The Dnieper, Southern Buh, Dniester, and Danube rivers are its principal feeders; the Don and Kuban rivers flow into the Sea of Azov. The rivers flowing into the northern part of the Black Sea carry much silt and form deltas, sandbars, and lagoons along the generally low and sandy northern coast. The southern coast is steep and rocky. The Black Sea has two layers of water of different densities. The heavily saline bottom layer has little movement and contains hydrogen sulfide; it has no marine life. The top layer, much less saline and richer in fish, flows in a counterclockwise direction around the sea. There is little tidal action.
Pollution in the Black Sea has spurred surrounding nations to cooperate in instituting environmental safeguards, but remains a problem; overfishing is also a significant problem and has altered the sea's ecosystem. The sea is subject to severe winter storms, and waterspouts are common in summer. Ice-free, it is the chief shipping outlet of Ukraine and Russia; Odessa in Ukraine, Sevastopol in Crimea, and Novorossiysk in Russia are major ports. Other important ports are Constanţa in Romania; Varna and Burgas in Bulgaria; and Trabzon, Samsun, and Zonguldak in Turkey. The Black Sea region, especially in the S Crimea and W Caucasus, is a popular resort area.
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