The Pannonian Plains are drained by the Danube and its four major tributaries, the Drava, Tisa (Hungarian spelling Tisza), Sava, and Morava. The Danube, extremely wide and deep at some points, varies greatly between high- and low-water seasons. The former is from March or April until May or June, the latter from mid-July to late October. The Danube also drains the interior highlands. The Vardar River and its tributaries drain the southeastern region into the Aegean Sea, and the Neretva drains the region into the Adriatic. The western alpine slopes parallel to the coast are drained into the Adriatic by surface and underground streams. Underground streams are formed when water seeps through the limestone of the alps into channels, then empties into the Adriatic below its surface.