I suggest that it may be from a prior, unstated, word, ending in T - which may mean slow-flowing or dark river. The "T" may be a Brythonic elision - where the second word changes so that the words "elide" together. There are so many examples that there must be a good reason - Trent, Thame, Thames, Till, Tamar, Torridge, Tyne. And, of course, rivers had names before communities did; the names were given 3000+ years ago.
Some Rivers' names that start with 'I' are: * Illinois River * Iowa River * India River * Indus * Irrawaddy
Lyne River in England, Li River in China and Lachlan River in Australia are famous water channels. They begin with the letter L.
Boy names that start with the letter G:GabrielGaryGavinGeorgeGeraldGilbertGlennGordonGregGregoryGriffinGusGuyGirl names that start with the letter G:GabrielleGabyGaleGenevieveGeorgiaGinaGloriaGraceGretaGwendolyn
Female names that start with the letter A:AbbyAbigailAdelineAddisonAgnesAlexAlexandriaAlexisAliceAliciaAlisonAlitaAlyssaAmandaAmberAmeliaAmyAnastasiaAndreaAngelaAngelinaAngieAnitaAnnAnnaAnnabelAnnabelleAnneAnnetteAnnieAntoinetteArleneAshleyAshtonAubreyAudreyAuroraAvery
Street names that start with the letter F:FactoryFairfieldFairwayFawnFederalFernFillmoreFisherFlagFleetFletcherFloridaFosterFoxFrankFrankfordFranklinFremontFrontFulton
Cities in Europe that begin with the letter Z:Zabrze, PolandZagreb, CroatiaZaragoza, SpainZurich, SwitzerlandZwickau, GermanyZwolle, NetherlandsRivers in Europe that begin with the letter Z:Zagyva River (Hungary)Zancara River (Spain)Zezere River (Portugal)
The Pokémon names confirmed to start with the letter Y are:YamaskYanmaYanmegaYveltal
Trout
Amazon River.
River Dee River Don River Deveron
Female names that start with the letter L:LaceyLakishaLanaLaraLatashaLatishaLatonyaLatoyaLauraLaurelLaurenLaurieLaverneLawandaLeaLeahLeannLeanneLeeLeighLeilaLelaLenaLenoraLenoreLeolaLeonaLeslieLethaLeticiaLidiaLilaLilianLillianLillieLillyLindaLindseyLisaLizzieLoisLolaLoraLoraineLorenLorenaLorettaLoriLornaLorraineLorrieLottieLouLouisaLouiseLuannLuciaLucileLucilleLucindaLucyLuellaLuisaLulaLydiaLyndaLynetteLynn
Street names that begin with the letter N:NassauNavahoNeptuneNevadaNewNewburyNewportNewtonNiagaraNobleNorfolkNormanNorthNorthamptonNorthbrookNorthfieldNorwayNorwood