Where sea and river meet, Nature draws no sharp dividing line. Here the indeterminate boundary zone is conspicuous. The fresh water stream merges into brackish estuary, estuary into saltier inlet and inlet into briny ocean. Closely confined sea basins like the Black and Baltic, located in cool regions of slight evaporation and fed from a large catchment basin, approach in their reduced salinity the fresh water lakes and coastal lagoons in which rivers stretch out to rest on their way to the ocean. The muddy current of the Yangtze Kiang colors the Yellow Sea, and warns incoming Chinese junks of the proximity of land many hours before the low-lying shores can be discerned.[630] Columbus, sailing along the Caribbean coast of South America off the Orinoco mouth, found the ocean waters brackish and surmised the presence of a large river and therefore a large continent on his left.[631]
The transitional form between stream and pelagic inlet found in every river mouth is emphasized where strong tidal currents carry the sea far into these channels of the land. The tides move up the St. Lawrence River 430 miles (700 kilometers) or half way between Montreal and Quebec, and up the Amazon 600 miles (1,000 kilometers). Owing to their resemblance to pelagic channels, the estuaries of the American rivers with their salty tide were repeatedly mistaken, in the period of discoveries, for the Northwest Passage to the Pacific. Newport in 1608 explored the broad sluggish course of the James River in his search for a western ocean. Henry Hudson ascended the Hudson River almost as far as Albany, before he discovered that this was no maritime pathway, like the Bosporus or Dardanelles, leading to an ulterior sea. The long tidal course of the St. Lawrence westward into the heart of the continent fed La Salle's dream of finding here a water route to the Pacific, and fixed his village of "La Chine" above the rapids at Montreal as a signpost pointing the way to the Indies and Cathay. In the same way a tidal river at the head of Cook's Inlet on the Alaskan coast was mistaken for a Northeast Passage, not by Captain Cook but by his fellow officers, on his Pacific voyage of 1776-1780; and it was followed for several days before its character as a river was established.[632]
Sea navigation merges into river navigation.
Rivers have always been the great intermediaries between land and sea, for in the ocean all find their common destination. Until the construction of giant steamers in recent years, sea navigation has always passed without break into river navigation. Sailing vessels are carried by the trade wind 600 miles up the Orinoco to San Fernando. Alexander's discovery of the Indus River led by almost inevitable sequence to the rediscovery of the Eastern sea route, which in turn ran from India through the Strait of Oman and the Persian Gulf up the navigable course of the Euphrates to the elbow of the river at Thapsacus. Enterprising sea folk have always used rivers as natural continuations of the marine highway into the land. The Humber estuary and its radiating group of streams led the invading Angles in the sixth century into the heart of Britain.[633] The long navigable courses of the rivers of France exposed that whole country to the depredations of the piratical Northmen in the ninth and tenth centuries. Up every river they came, up the Scheldt into Flanders, the Seine to Paris and the Marne to Meaux; up the Loire to Orleans, the Garonne to Toulouse and the Rhone to Valence.[634] So the Atlantic rivers of North America formed the lines of European exploration and settlement. The St. Lawrence brought the French from the ocean into the Great Lakes basin, whose low, swampy watershed they readily crossed in their light canoes to the tributaries of the Mississippi; and scarcely had they reached the "Father of Waters" before they were planting the flag of France on the Gulf of Mexico at its mouth. The Tupi Indians of South America, a genuine water-race, moved from their original home on the Paraguay headstream of the La Plata down to its mouth, then expanded northward along the coast of Brazil in their small canoes to the estuary of the Amazon, thence up its southern tributary, the Tapajos, and in smaller numbers up the main stream to the foot of the Andes, where detached groups of the race are still found.[635] So the migrations of the Carib river tribes led them from their native seats in eastern Brazil down the Xingu to the Amazon, thence out to sea and along the northern coast of South America, thence inland once more, up the Orinoco to the foot of the Andes, into the lagoon of Maracaibo and up the Magdalena. Meanwhile their settlements at the mouth of the Orinoco threw off spores of pirate colonies to the adjacent islands and finally, in the time of Columbus, to Porto Rico and Haiti.[636] [See map page 101.]
Historical importance of seas and oceans influenced by their debouching streams.
So intimate is this connection between marine and inland waterways, that the historical and economic importance of seas and oceans is noticeably influenced by the size of their drainage basins and the navigability of their debouching rivers. This is especially true of enclosed seas. The only historical importance attached to the Caspian's inland basin is that inherent in the Volga's mighty stream. The Mediterranean has always suffered from its paucity of long river highways to open for it a wide hinterland. This lack checked the spread of its cultural influences and finally helped to arrest its historical development. If we compare the record of the Adriatic and the Black seas, the first a sharply walled cul de sac, the second a center of long radiating streams, sending out the Danube to tap the back country of the Adriatic and the Dnieper to draw on that of the Baltic, we find that the smaller sea has had a limited range of influence, a concentrated brilliant history, precocious and short-lived as is that of all limited areas; that the Euxine has exercised more far-reaching influences, despite a slow and still unfinished development. The Black Sea rivers in ancient times opened their countries to such elements of Hellenic culture as might penetrate from the Greek trading colonies at their mouths, especially the Greek forms of Christianity. It was the Danube that in the fourth century carried Arianism, born of the philosophic niceties of Greek thought, to the barbarians of southern Germany, and made Unitarians of the Burgundians and Visigoths of southern Gaul.[637] The Dnieper carried the religion of the Greek Church to the Russian princes at Kief, Smolensk, and Moscow. Owing to the southward course of its great rivers, Russia has found the crux of her politics in the Black Sea, ever since the tenth century when the barbarians from Kiev first appeared before Constantinople. This sea has had for her a higher economic importance than the Baltic, despite the latter's location near the cultural center of western Europe.
Baltic and White Sea rivers.