little to our knowledge—and yet extensive, given the great detail of al-?abar??s account. The high salinity and frequent erosion and deposits of land by water in the marshlands (ba???i?) of southern Iraq mean that the site of the Zanj capital, al-Mukht?ra, is unlikely ever to be discovered. Yet this archaeological futility provides a key to rethinking the revolt using existing sources. The hydropolitics of the revolt can be mapped by drawing on approaches to materiality and non-human agency developed in STS, and on the anthropology and history of water, deltas, and marshlands. Structurally speaking, the marshes have long been a major agent in local resistance against centralized control, both because of their inaccessibility and because of the resources they provide, whether from alluvial agriculture, fishing, or the waters themselves. Nonetheless, as a space of transport, the region was also central to the trade routes, via the Persian Gulf to the Indian Ocean, whose control was important to authorities and elites upriver in Baghdad and Samarra. Joining structure and conjuncture, a major factor in the revolt was the hydrogeology of the region. Potentially fertile lands were covered with natron (sib?kh), a mix of types of sodium carbonate and related salts, removal of which was the task of the Zanj slaves mobilized by landowners, drawing on the approval provided by Islamic jurists for the cultivation of “dead lands”. The hydropolitics of the marshlands was thus also a “natropolitics”, joining water, sodium salts, property relations, ideological justifications, and forced labor in an attempt to fix and render profitable a fundamentally fluid ecology. Finally, the actual fighting was itself profoundly structured by water: it provided a battle “field”, whether the marshes or the canals of Basra; acted as an obstacle to the combatants; and was deployed as a weapon when one side diverted or obstructed waters to encumber its adversaries.