Equally, we may have completely the wrong river. For example, Camden* suggests, by way of Ptolemy's (see Other Works) River Trisantonis and a convoluted play on the names of Southampton and Andover, the River Test in Hampshire (for a discussion see Hants.Gazetteer*). The Test is now so altered as to be impossible to tell whether it had a bore, though the shape of Southampton Water, which it enters, is suitable and the river must have been partly tidal (the fields above Testwood still flood at Spring tides*).
In actual fact, the Trisantonis is more likely to be the Sussex Arun, who's old name appears in "Tarrant Street" in Arundel. It appears as the Tarente in a Saxon deed of c.725 CE*, a name that was still in use in 1279*. There is no evidence there was ever a bore (or anything that might give the wonder) on the river - nothing, for example, appears in the extensive description of the river from 1637*.
Whichever river is Ptolemy's Trisantonis it would only be one of the many rivers of this and similar names collected by Cole* (See Note on Trisanton). Of these (which include the Trannon), the strongest candidate would seem to be the major northern-British Trent. In addition, if we accept that it is a tidal bore is being described, of the rivers that carry one (other than the Severn) only the Trent would seem likely to have a bore that could be large enough and a name that was close enough, though Morecambe Bay might qualify through the fact that, of all of them, it is the only area that actually fills from top to bottom rapidly. In the case of the Trent, village names like Trentham suggest the river had a similar name at least as far back as the Saxons (as suggested by Camden* : Staffordshire). Bede (see Other Works), more closely to our wonder, records it as the Traanta or T[a?]'ranta*, while Ptolemy (2nd C) lists it as Gabranto-, which is easily a transcription error in the Greek from Tarranto-*. Even so, the Trannon, despite the immediate lack of a bore, seems closer, name wise.
Note, also, that Ptolemy also lists Moricambe (Morecambe Bay) suggesting this name was current well before the Historia was likely to have been written.
In general, bores are delicate things*, and it is possible the river discussed no longer carries one.