of water, and extend constantly its tributary area, does a river assume real historical importance.  It reaches its fullest significance at its mouth, where it joins the world’s highway of the ocean.  Here are combined the best geographical advantages—­participation in the cosmopolitan civilization characteristic of coastal regions, opportunity for inland and maritime commerce, and a fertile alluvial soil yielding support for dense populations.  The predominant importance of the debouchment stretch of a river is indicated by the presence of such cities as London, Rotterdam, Hamburg, Bremen, Bordeaux, Odessa, Alexandria, Calcutta, Rangoon, Bangkok, Hongkong, Canton, Nanking and Shanghai, Montreal and Quebec, New York, Philadelphia, New Orleans, Buenos Ayres and Montevideo.  This debouchment stretch gains in practical value and hence in permanent historical importance if it is swept by a scouring tide, which enables the junction of inland and maritime routes to penetrate into the land.  Even Strabo recognized this value of tidal reaches.[652] Hence in tideless basins like the Baltic and Caribbean, the great river ports have to advance coastward to meet the sea; and the lower course of even mighty streams like the Volga and Nile achieve a restricted importance.[653]

The control of a river mouth becomes a desideratum or necessity to the upstream people.  Otherwise they may be bottled up.  Though history shows us countless instances of upstream expansion, nevertheless owing to the ease of downstream navigation and this increasing historical importance from source to mouth, the direction of a river’s flow has often determined the course of commerce and of political expansion.

[Sidenote:  Location at hydrographic centers.]

The possibility of radial expansion, which we have found to be the chief advantage of a central location, is greatly enhanced if that central location coincides with a hydrographic center of low relief.  The tenth century nucleus of the Russian Empire was found about the low nodal watershed formed by the Valdai Hills, whence radiated the rivers later embodied in the Muscovite domain.  Here In Novgorod at the head of the Volchov-Ladoga-Neva system, Pskof on the Velikaya, Tver at the head of the navigable Volga, Moscow on the Oka, Smolensk on the Dnieper, and Vitebsk on the Duna, were gathered the Russians destined to displace the primitive Finnish population and appropriate the wide plains of eastern Europe.  Everywhere their conquests, colonization, and commercial relations have followed the downstream course of their rivers.  The Dnieper carried the Rus of Smolensk and Kief to the Euxine, into contact with the Byzantine world, and brought thence religion, art, and architecture for the untutored empire of the north.  The influence of the Volga has been irresistible.  Down its current Novgorod traders in the twelfth century sought the commerce of the Caspian and the Orient; and later the Muscovite princes pushed their conquest of the Tartar hordes from Asia.  The Northern Dwina, Onega, Mesen and Petchora have carried long narrow bands of Slav settlement northward to the Arctic Ocean. [See map page 225.] Medieval Russian trade from Hanseatic Pskof and Novgorod, and later Russian dominion followed the Narva and Neva to the Baltic.  “The Dnieper made Russia Byzantine, the Volga made It Asiatic.  It was for the Neva to make it European."[654]