river in western Venezuela. The major navigable tributary of the Orinoco River, it arises in the Cordillera de Mérida and flows for 510 miles (820 km) northeast and east through the heart of the Llanos (plains), Venezuela's most important cattle-raising area. The river's principal tributaries, including the Portuguesa and the Guárico, flow mainly from the north. During the rainy season, the Apure is navigable for small boats from Ciudad Bolívar up to San Fernando de Apure and Puerto de Nutrias. Since 1957 the highway connecting San Fernando de Apure with Calabozo and the densely populated north has reduced river traffic drastically.