To return to the river regarded as a _boundary_, it is subject to this rather interesting historical observation that it has been more of a boundary in highly civilised than in barbaric times.
One would expect the exact contrary to be the case. A civilised man can cross a river more easily than a barbarian; and in civilised times there are permanent bridges, where in barbaric times there would be only fords or ferries.
Nevertheless, it is true of the Thames, as of nearly every other division in Europe, that it was much more of a boundary at the end of the Roman Empire, and is more of a strict boundary to-day, than it was during the Dark Ages, and presumably also before the Claudian invasion. Thus we may conjecture with a fair accuracy that in the last great ordering of boundaries within the Roman Empire, which was the work of Diocletian, and so much of which still survives in our European politics to-day (for instance, the boundary of Normandy), the Thames formed the division between Southern and Midland Britain. It is equally certain that it did _not_ form any exact division between Wessex and Mercia.
The estuary has, of course, always formed a division, and in the barbarian period it separated the higher civilisation of Kent from that of the East Saxons, who were possibly of a different race, and certainly of a different culture. But the Thames above London Bridge was not a true boundary until the civilisation of England began to form, towards the close of the Dark Ages. It is perpetually crossed and recrossed by contending armies, and the first result of a success is to cause the conqueror to annex a belt from the farther bank to his own territories.
It is further remarkable that the one great definite boundary of the Dark Ages in England--that which was established for a few years by Alfred between his kingdom and the territory of the Danish invaders--abandons the Thames above bridges altogether, and uses it as a limitation in its estuarial part only, up to the mouth of the Lea.
With the definition of exact frontiers for the English counties, however, a process whose origin can hardly antedate the Norman Conquest by many years, the Thames at once becomes of the utmost importance as a boundary.