After a river has cut its channel to the base line, then its grading really begins, the inequali ties gradually disappear, or would disappear if there were no obstructions to the regular work. The constructional lakes and the waterfalls gradually disappear. The constructional lakes may disappear, but if the amount of detritus is large the channels lower down a stream, with a slight grade, may be filled, and thus the water will flow over a bed elevated above what was once its channel. Such channels exist in Ohio and in the plains of Lombardy. Any change in the depression or uplift of the land, whether from volcanic or other causes, will produce some change in the river. In various parts of Europe and the eastern part of North America rivers have changed because of increase of evapora tion caused by the continued deforestation of •their basins. New tributary rivers appear in the drainage area of a well-defined river. The valleys formed by such streams are called sub sequent valleys and the streams themselves are called • subsequent streams.
The main stream with all the lateral streams and all tributaries, whether lakes or rivers, is called a river system. The land drained by a river system is called its basin. The system is usually known by the name of the main stream or trunk The rate of fall of many of the large rivers is pot great. The Amazon has a descent of, only 101, feet 6f10 miles of its course — that is, one twenty-seventh part of an inch for every 1,000 feet of that distance. The Loire, in France, between Pouilly and Briare, falls one foot in 7,500 feet, but between Briare and Orleans only one foot in 13,596 feet. Even the rapid Rhine has a descent of no more than four feet in one mile between Schaffhausen and Strassburg, and of two feet between the latter place and the borders of Holland. The glaciers in France show a change in volume and a con sequent change in the size and rate of fall of the glacier-fed rivers. See NIAGARA FALLS; WATERFALLS.
Watersheds or Divides.— The line which separates the waters which flow into different rivers or different systems is called the water shed or the divide. The continental divide in the United States is the line which parts or separates the streams which flow into the Pacific from those which flow into the Gulf of Mexico through the Mississippi. Divides change from various causes. The subsequent rivers some times take the water from one system to an other. The gradual removal of a constructional lake may change the divide, as in New York State between the Mohawk River and Lake Ontario; • in the Northwest, between the Red River of the North and the Mississippi. The divides in many places are very low, as between the Amazon and Orinoco and the Amazon and the Paraguay. The Orinoco at some distant period reached the basin of the Amazon, and has been subsequently connected with it through what is now an important branch, the Cassi. oniare, which after a course of 120 miles from the main river discharges itself into the Rio Negro, a branch • of the Amazon; and as the navigable waters of the Amazon approach within • three miles to those of the Paraguay there is with only this interruption a continuous communication by navigable rivers from the mouth of the Orinoco, in lat. 9° N., to the mouth of the Paraguay, in lat. 35° S.
Flood-Plains.— For various reasons many rivers overflow their banks at annual periods others at irregular intervals. Such rivers usu ally carry a large amount of silt which at the time of an overflow is deposited on the land, thus enriching it. Great damage sometimes results from this. overflow (see LEVEE; MISSIS‘
mien), and various means have been devised to protect the lands adjacent to such rivers from inundation. The flood-plains of the Nile are noted for their fertility; the Mississippi, Ama son, and many others have extensive flood-plains. The large rivers of Siberia have vast flood-plains caused by the thaws at the sources of the rivers when the lower portions are closed by ice. The descending waters sweep over the frozen surface carrying with them vast quantities of soiletritus and even N forests. The Mackenzie in North America has flood-plains from the same sources as the Si berian rivers.
Mouths of Rivers.— The river which en ters the ocean may have its channel submerged and the mouth becomes an estuary or a fiord. In either case there is a constant battle for supremacy between the fresh water from the landmass and the salt waters of the sea. Some of the sediment of the rivers is brought to the sea and either carried out into the ocean or deposited at the mouth of the river, where it forms deltas. In some cases, as the Mississippi, the delta grows rapidly, extends out into the sea and becomes a part of the land-mass.. The sediment annually brought down the Mississippi has been estimated as equal to a deposit of a foot in thickness over 12 square miles. The waters of the Ganges and Brahmaputra come more highly charged with sediment on account of their more rapid descent and the more vio lent rains that fall about their sources, and their deposits exceed many times those of the Missis sippi. The sediments are spread out to a dis tance of 100 miles or more from the land, the waters of the Bay of Bengal being discolored by them even at this great distance. The quan tity annually discharged from the mouth of the river has been computed equal to a laver one foot thick over a tract of 225 square miles. The Nile has a vast delta. The Mississippi, Nile, Ganges, Po and many other rivers enter the sea by several channels. The Amazon enters by only one large channel, divided by an island, but the river broadens out into a bay 180 miles wide. The delta-lands are very fertile, and where the river channels have assumed stability, deltas have become valuable additions to the land-mass.
Economic Relation of Rivers.— The effect rivers have in supplying moisture to the adjacent lands, either by natural or artificial (irrigation) means, is recognized by all nations. Their uses as contributing to the healthfulness of the climate and to modifications of tem-, perature are well known. In the early history of all the nations on the globe, the waterways were the great thoroughfares which furnished means of intercommunication, and the rivers were largely instrumental in determining the location of the important commercial and indus trial centres of the world. The great rivers of Europe and Asia, such as the Rhine, Danube. Volga, Indus, Ganges, Brahmaoutra, Yangtse and Ob afford enormous populations access to the sea. The Amazon, with its plain track extending for nearly 3,000 . miles, is in many ways less like a river than a fresh inland sea; but the Mississippi and Saint Lawrence, though less extensive, are of greater value for carrying sea traffic to inland places. In their upper valley tracts, rivers are of use chiefly for transport ing timber and driving machinery. It is in teresting to note that in Switzerland, Norway and Sweden, where there is no coal, there exist exceptional facilities for the use of water power on account of numerous mountain torrents.