Floods in rivers are occasioned from various causes. Of course the primary cause is the same in all cases, that is, from precipitation of moisture in the form of rain or snow. Some rivers are so related to the area of rainfall and to the permeability of the soil that there is but little variation in the amount of discharge throughout the year. The great river of South America, the Amazon, is an instance of a river of this class. A certain number of the smaller rivers that feed it lie in the area of rainfall during the whole of the year; for instance, the streams of the upper Amazon are being fed by rains at one season of the year, when those feeding the river lower down are at the lowest stage. When the rainy season prevails in the upper section of the river the dry season prevails farther down, while at another season of the year these conditions are reversed. Therefore, though the Amazon has a larger drainage basin than any other river in the world, and in some parts the yearly rainfall is 280 inches, there is no very great fluctuation in the stages of water. The Orinoco River, which flows through Venezuela, and whose drainage area is largely covered with mountains, has a greater fluctuation than any other river, the difference between high and low water amounting to seventy feet. The River Nile has an annual rise of from fourteen to twenty-six feet. This river is the sole dependence of the inhabitants of lower Egypt, and their sustenance depends upon the height to which the river rises; if it does not rise high enough the agricultural lands are not sufficiently irrigated, and if it rises too high their crops are destroyed by the floods. In this section they depend entirely upon the overflow of the Nile for irrigation, and not upon the rainfall. There is scarcely ever a rainfall in lower Egypt except about once a year on the coast of the Mediterranean. After ascending the river for a short distance we come into an area of no rain for a distance of 1500 miles along the river. Egypt has a superficial area of about 115,200 square miles, and only about one-twelfth of this area is in a position to be cultivated. As there is no rainfall in this region, the sole dependence for agricultural purposes is from the River Nile when it rises to a sufficient height to admit of irrigation. The river brings down quantities of rich earth which during the overflow is deposited, and thus the agricultural regions are refertilized annually.