Rivas
capital of the department of Rivas, Nicaragua, 49 miles S. E. of Managua. The town is picturesque and is a development of the older Indian town of Nicarao, which was of importance as a stronghold and center of traffic. The surrounding region is agricultural, the chief produce being cacao, the production of which gives employment to many. Pop. about 15,000.
RIVER. Water falling on the land in the form of rain, or resulting from melting snow, or rising to the surface in springs, flows over the surface to a lower level. Where two slopes of land dip together the surface drainage collects to form a stream, and when evaporation is not very rapid several such streams ultimately unite and the volume of water they carry flows to the sea or to a salt lake. Small streams are termed runnels, rivulets, rills, brooks, becks, or burns; large streams are termed rivers, but the word has no precise reference to the magnitude of the stream to which it is applied.
RIVER source to the end, which if occurring in a lake or the sea is termed its mouth.
The connected streams which unite in one river form a river system. The series of convergent slopes down which a river system flows the land which it drains-forming its basin or catchment area, and the name watershed is also sometimes erroneously applied to it. The names watershed, waterparting, and divide are used to designate the boundary line separating adjacent basins. A watershed is always the meeting-place of the highest part of divergent divergent slopes, and from the characteristic form of continents the main watershed of a continent is almost always the crest of a range of mountains. In many cases, however, the diverging slopes meet in a low plain the summit of which may be occupied by a great marsh whence rivers creep away in opposite directions. The basins of all the rivers draining into the same ocean are called collectively the drainage area of that ocean. The main river to which the others are said to be tributary gives its name to the whole river system. It is often difficult to decide which of several converging streams is entitled to carry the name of the main river to its source. Some geographers give this distinction to the longest, others to that with the highest source, and others to that with the most direct course. The course of a typical river has been divided into three parts, though these are not represented in all cases. The torrential or mountain track is the steepest, its gradient usually exceeding 50 feet in a mile, and the velocity of its current being very great. The valley or middle track has a gradient which is rarely greater than 10 feet and often less than 2 feet in a mile. The plain track nearest the mouth of a river has a gradient of only a few inches in a mile. Rivers such as the Amazon, Mississippi, Ganges, Volga, and the long rivers of Siberia, in which the plain track is of very great length, are the most valuable for navigation, the limit of easy navigability being a gradient of about 1 foot in a mile.
The velocity of a river is proportional to the slope of the bed, but it also bears a relation to the depth of the channel and the volume of water flowing in it.
On account of friction on the bottom and sides of the channel retarding the stream, the water flows fastest on the surface and in the middle. The carrying power of a river for suspended solid particles and for stones and gravel pushed along the bed depends on the velocity alone. The following table shows how rapidly the carrying power falls off as the velocity diminishes.
The beginning of a stream-whether brook or river is called its source, and may be a spring issuing from underground, a lake or marsh in which rainfall accumulates, melting snow, or simply the gathering tricklings from falling rain. The path of a stream is its course, and is the line of lowest level from the E-Cyc Vol 8 RIVER 0.170 mile per hour will just begin to work on fine clay. lift sand as coarse " " " lift fine sand. " " " 0.682 66 " " 1.364 miles " 66 2.045 " " " as linseed. sweep along fine gravel. roll along rounded pebbles one inch in diameter. 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 torrential and valley tracks, and the character of the valleys or gorges produced depends largely on the geological structure of the region. The course of a river is frequently determined by lines of faults, but perhaps more often it appears to be independent of the nature of the strata. Some great rivers, notably 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.
RIVER 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 sweep along slippery angular to the formation of caves and understones as large ground rivers, which, as a rule, emerge from their subterranean channels on lower ground. Sometimes they do not reappear 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 toward 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. During 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 simultaneously 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 Louisiana. 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 cañon-like gully for the bed of the stream itself; a similar condition occurs where the Rhône enters the Lake of Geneva.
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 cañons are cut to a great depth across high plateaus; in rainy regions subaërial denudation 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 waterfalls 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 monuments of its erosive power. When a river is fairly established in its valley it is, geologically speaking, a more permanent feature than lakes or mountains. Upheaval, which acts very slowly, may even elevate a range of mountains across 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 composed of impermeable rocks all the rainwater 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 rainfall, 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.
