Fig. 4.—Sketch-map to illustrate river-capture.
A very simple example of this widespread phenomenon may be taken in illustration. The accompanying [sketch-map], drawn by Mr. Lionel Hinxman, shows part of the course of the River Feshie, one of the tributaries of the Spey, and part of the Geldie Burn, one of the tributaries of the Aberdeenshire Dee. It will be noted that the Feshie shows a very curious bend, or elbow. Mr. Hinxman points out that this curious condition can be explained on the supposition that the River Eidart, shown on the map to the north of the bend, once formed the headwaters of the Feshie, which cut its valley back until it captured the headwaters of the Geldie, and thus brought water which formerly flowed into the Dee into the Spey valley. The boundary between the two counties of Aberdeen and Inverness is shown on the map by a dotted line, and it is seen that the march follows the watershed, which between the present Geldie and the bend on the Feshie is very low. Formerly, however, this watershed lay much further to the west, and its shifting is due to the capture.
A careful study of large scale maps will show many examples of similar river-capture, some old and some recent. A sharp bend, the so-called elbow of capture, on a river in close proximity to another stream affords in itself a certain presumption that capture has taken place, though this presumption can only be verified by study on the spot.
It may be noted that before the capture is finally accomplished there may be an intermediate stage when the water has the choice of two channels, both of which may be utilised in a time of flood. A very curious case is that of the Casiquiare, a river in South America which connects together the two systems of the Amazon and the Orinoco, while another is the connection recently discovered by Captain Lenfant, a French explorer, between the systems of the Shari and the Niger in Africa. Such conditions are obviously unstable, for one stream must sooner or later predominate over the other, and deprive it even of flood water.
Another example may help to explain the evolution of a complex river system with many tributaries. A glance at the map of England (see [diagram]) shows that while the rivers of Northumberland and Durham flow independently into the sea, those of Yorkshire are united into a characteristic bunch, and all reach the ocean by means of the Humber. This estuary breaks through the high ground formed by the Wolds of Yorkshire and Lincolnshire, which consists of hard rock. At one time it is probable that the rivers of Yorkshire entered the sea separately, while the other great factor of the Humber, the Trent, mingled its waters with the present Witham. At this time the weathering of the land surface had not reached its present stage so the land would lie higher. In what is now the vale of York the rocks are much softer than where the Wolds now stand, and the present Ouse, which was at first a longitudinal tributary of a transverse stream, eating its way back through these soft rocks, tapped successively the streams flowing eastwards from the Pennines, and with the help of the abundant water so obtained was enabled to cut out the wide estuary of the Humber.