The River Nile is what is called a tropical river and is fed by the rains in upper Egypt caused by the monsoon winds that prevail in that section of Africa during the summer season, as they do in India. As has been explained in a former chapter, the monsoon winds blow steadily for about six months from off the southern ocean. These winds are highly charged with moisture, which is not precipitated till it strikes the mountainous regions of the interior. Here the high mountains, which are often snow-capped, cause a profuse precipitation, which runs off into the various feeders of the Nile, causing a gradual rise in the river that reaches the highest point about September of each year. If the Nile should dry up, or if the annual floods should materially change in height, it would make a desert region of all that portion of Egypt now so productive. The great rivers of China, the Yang-tse-Kiang and the Hwangho, are also tropical rivers and have an annual flood. Sometimes the rise is as much as fifty-six feet. These annual floods are also caused by the monsoon winds that carry moisture from the ocean, which is condensed and precipitated in the mountains of central Asia. The conditions are substantially the same as those which prevail at the sources of the Nile in Africa. Rivers are produced from all sorts of causes, some of them flowing only during the rainy season, while others are fed by melting snow from the higher mountains, and as the snow is rarely melted away entirely during the summer, in the high mountains, there is a continual flow from this source. The snow forms a system of storage, so that the water is held back and is gradually given up as it melts. If this were not true mountainous regions would be subjected to disastrous floods. If the precipitation were always in the form of rain it would immediately run off instead of being distributed over a whole season. The Platte is an instance of a river largely fed by the melting snows--of the Rocky Mountains. In the region of glaciers in the mountains of Alaska and Switzerland rivers are fed by the melting ice. These rivers are usually of a milky color occasioned by the pulverization of rock caused by the grinding of the great glaciers as they flow down the gulches in the mountain side. In some regions these glacial rivers have a diurnal variation. This is caused by the fact that the glacier is so situated that it freezes at night, which checks the flow, and thaws in the daytime, which increases it.