Between the mountain chains, besides the narrow valleys are limited plateau regions, the latter occupying about 900 square miles; while more than half of the country, an immense tract east of the Andes, broadening towards the southern boundary, is a great plain slightly inclining towards the east and south: the northern part belonging to the Orinoco Basin, the larger section at the south to that of the Amazon. This Amazon region has an area equal to that of the entire State of California. Its higher portion, as well as most of the Orinoco Basin in Colombia, where there are wet and dry seasons, is composed chiefly of grassy plains called llanos. Nearer the Amazon, where it rains a good part of the year, the country is heavily forested. RIVERS =Rivers entering the Caribbean Sea.= Most important at present as also best known are the rivers which flow into the Caribbean Sea. Chief of these is the Magdalena, 1020 miles long, the principal route to the interior. The most important affluent of the Magdalena is the Cauca, which enters it about 200 miles from the sea, after descending nearly 15,000 feet in a distance of 810 miles. The Magdalena has many other tributaries, 500 or more, a few of which, entering from the east, are navigable for small steamers. The Atrato River, 340 miles long, flows north between the highlands of the West Cordillera and the Coast Range, later turning east into the Gulf of Urabá. Of smaller streams flowing into the Caribbean, the Sinú bears considerable traffic. Besides these, there are the navigable Zulia, 120 miles, and the Catatumbo, 108 miles, which by way of Lake Marcaibo in Venezuela also enter the Caribbean. =Rivers entering the Pacific.= Into the Pacific flow many streams carrying much water, as the rainfall of the region is excessive; but the courses are mostly so short and the fall is so steep that few are navigable for any considerable distance. The longest of them, the Patía, 270 miles, is the only one which rises on the east side of the West Cordillera. Worth noting is the fact that this river and four others, the five belonging to three different basins, rise very near together in the highlands of southern Colombia; the Cauca and Magdalena going north to the Caribbean, the Putumayo and Caquetá southeast to the Amazon. The Patía penetrates the West Cordillera by a remarkable gorge with perpendicular walls several hundred feet in height. On the swampy lowlands the river channels are navigable. The San Juan River, 180 miles long, is navigable for 140 miles, as it, like the Atrato, flows a long distance parallel with the coast between the Baudó Range and the Cordillera, until it turns west into the Pacific.