Page:Collier's New Encyclopedia v. 08.djvu/88

LEFT RIVER 62 BIVER 0.170 mile per hour will just begin to work on fine clay. 0.340 " " " lift fine sand. 0.454 " " " lift sand as coarse as linseed. 0.682 " " " sweep along fine gravel. 1.364 miles " " roll along rounded pebbles one inch in diameter. 2.045 " " " sweep along slip- pery angular stones as large as an egg. Rivers in flood, even in the plain track, sometimes attain a velocity of over 5 miles an hour, and torrents may even flow as fast as 20 miles an hour. The course of a river is gradually carved out and shaped by the flow of the water. The sediment and stones carried along are powerful erosive agents in the tor- rential and valley tracks, and the char- acter of the valleys or gorges produced depends largely on the geological struc- ture of the region. The course of a river is frequently determined by lines of faults, but perhaps more often it ap- pears to be independent of the nature of the strata. Some great rivers, nota- bly the Volga, press against the right bank, cutting it into a steep cliff, while the left bank is left as a very gentle slope. This is explained by the directive influence of the earth's rotation. Rivers are of very great importance as agents of change in dynamic geology, the form of valley they excavate being determined partly by the nature of the rocks, partly by the climate. In rainless or arid regions steep-walled canons are cut to a great depth across high pla- teaus; in rainy regions subaerial de- nudation leads to the formation of wide valleys of much gentler slopes. Bars of more durable rock crossing the course of a stream lead to the formation of water- falls or rapids from the rapid erosion of the softer strata below. The river above the obstruction is reduced to what is termed the base level of erosion; the velocity of the current is checked, and wide alluvial deposits are laid down on either side. In course of time the bar of hard rock is completely cut through by a gorge, and the gradient of the stream is ultimately rendered uniform. In this way the common features of gorge and meadow are produced again and again along the course of a stream. The deposits of alluvium form terraces along the valley track of a river, and as the stream cuts its channel deeper they are left at various heights as mon- uments of its erosive power. When a river is fairly established in its valley it is, geologically speaking, a more per- manent feature than lakes or mountains. Upheaval, which acts very slowly, may even elevate a range of mountains across its course, yet all the while the river, cutting its way downward, remains at the same absolute level. The Uintah mountains, as they were upheaved, were divided in this way by the Green river, the chief tributary of the Colorado. In limestone regions the solvent power of river water on carbonate of lime leads to the formation of caves and under- ground rivers, which, as a rule, emerge from their subterranean channels on low- er ground. Sometimes they do not re- appear on land, but discharge their fresh water through openings in the bed of the sea. Such submarine river entrances are not uncommon along the shores of the Adriatic, off the coast of Florida, and in other calcareous regions. When a river advances along a nearly level plain to- ward the sea its carrying power falls off; gravel, sand, and finally mud are deposited on its margin, and the stream pursues a peculiar winding course. Dur- ing a flood the swift and muddy stream rises, overflows its banks, and widens out on the level land. The current is at once checked and a long bar of deposit forms along each margin. These are increased in height by each successive flood, and, the river-bed being simulta- neously silted up, broad muddy rivers like the Mississippi, Po, and Hoang-ho come in time to flow along the top of a gently sloping natural embankment, the sides of which are termed levees in Lou- isiana. The entrances of rivers into lakes or the sea are usually marked by great banks of deposit, or by bars of gravel or sand. In some cases, however, such as the River Plate, the Thames, and Tay, the mixture of river and sea water is gradual, and the sandbanks are spread over a very large area, but not built up into a delta at any one place. In a few instances, such as the Forth, rivers enter deep arms of the sea in which neither banks nor bars are formed. The Congo sweeps directly into the ocean, throwing down great banks of deposit along the continental slope to right and left, but leaving a deep canon-like gully for the bed of the stream itself; a similar con- dition occurs where the Rhone enters the Lake of Geneva. The ultimate source of all rivers is the condensation of water vapor from the atmosphere in the form of rain, snow, and even dew. If the land were com- posed of impermeable rocks all the rain- water not lost by evaporation would run off directly over the surface, and rivers would only flow during and immediately after showers. A large part of the rain- fall, however, soaks into the soil, which retains it as in a sponge, especially if the land be marshy, and allows it to flow off gradually as superficial springs