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The Main European Watershed is the drainage divide which separates the basins of the rivers that empty into the Atlantic Ocean, the North Sea and the Baltic Sea from those that feed the Mediterranean Sea, the Adriatic Sea and the Black Sea. It stretches from the tip of the Iberian Peninsula at Gibraltar in the southwest to the endorheic basin of the Caspian Sea in Russia in the northeast.
The Watershed runs northwards through Spain in the Iberian Peninsula towards the Bay of Biscay along the basins of Guadalquivir, Guadiana, Tagus and Douro in the wast to the sources of the Ebro in the Basque Country. It then follows the main ridge of the Pyrenees eastwards up to Andorra, where it again turns to the north through France along the catchment areas of Garonne (with the summit of the Canal du Midi at Seuil de Naurouze), Loire and Seine in the west and that of the Rhone in the east, marked by the Cévennes mountain range between the sources of the Allier and Ardèche tributaries.
At the Belfort Gap south of the Vosges Mountains it separates the Rhone basin in the southwest from the drainage of Meuse and Rhine in the northeast, turns southwards to the Swiss Jura Mountains and passes between Lake Geneva and Lake Neuchâtel. It reaches the Bernese Alps where it runs along the Alpine divide to the Gotthard Pass. Mt. Witenwasserenstock marks the triple watershed of the Rhone, Rhine and Po rivers, further to the east along the Grison Alps it reaches the tripoint of the Rhine, Po and Danube basins at the "roof of Europe" Piz Lunghin west of the Maloja Pass.
The divide continues northwards along the Albula Alps to Julier Pass, Albula Pass and Flüela Pass south of Davos between the catchment area of the Rhine, which empties into the North Sea via the Netherlands, and the Danube, which flows eastward emptying into the Black Sea. It enters Austria at Piz Buin in the Silvretta Alps running along the Arlberg massif and forming the border between the states of Tyrol and Vorarlberg. It reaches Bavaria, Germany via the crest of the Allgäu Alps and leaves the High Alps at the Adelegg range near Kempten.
Within the Alpine foothills it runs north of the Schussen catchment area of Lake Constance towards the sources of the Brigach and Breg headwaters of the Danube in the Black Forest. It then follows the main ridge of the Swabian and Franconian Jura with the crossing of the Rhine–Main–Danube Canal northeastwards up to the Fichtelgebirge and southeastwards along the Bohemian Forest, forming the border between Germany and the Austrian Mühlviertel and Waldviertel regions with the Czech Republic, where the large Bohemian basin is drained by the Elbe River. It again turns northeastwards along the Bohemian-Moravian Highlands to the Śnieżnik Mountains of the Eastern Sudetes at the border with Poland, where the tripoint of Danube, Elbe/North Sea and Oder/Baltic Sea is located at the peak of Králický Sněžník.
The Watershed continues in eastern direction along the Jeseníky Mountains down to Moravian Gate and uphill into the Beskids, with the Oder and Vistula basins in the north, reaching the tripoint of Poland with Slovakia and Ukraine in the Bieszczady Mountains. Then, the Watershed goes northeast near the Poland–Ukraine border, passes through the city of Lviv and, via Volyn, enters Belarus. Then it runs east-northeast along Belarusian Ridge and continues to the Smolensk Oblast of Russia, ending at the tripoint with Caspian drainage basin in the Sychyovsky District, near the midpoint of Western Russia's cities Smolensk and Tver.
The watershed is not a clearly defined divide. Tectonics, especially in the area that is now the Upper Rhine Plain, created the River Rhine. As its headward erosion is much stronger than that of the much older River Danube (see the upper river geology) (due to the Rhine’s gravity[clarification needed] on its much shorter way to the North Sea), the Rhine and its tributaries intrude deeper into phreatic zones of the Swabian Karst and even capture the upper Danube and its surface tributaries.
It is expected that the Danube's upper course will one day disappear entirely in favour of the Rhine ("stream capture").