From the uniting power of rivers it follows that they are poor boundaries. Only mountains and seas divide sharply enough to form scientific frontiers. Rivers may serve as political lines of demarcation and therefore fix political frontiers; but they can never take the place of natural boundaries. A migrating or expanding people tend always to occupy both slopes of a river valley. They run their boundary of race or language across the axis of their river basin, only under exceptional circumstances along the stream itself. The English-French boundary in the St. Lawrence Valley crosses the river in a broad transitional zone of mingled people and speech in and above the city of Montreal. The French-German linguistic frontier in Switzerland crosses the upper Rhone Valley just above Sierre, but the whole canton of Valais above the elbow of the river at Martigny shows fundamental ethnic unity, indicated by identity of head form, stature and coloring.[692] Where the Elbe flows through the low plains of North Germany, its whole broad valley is occupied by a pure Teutonic population—fair, tall, long-headed; a more brunette type occupies its middle course across the uplands of Saxony, and speaks German like the downstream folk; but its upper course, hemmed in by the Erz and Riesen Mountains, shows the short, dark and broad-headed people of the Bohemian basin, speaking the Czech language.[693] On the Danube, too, the same thing is true. The upper stream is German in language and predominantly Alpine in race stock down to the Austro-Hungarian boundary; from this point to the Drave mouth it is Hungarian; and from the Drave to the Iron Gate it is Serbo-Croatian on both banks.[694] Lines of ethnic demarcation, therefore, cut the Elbe and Danube transversely, not longitudinally. [See map page 223.]
The statements of Cæsar and Pliny that the Seine and Marne formed the boundary between the Gauls and Belgians, and the Garonne that between the Gauls and Aquitanians, must be accepted merely as general and preliminary; for exceptions are noted later in the text. Parisii, for instance, were represented as holding both banks of the Seine and Marne at their confluence, and the Gallic Bituriges were found on the Aquitanian side of the Garonne estuary.
Scientific river boundaries.
Only under peculiar conditions do rivers become effective as ethnic, tribal or political boundaries. Most often it is some physiographic feature which makes the stream an obstacle to communication, and lends it the character of a scientific boundary. The division of the Alpine foreland of southern Germany first into tribal and later into political provinces by the Iller, Lech, Inn, and Salzach can be ascribed in part to the tumultuous course of these streams from the mountains to the Danube, which renders them useless for communication.[695] The lower Danube forms a well maintained linguistic boundary between the Bulgarians and Roumanians, except in the northwest corner of Bulgaria, where the hill country between the Timok River and the Danube has enticed a small group of Roumanians across to the southern side. From this point down the stream, a long stretch of low marshy bank on the northern side, offering village sites only at the few places where the loess terrace of Roumania comes close to the river, exposed to overflows, strewn with swamps and lakes, and generally unfit for settlement, has made the Danube an effective barrier.[696] Similarly, the broad, sluggish Shannon River, which spreads out to lake breadth at close intervals in its course across the boggy central plain of Ireland, has from the earliest times proved a sufficient barrier to divide the plain into two portions, Connaught and Meath,[697] contrasted in history, in speech and to some extent even in race elements.[698] A different cause gave the Thames its unique rôle among the larger English rivers as a boundary between counties from source to mouth. London's fortified position at the head of the Thames estuary closed this stream as a line of invasion to the early Saxons, and forced them to make detours to the north and south of the river, which therefore became a tribal boundary.[699]
Where navigation is peculiarly backward, a river may present a barrier. An instructive instance is afforded by the River Yo, which flows eastward through northern Bornu into Lake Chad, and serves at once as boundary and protection to the agricultural tribes of the Kanuri against the depredations of the Tibbu robbers living in the Sahara or the northern grassland. But during the dry season from April to August, when the trickling stream is sucked up by the thirsty land and thirstier air, the Tibbu horsemen sweep down on the unprotected Kanuri and retreat with their booty across the vanished barrier. The primitive navigation by reed or brushwood rafts, practiced in this almost streamless district, affords no means of retreat for mounted robbers; so the raiding season opens with the fall of the river.[700]
Rivers as political boundaries.
For political boundaries, which are often adopted with little reference to race distribution, rivers serve fairly well. They are convenient lines of demarcation and strategic lines of defense, as is proved by the military history of the Rhine, Danube, Ebro, Po, and countless other streams. On the lower Zambesi Livingstone found the territories of the lesser chiefs defined by the rivulets draining into the main river. The leader of the Makololo formally adopted the Zambesi as his political and military frontier, though his people spread and settled beyond the river.[701] Long established political frontiers may become ethnic boundaries, more or less distinct, because of protracted political exclusion. To the Romans, the Danube and Rhine as a northeastern frontier had the value chiefly of established lines in an imperfectly explored wilderness, and of strategic positions for the defense of an oft assailed border; but the long maintenance of this political frontier resulted in the partial segregation and hence differentiation of the people dwelling on the opposite banks.
Poor as a scientific boundary, a river is not satisfactory even as a line of demarcation, because of its tendency to shift its bed in every level stretch of its course. A political boundary that follows a river, therefore, is often doomed to frequent surveys. The plantations on the meanders of the lower Mississippi are connected now with one, now with the other of the contiguous states, as the great stream straightens its course after the almost annual overflow.[702] The Rio Grande has proved a troublesome and expensive boundary between the United States and Mexico. Almost every rise sees it cutting a new channel for itself, now through Texas, now through Mexican territory, occasioning endless controversies as to the ownership of the detached land, and demanding fresh surveys. Recent changes in the lower course of the Helmund between Nasralabad and the Sistan Swamp, which was adopted in 1872 as the boundary between Afghanistan and Persia, have necessitated a new demarcation of the frontier; and on this task a commission is at present engaged.[703] In a like manner Strabo tells us that the River Achelous, forming the boundary between ancient Acarnania and Aetolia in western Hellas, by overflowing its delta region, constantly obliterated the boundaries agreed upon by the two neighbors, and thereby gave rise to disputes that were only settled by force of arms.[704]