Conversely, in some streams of peculiar breadth and constancy very shallow upper reaches may have early been converted to the use of man. The matter is only to be determined by the experience of what the inhabitants of a river valley have actually been able to do under the local circumstances, and when we examine this we shall usually be astonished to see how far inland a river was used until the history of internal navigation was transformed by the development of canals or partially destroyed by the development of railways. Thus it is certain that so small a stream as the Adur in Sussex floated barges up to the boundaries of Shipley Parish; that the Stour was habitually used beyond Canterbury; that so tiny a tributary as the Ant in Norfolk was followed up from its parent Bure to the neighbourhood of Worsted. In this connection the Thames is of an especial interest, for it had, in proportion to its length, the greatest section of navigable non-tidal water of any of the shorter rivers in Europe. Until the digging of the Thames and Severn Canal at the end of last century it was possible, and even common, for boats to reach Cricklade, or at any rate the mouth of the Churn. And even now, in spite of the pumping that is necessary at Thames head and the consequent diminution of the volume of water in the upper reaches, the Thames, were water carriage to come again into general use, would be a busy commercial stream as high up as Lechlade. This exceptional sector of non-tidal navigable water cutting right across England from east to west, and that in what used to be the most productive and is still the most fertile portion of the island, is the chief factor in the historic importance of the Thames. From Cricklade to the navigable waters of the Severn Valley is but a long day's walk; and one may say that even in the earliest times there was thus provided a great highway right across what then was by far the most thickly populated and the most important part of the island. A third section in all such rivers (and, from what we have said above, a short and insignificant one in the case of the Thames) may be called the _head-waters_ of the river: where the stream is so shallow or so uncertain as to be no longer navigable. In the case of the Thames these head-waters cover no more than ten to fifteen miles of country. With the exception of rivers that run through mountain districts this section of a river's course is nearly always small in proportion to the rest; but the Thames, just as it has the longest proportion of navigable water, has also by far the shortest proportion of useless head-water of all the shorter European rivers.