There is a further discussion as to what is the true source of the Thames, and which streams may properly be regarded as its head-waters: the Churn, especially since the digging of the canal, having a larger flow than the stream from Thames head; but whichever is chosen, the non-navigable portion starts at the same point, and is the third of the divisions into which the valley ranges itself when it is considered in its length, as a highway from the west to the east of England. The two limits, then, are at London Bridge and at Cricklade, or rather at some point between Lechlade and Cricklade, and nearer to the latter.
But a river has a second topographical and historic function. It cannot only be considered longitudinally as a highway, it can also be considered in relation to transverse forces and regarded as an obstacle, a defence, and a boundary.
This function has, of course, been of the highest importance in the history of all great rivers, not perhaps so much so in the case of the Thames as in the case of swifter or deeper streams, but, still, more than has been the case with so considerable and so rapid a river as the Po in Lombardy or the uncertain but dangerous Loire in its passage through the centre of France. For the Thames Valley was that which divided the vague Mercian land from which we get our weights, our measures, and the worst of our national accent, and cut it off from that belt of the south country which was the head and the heart of England until the last industrial revolution of our history.
The Thames also has entered to a large, though hardly to a determining, extent into the military history of the country; to an extent which is greater in earlier than in later times, because with every new bridge the military obstacle afforded by the stream diminished. And finally, the Thames, regarded as an obstacle, was the cause that London Bridge concentrated upon itself so much of the life of the nation, and that the town which that bridge served, always the largest commercial city, became at last the capital of the island.
We have already said that the establishment of the site of London Bridge was a capital point in the history of the river and the principal line of division in its course. What were the topographical conditions which caused the river to be crossed at this point rather than at another?