2Q0 PHYSICAL GEOGRAPHY OF
forest ends, it is not more than a hundred yards wide ; whereas the Uaupes, at the furthest point the traders have reached, is still a large river, from a quarter of a mile to a mile in width. The Amazon and all its branches are subject, like most tropical rivers, to an annual rise and fall of great regularity. In the main stream, and in all the branches which flow from the Andes, the waters begin to rise in December or January, when the rains generally commence, and continue rising till June, when the fine weather has just set in. The time when the waters begin to fall is about the 21st of June, — seldom deviating more than a few days from this date. In branches which have their sources in a different direction, such as the Rio Negro, the time of rising does not coincide. On that river the rains do not commence steadily till February or March, when the river rises with very great rapidity, and generally is quite filled by June, and then begins to fall with the Amazon. It thus happens that in the months of January and February, when the Amazon is rising rapidly, the Rio Negro is still falling in its upper part ; the waters of the Amazon therefore flow into the mouth of the Rio Negro, causing that river to remain stagnant like a lake, or even occasionally to flow back towards its source. The total rise of the Amazon between high and low water mark has not been accurately ascertained, as it cannot be properly determined without a spirit-level ; it is, however, certainly not less than forty, andiprobably often fifty feet. If therefore we consider the enormous water surface raised fifty feet annually, we shall gain from another point of view an idea of the immense quantity of water falling annually in the Amazon valley. We cannot take the length of the Amazon with its main tributaries at less than ten thousand miles, and their average width about two miles ; so that there will be a surface of twenty thousand square miles of water, raised fifty feet every year. But it is not only this surface that is raised, for a great extent of land on the banks of all the rivers is flooded to a great depth at every time of high water. These flooded lands are called, in the language of the country, "gapd," and are one of the most singular features of the Amazon. Sometimes on one side, sometimes on both, to a distance of twenty or thirty miles from the main river, these gapds extend on the Amazon, and on portions of all its great branches. They are all covered with a dense virgin forest of
