Attested:  (1) Ptolemy 2,3,4 Τρισαντωνος river mouth
(2)  Possibly Tacitus Annals 12,31 castris antonam emended to cis trisantonam

Where:  (1) The river formerly called Tarrant, but now Arun, in Sussex, which reaches the sea by Littlehampton, at TQ027011.
(2)  The river Trent, variously spelled Treenta, Treanta, Treontan etc by Bede in AD 731

Name Origin:  Greek τρισ ‘threefold, triple’ plus αντιος ‘opposite’ plus the common river-name ending -ona yields a meaning much like ‘three tributaries’.  Much the same could have arisen in Latin and other European languages.  See the discussion under Aventio.

Notes:  The Sussex Arun is formed by convergence of three main rivers at its former limit of navigation (and Roman road junction) near Pulborough: the Rother, the Chilt, and the upper Arun; see map here.  Exactly how it reached the sea in Roman times is uncertain, but the flood-risk map for that area shows two more drainage routes to the sea through the Bognor-Littlehampton conurbation, and two side-streams converging on a point that might have been the head of an ancient estuary.  Three possible ways to explain why Trisantona was a triple!
  Much the same holds for the Trent.  At the Roman-era head of the river Humber three rivers converged: the Trent, the Ouse, and the Don (before it was diverted by the Romans into joining the Aire and thence into the upstream Ouse, as described by Jones, 1995).  Each of those individual rivers results from a fractal pattern of inflowing tributaries, but the Trent has a particularly definite threefold merging of the major rivers Derwent, Soar, and upper Trent roughly where the modern M1 crosses the Trent. 
  This analysis rejects the name division Tri-santona and Celtic origin discussed by Ekwall (1928:415-8) and picked up by R&S pp 476-8, who envisaged a first part meaning ‘very’ or related to ‘trans’ and a second part from PIE *sent- ‘to go’.  Coles (1994) thought that the name Trisantona arose from these rivers' ancient importance as transport routes.  That emendation to Tacitus is highly debatable and Trent might perhaps be linked (etymologically or by later confusion) to torrent, derived from Latin torrens, and possibly referring to tricky tidal currents at Humberhead.

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Last Edited: 14 December 2017