Note 695 — Chapter XI · —The Anthropo-Geography Of Rivers

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