This article investigates hydrological lexicon in Jahai, a Mon-Khmer language of the Malay Peninsula. Setting out from an analysis of the structural and semantic properties as well as the indigenous vs. borrowed origin of lexicon related to drainage, it teases out a set of distinct lexical systems for reference to and description of hydrological features. These include (1) indigenous nominal labels subcategorised by metaphor, (2) borrowed nominal labels, (3) verbals referring to properties and processes of water, (4) a set of motion verbs, and (5) place names. The lexical systems, functionally diverse and driven by different factors, illustrate that principles and strategies of geographical categorisation can vary systematically and profoundly within a single language. (Contains 2 figures and 1 table.)