WATERSHED. It has been observed, in the article RIVERS, that the margin of a river-basin generally lies contiguous to the basin of another river, and therefore constituting the boundary-line of two basins, the waters descend on both sides into their respective basins ; and hence the whole line of these margins, it has been stated, is called a watt/lard—properly, the trater-parting.
It might be supposed that this is simply a matter of nomenclature, relating to a single and definite elementary phenomenon of physical geography. But this is far from being the case. The term we are explaining, originally introduced in the sense just described, has been extended, apparently, to every physical locality in which tributary streams having their confluence in a river-basin issue from, or descend on the face of, sloping land, without reference to those on the other side of a ridge, or on the counter-slope ; and this extensive application often obscures, in geographical description, the original sense of the tetra. Of these various applications, examples will be found in the 'citations which follow.
The Rev. C. G. Nicolay, in the Manual of Geographical Science' (vol. i., Terminology, pp. 422.3), enters into the following 'elaborate and critical discussion of the manner in which this term is to be under stood :—" The tendency of water to seek its level makes the position and quantity, as well as character, of the waters of the globe, dependent on its contour ; every conical projection, every ridge, in short, every elevation of what sort soever it may be, becomes a water shed; and that knowledge of the height, slope, and direction of the various watersheds of the earth's surface the first step to its general contour. The word watershed, in geographical definition, implies the line by which any waters aro divided from each other, and the water shed of any country is no doubt such a lino; but as every slope sheds water, and many rivers have their rise on slopes below the main water shed, some further division of the word—some classification of the districts to which it is applicable—appears highly desirable. As this does not seem to have been ever attempted, the following is offered as a suggestion.
Host there is a line in every country which may be termed its principal watershed, will not be disputed; every country has some one district, usually in the direction of its greatest length, more elevated than another, from the sides of which the waters collected from snows, dew, rain, and springs pour down, until they aro received into the basin of some inland water, or at last into the sea; this may, there fore, be properly termed its primary watershed ; but as the mountains of the world cannot be satisfactorily considered, except in their relative connection, the highest ranges extending through the greatest length of the continental masses, the term primary watershed should be confined to these; beyond them others of leas considerable elevation are found, the slopes of which are presented towards the primary watershed and form with it deep hollows, into which their united waters are poured, while from the opposite elope the waters collected descend in a differ ent direction. These may, not inaptly, be termed secondary watersheds, as paying the tribute of part of their waters to the primary, and forming the inferior limit to the principal river basins ; while others rising beyond may be called tertiary. It will be observed that this classification affords not only a systematic division of the elevated land, but also of the waters of the globe, as appertaining to any of its parts ; rivers having their rise in the primary watersheds may also receive a similar designation, as may their basins; others may be termed secondary or tertiary, according to their position and the watersheds to which they belong." In Dr. Beke' records of travel and geographical works, the term "water-parting" Is substituted for "watershed," for which change he assigns the following reasou :—" Tho lino of division and separation between the contiguous basins of two rivers, allied by the ancients dirortio aquarum, the parting (or flowing In opposite directions) of the waters, is in Gerueur called dis Wasserscheide, which means literally the same. English geographers, following the example of geologists, have
adopted the expression watershed,' which Is evidently a corruption of the German Wasserscheide, and was probably first introduced among us by miners from Germany. The term is, however, objectionable; because, to the mere English scholar, it appears to be a native compound of the words ' water' and ' shed,' as if meaning that the water is shed in oppo site directions, and hence leads to the belief that the side of the basin of a river, rather than the division between the adjoining basins of two rivers, is intended. In fact, the expression has of late years been frequently used in that sense. The substitution of the term ' water parting ' renders the idea intended to be conveyed intelligible to all, and exactly expresses the Latin divortio aquarum, and the German Wasserscheide." (` Sources of the Nile,' p. 3, note.) Dr. Beke's remarks are amply justified, it will be seen, by the previous discussion of Mr. Nicolay, and also in the following statement on the philosophy of the subject as one of the configuration of the globe, by Sir J. F. W. Herschel, who, it will be observed, while adopting by previous implication the term watershed, in its primary signification, evidently connects it also with the meaning of the English word to shed. " Possessed of a knowledge of the heights of stations above the sea, we may connect all stations at the same altitude by level lines, the lowest of which will be the outline of the sea-coast and the rest will mark out the successive coast-lines which would take place were the sea to rise by regular and equal accessions of level over the whole world, till the highest mountains were sub merged. The bottoms of valleys and the ridge-lines of hills are determined by their property of intersecting all these level lines at right angles, and being, subject to that condition, the shortest and longest, that is to say, the steepest and the most gently sloping courses respectively, which can be pursued from the summit to the sea. The former constitute the watercourses' of a country ; the latter its lines of `watershed; by which it is divided into distinct basins of drainage." (` Outlines of Aatronomy; par. 289.) Again, Captain H. Strachey, and his brother Lieut.-Col. R. Strachey, to whose contributions to scientific geography we have so often referred, designate the northern and southern faces, or slopes of the great table-land of Thibet [PLArss] as the Turkish, and Himalayan or Indian 'watersheds' respectively ; meaning the entire inclination, between the table-land and the low plains to the north and south, occupied by successive ranges of mountains, down and through which the great rivers maintaining a course along the summit of the table land, and receiving the drainage of its corrugations, flow into those plains, namely, the plains of Hindostan on the one hand, and those of Turkistan or Yarkend on the other. (` Phil. Trans.', 1859, pp. 776, 777.) Here we have another modification of the sense in which the term we are discussing is to be understood, referring to a slope, not as giving rise to streams, or separating them, but as merely giving them passage. In the signification of Wasserscheide, the water-parting, the highest part of the table-land itself is the uraterslied. But LieutsCoL Strachey defines the meaning in which he uses the term by referring to "the crest of the Indian watershed," which is merely the summit of the slope.