A hydronym (from Greek: ὕδωρ, hydor, "water" and ὄνομα, onoma, "name") is a proper name of a body of water. Hydronymy, a subset of toponymy or the taxonomic study of placenames, is the study of the names of bodies of water, the origins of those names and how they are transmitted through history. Hydronyms can include the names of rivers, lakes, and even oceanic elements.
More than most toponyms, as linguistic items hydronyms are very conservative, with people who move into an area often retaining the existing name of a body of water rather than renaming it in their own language. For example, the Rhine river in Germany bears a Celtic name, not a German name.  The Mississippi river in the United States bears an Anishinaabe name, not a French or English one. The names of large rivers are especially conserved, while the local names of small streams are less so.
Therefore, hydronomy can be a tool to reconstruct past cultural interactions, population movements, religious conversions or older languages. For example, history professor Kenneth Jackson identified a river-name pattern against which to fit the story of the Anglo-Saxon invasion of Britain and the pockets of survival of native British culture. His river map of Britain divided the island into three principal areas of English settlement: the river valleys draining eastward where surviving British names are limited to the largest rivers and Saxon settlement was early and dense, the highland spine, and a third region where British hydronyms apply even to the smaller streams.
Often a given body of water will have several entirely different names given to it by different peoples living along its shores. For example, Tibetan: རྫ་ཆུ་, Wylie: rDza chu, ZYPY: Za qu and Thai: แม่น้ำโขง [mɛ̂ː náːm kʰǒːŋ] are the Tibetan and Thai names, respectively, for the same river, the Mekong, in southeast Asia.
Hydronyms from various languages can all share a common etymology. For example, the Danube, Don, Dniester, Dnieper and Donets rivers all contain the Scythian name for "river" (cf. don, "river, water" in modern Ossetic).
It is possible for a toponym to become a hydronym: for example, the River Liffey takes its name from the plain on which it stands, called Liphe or Life; the river itself was originally called An Ruirthech. An unusual example is the River Cam - it was originally called the Granta, but when the town of Grantebrycge became Cambridge, the river's name changed to match the toponym.
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- Jackson, Language and History in Early Britain, Edinburgh, 1953:220-23, summarized in H.R. Loyn, Anglo-Saxon England and the Norman Conquest , 2nd ed. 1991:7-9.
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