Page:The New International Encyclopædia 1st ed. v. 17.djvu/190

KIVER. 170 RIVER. plain. Tliis liill'i'iiMU'e in slope is due to the fact that, in the normal development of its val- ley, a stream does its earliest and most ef- fective work near the lower portion, where the volume is greatest, while the rills and creeks of the headwaters have had less time for their work. They also have less water with which to work, and, "being higher, they have a greater task to perform in cutting down their slope. Hence the headwater streams may be vigorously at work excavating their valleys long after the lower course has been reduced to its projih' of equilih- rium — that is, the easiest slope down which the river water with its sediment load may pass. The condition of the river slope, the valley form, and most of the peculiarities of rivers de- ])end in such large measure upon the stage in development which the river has reached in its task of valley formation that it seems quite es- sential, in attempting to give an adequate state- ment of the variations in rivers, that we should first of all consider the question of river-valley development. Let us imagine a new land for the first time exposed to the air. The rain that fell upon it would run off down the easiest slope and quickly carve a channel which would necessarily he steep-sided. Such a condition as this is illus- trated in southern Florida, where the raised sea bottom is so level that the run-off is retarded and the rivers expand into many shallow lakes and swamp tracts. It is also illustrated on the coastal plains of Texas, where shallow, steep- sided valleys are cut in the soft strata of the low-lying plains. At tlie same time that the river is rapidly excavating along its bed, the weather — rain, frost, etc. — is much more slowly attacking the valley walls; but so long as a stream can cut along its channel the deepening will proceed with much more rapidity than the widening. That is to say, the valley form will be that of a gorge. When, however, the stream has reached the limit of its power to cut vertically, that is when it has reached its base JercJ, the slow process of broad- ening under the action of weathering, being in excess, reduces the slope of the valley walls. Therefore the river valley broadens out. Natur- ally the rate of broadening of a valley will vary according to many conditions, two of the most important of which are the nature of the rock and the climate. JIany of the scenic features of river valleys depend upon the influence of rock structure in retard- ing or accelerating weathering. The Colo- rado Canon of Utah and Arizona furnishes numerous examples of this; and it also stands as a type of the effect of climate in retarding valley development. The Colorado is topographi- cally a young stream, but its valley is much less broad and much steeper than it would be had it been formed in a moist climate. In the course of erosion a river excavates more rapidly in the soft than in the hard layers. It therefore locally so increases its slope as to introduce rapids or even falls in its course. The Niagara gorge and falls offer an excellent illus- tration of this phase. There are n uerous other causes for waterfalls than this most common one; for example, the two Yellowstone falls oc- cur where two hard vertical dikes occur in the softer, partly decomposed lava. The Yosemite falls are apparently due to excavation of the main Yosemite valley by a glacier which passed down that valley; and in the Al|)s and the fiords of Norway falls of similar origin abound. Where lava flows have interfered with the .stream courses waterfalls have resulted by the action of the river in excavating a new valley in the lava, as at the Shoshone Kails of Idaho and Spokane Falls of Washington. The glacial interference with rivers is i-espon- sible also for the lakes which abound in Northern Europe and America. It is to this cause that the peculiarities of the Saint Lawrence system, by wiiich there are alternate expansions of water and narrow river-like stretches, are due. The im|)ortance of these lake expanses of ri'ers is not confined to their usefulness in navigation; they also serve to regulate the flow of water. The rise of a few feet in a lake requires a long time for the corresponding discharge into the river to be completed. This checks the floods and furnishes an explanation of the fact that such a river as the main stream of the Saint Lawrence system is free from destructive floods. A lake also acts as a filter to river water, and the outflowing stream is therefore practically free from all mineral load excepting that held in solution. By this means the river has its power of excavation greatly decreased, since the tools with which it works in corrasion are rock frag- ments in suspension. It thus happens that the outlets of lakes are rarely deep valleys of erosion. Ordinary rivers are subject e<l to variations in the depth of water and in the quantity of dis- charge per minute. With the rapid melting of the snow in spring, or at times of heavy rains, the volume of the river is greatly increased and its erosive power is very much greater than at ordinary times. In a large river with many branches a great rise is usually the result of the combination of marked increases in the volume of water supplied by nmnerous branches. At such times the river commonly rises until the channel is no longer able to hold it. Spreading out over the surrounding country, it floods the land, and, instead of a single thread of water, there may be a vast sheet miles in width, as in the case of the lower Mississippi valley. When the flood sub- sides a thin layer of sediment is left behind, and this, in the course of time, builds up a broad flat plain, known as the flood plain (q.v.). whose level is just below that of the level of the ordi- nary floods. The flood-plain soil is so fertile and productive that river flood plains are among the most densely populated parts of the earth; and for protection from the floods the people have found it necessary to build levees to confine the river to its channel. Such control of rivers cannot be made permanently successful, since the sediment that accumulates on the flood plain is then in part deposited in the channel, thus build- ing it up. After a while, therefore, the river must leave its higher channel for the low ground to one side. It is because of the frequent changes of this sort in the Yellow River of China, accom- panied by terrible destruction of life and prop- erty, that the Yellow River has been called 'China's Sorrow.' See Ini^ndation. The flood plains of rivers often merge into a delta (q.v.). In fact, some flood-plain sections, as the lower Nile, were first built as deltas. Wherever a stream carries sediment into the sea the accumulation that settles tends to pro- duce a delta; and if the coast line remains at a