=Amazon Tributaries.= The Amazon receives two large tributaries from the southern part of Colombia: the Putumayo, 840 miles; and farther east the Caquetá, 1320 miles, the last also called the Yapurá, especially in Brazil. These rivers are navigable by canoe and by steamers of shallow draft for hundreds of miles, though with interruptions in places from difficult rapids. The Putumayo is the better, having been ascended a distance of 800 miles from the Amazon in a steamer drawing six feet. (The entire length of the Hudson is 350 miles.) Smaller rivers, the Guainía and the Vaupés, unite with the Casiquiare from Venezuela to form the Rio Negro, another important affluent of the Amazon. These rivers have many smaller tributaries, but the section has been little explored save for going up or down the main stream. =The Orinoco River=, which part of the way forms the boundary between Colombia and Venezuela, receives several important tributaries from the former country: the Guaviare, 810 miles long, the Vichada, 312 miles, the Meta, 660 miles, and the Arauca, 480 miles. Though all are more or less navigable the Meta is the most important. Joining the Orinoco below the Maipures cataract and the Atures rapids, which higher up obstruct the greater river, it permits continuous navigation to the Atlantic Ocean. Where joined by the Meta the Orinoco is a mile wide. The Meta is navigable for 150 miles above the junction, in the rainy season 500 miles, to a point but 100 miles from Bogotá. CLIMATE It has already been noted that the altitude of a district as well as its latitude affects the climate, which may be modified further by the direction of prevailing winds and by ocean currents. The extensive and lofty mountain ranges of Colombia therefore give the country a greater variety of climate than it would otherwise enjoy, with temperatures agreeable to every taste and suited to products of almost every character. The configuration of the mountain ranges and valleys causes a further difference in temperature and in rainfall among points at the same altitude; the elevations being responsible not only for their own lower temperatures, but for the greater heat of secluded valleys, and for other variations.