Ecgfrith's Battle Against the Picts in the Vita Sancti Wilfrithi (with a Note on Nechtansmere)

"The Battle of Two Rivers was fought between the Picts and Northumbrians in the year 671. The exact battle site is unknown. It marked the end of the Pictish rebellion early in the reign of Ecgfrith, with a decisive victory for the Northumbrians. Attestation of the battle is limited to the account in Stephen of Ripon's Vita Sancti Wilfrithi."

That from Wikipedia - and it is really all that can be said about this mysterious battle-site location.

Well, almost.  There is another very important detail found in the Vita which I have not seen discussed before.  The following are the Latin and English text passages on the battle from the Vita itself:

Note the agger the Picts are said to build to protect their homeland/country (a better translation than 'house').  We all know what an agger is:

From the Lewis and Smith Latin Dictionary -

agger, ĕris, m. [ad-gero].

I. Things brought to a place in order to form an elevation above a surface or plain, as rubbish, stone, earth, sand, brushwood, materials for a rampart, etc. (in the histt., esp. Cæs., freq.; sometimes in the poets): ab opere revocandi milites, qui paulo longius aggeris petendi causā processerant, Caes. B. G. 2, 20: aggere paludem explere, id. ib. 7, 58; cf. id. ib. 7, 86: longius erat agger petendus, id. B. C. 1, 42; 2, 15 al.: superjecto aggere terreno, Suet. Calig. 19; cf. id. ib. 37: implere cavernas aggere, Curt. 8, 10, 27: fossas aggere complent, Verg. A. 9, 567: avis e medio aggere exit, from the midst of the pile of wood, Ov. M. 12, 524.

But far oftener,

II. Esp.

A. The pile formed by masses of rubbish, stone, earth, brushwood, etc., collected together; acc. to its destination, a dam, dike, mole, pier; a hillock, mound, wall, bulwark, rampart, etc.; esp. freq. in the histt. of artificial elevations for military purposes

The odd thing about the agger construction is that it seems precisely this which so angers the English king.  Yet to the best of our knowledge, the Picts did not build such things.  The various dykes found across Scotland, although sometimes colloquially called Picts Dykes, have been shown by the archaeologists to belong to other periods.

However, it is not at all impossible that at the time the Life of St. Wilfrid was written, or at the time of the actual event itself, an impressive dyke had been used by the Picts as an ad hoc defensive structure.  Such a structure would have predated the Picts themselves.

With this possibility in mind, I went looking for a major dyke adjacent to two rivers in Pictish territory or on the border of that territory.  These features were not hard to find.

I began by assuming that the English of Northumbria would have likely headed north into Pictland along the old Roman route, which led across the Forth and thence followed the Gask Ridge.  When I got to the one-time Roman legionary fortress at Inchtuthil, I noticed something called the Cleaven Dyke.  This massive earthwork was near the Roman fort, the Roman road and literally spanned much of the space between the Rivers Tay and Isla. It has been dated around 6,000 B.C.

Research on the site soon bore fruit.

https://scarf.scot/regional/pkarf/perth-and-kinross-archaeological-research-framework-case-studies/the-cleaven-dyke/

https://www.academia.edu/25082288/The_Cleaven_Dyke_and_Littleour_Monuments_in_the_Neolithic_of_Tayside_Society_of_Antiquaries_of_Scotland_Edinburgh_1998_Monograph_Series_13

https://portal.historicenvironment.scot/apex/f?p=1505:300:::::VIEWTYPE,VIEWREF:designation,SM1602

Most helpful were the various maps I could find of the dyke.

In my opinion, as a "best guess", this location at the Cleaven Dyke and the Rivers Tay and Isla (although note also the Lunan Burn hard by) very nicely fits the duo fluminum battle of Ecgfrith. 

A NOTE ON NECHTANSMERE

Two main theories have been proposed for the Battle of Nechtansmere, in which Ecgfrith later fell: a lake at Dunnichen or Loch Insh at Dunachton. The latter has been favored lately because Bede's description of the wild, mountainous Scottish landscape seems to fit Dunachton better.

However, if I'm right about locating the Two Rivers Battle at the Cleaven Dyke near the Tay and the Isla, then Dunnichen is surely the right place.  If we continue NE along the Roman road from Inchtuthil, along the line of the forts, we see that Dunnichen is hard by and a likely location for Nechtansmere.