Trat, Trath, Treroit, Treuroit, Tribruit
An unidentified river in Britain that, in Nennius, was the site of Arthur’s tenth battle against the Saxons. As in all of Arthur’s twelve battles, Arthur was victorious.
Nennius may have intended the river Ribroit in Somersetshire or the river Ribble in Lancashire. A Welsh poem alludes to a fight at a river called Tryfrwyd, which may be identical.
Source
Historia Brittonum | Probably Nennius, early 9th century