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, the taxonomic study of place-names, is the study of the names of bodies of water, the origins of those names, and how they are transmitted through history. Hydronyms may include the names of rivers (potamonyms), lakes, and even oceanic elements.
Compared to most other toponyms, hydronyms are very conservative linguistically, and people who move to an area often retain the existing name of a body of water rather than rename it in their own language. For example, the Rhine 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 even more conservative than the local names of small streams.
Therefore, hydronomy may be a tool used to reconstruct past cultural interactions, population movements, religious conversions, or older languages. For example, history professor Kenneth H. Jackson identified a river-name pattern against which to fit the story of the Anglo-Saxon invasion of Britain and pockets of surviving native British culture. His river map of Britain divided the island into three principal areas of English settlement: the river valleys draining eastward in which surviving British names are limited to the largest rivers and Saxon settlement was early and dense; the highland spine; and a third region whose 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: ་, 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. And the Tibetan term Za Qu (rdza chu) refers to three other rivers as well.
Hydronyms from various languages may 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). A similar suggestion is that the Yarden, Yarkon, and Yarmouk (and possibly, with distortion, Yabbok and/or Arnon) rivers in the Israel/Jordan area contain the Egyptian word for river (itrw, transliterated in the Bible as ye'or).
It is also 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 originally was called An Ruirthech. An unusual example is the River Cam, which originally was called the Granta, but when the town of Grantebrycge became Cambridge, the river's name changed to match the toponym.
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