The Elvish root kel- meant 'run' or 'flow', especially of water, and in The Etymologies (in volume V of The History of Middle-earth) this gives rise to a noun celon meaning 'river'. This initial simple idea underwent some development, and ultimately Tolkien seems to have intended to reject the name Celon altogether. In volume XI of The History of Middle-earth, we have a note that the name Celon needed revision to make full etymological sense, and even a comment that 'Celon is too hackneyed a river-name', with the stated intention to replace it with Limhir ('clear/sparkling river'). Nonetheless the older form Celon is the one that survived into the published Silmarillion.