It has been observed, that the outer borders of river-basins are the most elevated parts which occur in some given places between their respective beds, though it is not always the CAM) that the water parting is formed by mountain-ridges. Owing to such a disposition of the surface, the waters which are collected on or near the borders run to one or the other of the two rivers. Up to the commencement of this century it was thought improbable, if not impossible, that two different river systems or basins could be united by a natural water communication ; but it is now ascertained that a low tract of country or a deep depression of the surface may occur, by which a portion of the water of a river, after being diverted from its own channel, may join a river which otherwise is not connected with that river from which the water branches off. The instances in which this occurs are very few, and we shall therefore enumerate those whose existence is beyond all doubt. The river Anio, in Tuscany, in that part where it runs between the high ridges of the Apennines and approaches the town of Arezzo, sends an ann southwards through a narrow valley, under the name of Chian, hich falls into the Chiare, an of tho Tiber. The Chiang had been filled with sand, but its course has been re-established by artificial means. Another case occurs in the kingdom of Hanover, a few miles east of the town of Osnabrfick, where the river hlaase divides into two branches, of which one, running west to Osnabritek, preserves its name, and, after a course of many miles, joins the Ems ; the other, running east under the name of Els, falls, after a abort course, into the Werre, an affluent of the Weser. In Sweden, two large rivers fall into the northern extremity of the Gulf of Bothnia—the Tornca Elf and the Celia Elf. About 100 miles from the sea, the last-mentioned river sends off an arm to the south-east, which, after a course of about twelve or fifteen miles, falls into the Tornea Elf : this arm is called Tarenda.Elf. In theme cases the rivers thus united by a natural water communication flow in the same direction, or nearly PO. But in South America, two large rivers, the Orinoco and the Amazonas, are united in this way in a part of their extensive courses, where the Orinoco runs west and the Amazonas mat. The branch of the Orinoco by which this natural water communication is effected is called the Cassiquinre. [Omoco, in
Cow. Div.1 This phenomenon is otherwise described as the bifur cation of a river high up its course, of which the Mareb, one of the tributaries of the Nile, already mentioned, affords another example.
It is a kind of established rule, that the whole course of a river should bear the same mune, and that this name should be continued to that branch or head-stream whose sources aro farthest from the month. But practice is frequently at variance with this rule, and in a manner establishes another. The inhabitants of a country preserve the name of that river which does not undergo any deflection of its course. At the confluence of the Mississippi and the Missouri, the latter is the larger river, 'mil has had a course of above 1000 miles more than the former, but it door not deflect the course of the Mississippi by its junction, and the name of the last-mentioned river is preserved. The same occurs in Smith America as to the Amazonas and Madeira, where we find that the last-mentioned river changes the direction of its course to meet the Antaxcnas, whose name is preserved. In Europe, the Rhine is joined by the Aar in Switzerland, above Laufenhurg. The Aar is the larger river, and brings down a greater volume of water ; but the Rhine, where it is joined by it, continues its westerly course, and its name is preserved. There are also two rules for determining which of the various head straatms of a river is entitled to be regarded as its upper course, and consequently to bear the name borne by the united stream lower down : the one rule is at the same time theoretical and natural ; the other is practical or conventional. By the former, the greater length and size, and the general direction of the valley or basin of the river, are the main considerations. By the latter, it is the first acquaintance which the inhabitants or discoverers of the valley of the main stream may make with one of Its branches (or the converse) that causes the name of the former to be extended to the latter.
The extent of a few river basins is here given in round numbers, but they must only be considered as rough approximations:— According to Humboldt the area of drainage of the Amazonas measures 3,000,000 of square miles ; but in this he, in all probability, includes that of the Tocantins (Brum, in (Iwo. Dlr.) which Mr. Wallace, correctly in our opinion, regards as a distinct river.