The underlying theme of this essay is how intelligence was gathered and expertise dispersed in an emerging colonial environment in Africa, and how that knowledge was captured, credited and distributed between local Africans and (largely) itinerant Europeans. It sets that discussion within a more recent debate on the mechanics of European exploration during the wider nineteenth century. The expanded population of Europeans (officials, merchants, missionaries) that arrived in the later part of that century to consolidate the colonial enterprise in German East Africa often moved with initial uncertainty through the landscape, triggering a demand for topographical knowledge to become commodified and commercialised, to become less dependent on the knowledge of individuals. This demand fuelled the production of an innovative series of standardised grid maps. At a time when slavery was still legal, when the local workforce was increasingly discussed in colonial circles in terms of unskilled plantation labour, our essay explores two case studies that demonstrate how certain African experts came to exert key technical and management influence within long-term scientific and commercial projects unfolding in the southeast corner of what is today Tanzania. The matter of water flows through this essay, and does so with deliberate intent.