While severe drought is experienced over much of the Llano region in summer, the heavy rains, particularly in the western districts, produce floods over the low-lying plains, the mesas being dry at all times. The whole area is traversed by numerous streams and rivers, which rise either on the southern slopes of the Cordillera or in the mesas. North of the Meta, in addition to the large number of smaller streams which here and there broaden out into marshy lakes or _cienagas_, we have the navigable rivers Arauca (the main waterway to eastern Colombia) and Apure, flowing from the Andes to the Orinoco in an easterly direction. The Apure receives many tributaries on its left bank from the Venezuelan Andes, most important of which are the Portuguesa, rising in the plains of the same name south of Barquisimeto and joining the Apure at San Fernando, and the Guárico, whose mouth is east of the same town, flowing from south of Carácas through the State to which it gives its name, and receiving from the east the waters of the Orituco, whose source is less than thirty miles from the coast in longitude 66°. Of the Orinoco tributaries from the north beyond the Apure the most important is the Manapire, all the streams east of this rising in the mesas and having but short courses. The greater part of the eastern Llanos drains northward by the Unare and Aragua into the Caribbean Sea. A few large rivers rise on the east of the mesas, but flow for short distances only through the plains, emptying, not into the Orinoco itself but the _caños_ of the Delta. This last-mentioned region of inundated forest, savannah and mangrove swamp, occupies about 11,500 square miles, bringing the total area of the central plains up to 119,800 square miles. The third great tract, the north-eastern spur of the Andes, divides itself naturally into three parts—the Caribbean Hills, along the shores of the sea of the same name; the Segovia Highlands, linking the former to the higher mountains of western Venezuela; and the Cordillera of Mérida, or the Venezuelan Andes. The total area occupied by these mountain and hill tracts is about 41,800 square miles.