RIVER Some also percolates deeply into the rocks, ultimately emerging as deep-seated springs at a great distance. When a river flows toward a region of great evaporation and small rainfall, such as exists in the interior of each of the great continents, evaporation removes more water than is supplied by the remote tributaries, and the stream may fail to fill the hollow it enters, and therefore cannot overflow into the sea. This is the case with the Oxus entering the Aral sea, and the Volga entering the Caspian.
It may be that evaporation is so far in excess of contributions from distant rainfall or snow-melting that the river dries up as it flows, and its last remnant is absorbed in the desert sand. This is the fate of the Murghab, the Heri-rud, the Zerafshan, and many other rivers of central Asia.
The annual inundations of the Nile are due to the monsoon rainfall on the great mountains of Abyssinia. The Orinoco is another instance of seasonal rains producing tremendous inundations, over 40,000 square miles of Llanos being said to be laid under water by the summer rains. The Amazon is an instance of a river which is always more or less in flood as the various tributaries attain their greatest height at different seasons. The Ganges overflows its banks in summer when the monsoon rainfall is reinforced by the melting of snow on the Himalayas. Where the seasons of maximum rainfall and of snowmelting are different, as in the Mississippi, the Tigris, and Euphrates, there are two regular floods in the year.
The danger of flooded rivers arises from the suddenness with which the water rises and overflows narrow valleys or even plains. Frightful devastation follows the bursting of glacier obstruction lakes in mountain valleys. The most serious floods in the Danube and Theiss have resulted from the constriction of the channel at the Iron Gates, which prevents the flood water from passing away as rapidly as it comes down; the current of the Theiss is sometimes reversed for many miles. Great rivers which have embanked their course above the level of the plain are the most dangerous of all when flooded. The damage caused by the bursting of the levees on the lower Mississippi necessitates a great expenditure in strengthening the embankments, and the most disastrous inundations recorded in history have followed the bursting of the banks of the Hoang-ho and its consequent changes of course.
River water is spoken of as fresh, but it always contains a certain amount of solid matter in solution, varying from RIVER two grains in the gallon or less in rivers draining hard crystalline rocks to 50 grains in the gallon or more in lime stone districts.
The temperature of rivers, as a rule, follows that of the air, but is subject to variations on account of the effect of rain.
The great rivers of Europe and Asia, such as the Rhine, Danube, Volga, Indus, kiang, afford' dus, Ganges, Brahmaputra, Yang-tseafford access to the sea to enormous populations. 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 St. Lawrence, though less extensive, are of greater value for carrying sea traffic to inland places. In their torrential and upper valley tracks rivers are of use chiefly for transporting timber and driving machinery. It is interesting 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. In hot countries rivers are of the utmost service in irrigating agricultural land; the Zerafshan and Murghab are entirely consumed in that service, and since the completion in 1890 of the barrage on the Nile no water escapes to the Mediterranean in the low Nile months except along irrigation canals.
The largest rivers of the world, with their length in miles, are: Amazon, 4,000; Nile, 3,766; Yangtse, 3,400; Yenisei, 3,300; Mississippi, 3,160; Missouri, 3,000; Congo, 3,000; Lena, 2,800; Niger, 2,900; Ob, 2,300; Hoang-ho, 2,600; Amur, 2,500; Volga, 2,300; Mackenzie, 2,525; La Plata, 2,300; Yukon, 2,300; St.
Lawrence, 2,150; Rio del Norte, 1,800; São Francisco, 1,200; Danube, 1,725; Euphrates, 1,700; Indus, 1,700; Brahmaputra, 1,680; Zambesi, 1,600; Ganges, 1,500; Mekong, 2,500; Amu Daria, 1,500; Ohio, 950.
The pollution of rivers has of late years, in consequence of the extension of manufactures, caused serious concern.
No person has a right to poison or pollute a stream, and if he do so any of the persons whose lands abut on the stream lower down may bring an action to recover damages. At common law, indeed, in every question of river pollution, the real question of fact is whether there has been any material increase of pollution beyond that which is natural to the particular stream, or beyond that which has existed there for the prescriptive period. Questions of river pollution are eminently fitted for submission to a jury, and are generally disposed of in that way.
RIVER CRAB In the United States the common law of England was at first followed; but in some of the States it is expressly declared that the common law is inapplicable. Mining rights have been specially determined in some districts; and the laws as to irrigation rights have been elaborately defined in Colorado and elsewhere.