INUNDATION (Lat. inundatio, from inun dare, to overflow. from in, in + undarc, to rise in waves, from undo. wave: connected with 011G. undea, undo, AS. Sp, wave). :Many large areas of law ground near rivers or the ocean are in danger of inundation front various Of these inundations the most catmint. and probably also the most destructive, are the river floods that result from unusual rains or melting snows. Most rivers are subjected to rising and falling by these causes. and those that flow through flood plains or deltas may rise and overflow the bordering plains. The best-known instance of such river flooding is supplied by the Nile. which makes possible the existence of millions of people on the Nile in the broad African desert. The rising of the waters begins in .June in the lower Nile. the accumulation having taken weeks to down eitallIWI from the tipper Waters. II continues till September, and at Cairo the water rea•hes a height of 2.5 feet abate the normal river level. .1 vast tract of flood plain and delta is inundated. and this water makes agriculture possible in the des ert, and is the basis upon which the ancient Egyptian agriculture was founded, The sedi ment deposited by the floods fertilizes the land, and is raising the level of the flood plain at a rate of over four ladle, a century. The Mississippi also supplies an illustration of great river floods. .1n area of :30,000 square inhabited by a million people, is liable to floods. Low embankments, built by the floods, and called natural keep the ricer in its channel, excepting during very high Water, and levees added by Mall are built from the Gulf to Cairo in the effort to hold the extraordinary floods. But even these artificial levees are soffile tillieS ineffective. Crevasses 50 to 500 feet wide break through the levees and a vast area is inundated. These great (hoods come usually be tween February and :Slay, and are caused by the coincidence of 6eaty rains, 01• melting snows, or both. in the lissouri, Mississippi, and I Mill During the great floods the diseharge of the 'Mississippi reaehes 2.000.000 eubie feet per second, and is far in excess of the capacity of the channel. The water-level then rises 40 .50 feet. and great destruction of life and prop erty results. Great inundations by the .N1 is,js ,,ippi are reoorled in 1828, 1844. 18-19, 1850. 1853, 1559, 181;3, 1807, 1870, 1874, 1882, 1884, 1890, 1892, I593, and 1807. The Iloang•ho of China flows out of its mountainous course over a low alluvial fan delta is rapidly growing out into the sea. The town of Putai, was
on the seacoast about 2000 years ago, is now over 40 miles inland. Not only is the alluvial fan delta growing outward. hut upward, and when the river bed is built up above the level of the surrounding plain the course of the stream he exceedingly unstable and liable to shifting in time of flood. The river then sometimes bursts through levees fully 70 feet high, erected more than half a nine from the banks. It has shifted its efaIrse nine times in 2500 years, laying waste a region as large as (4eat Britain, Its mouth has changed position fully 200 miles by these shiftings. In the flood of 1887. which covered an area of 50.000 square miles, densely populated, a million people weir drowned, all() still more lives Were lost by the famine and disease that followed the disaster. The river has been used a- a weapon of war as far hack as the year 1209. Hundreds of thousands of people have thus been drowned by the of the river thirds against then'. Smaller river floods are of eo lllll ton occurrence, and are frequently caused by ice jams. which form a temporary dant and hold the water lock. The bursting of the (lams of artitkial ponds and lakes also causes destructive floods.
Inundations of the sea are far less widespread than those of rivers, because some coasts are to-. high. and others are not frequently visited by high sea or 'tidal waves.' The great number of river lowlands, and the frequency of the causes of a rise in the river water. makes these river more than those of the sea. But where low coast:: are exposed to inundations of the sea, the results are terribly destructive, since the coasts are populated. There are various causes of inundations by the sea; the simplest are Hitt-• hated ted by t hose count ries which are actually below the level of the highest tides, and are pro. tested from inundation by sand dune hills and dikes. The Netherlands offer the hest illustra tion. This plain is the delta of the Rhine and Aleuse, and is open to floods 11'4)111 ricer and :11111 parts Of it 1 le well below lbe level of the sea. At various times the dikes, which were fman•rly less solidly constructed than at present. have given way, and large tracts have been submerged, causing a vast destruction of life and property. For example, in 1421 a flood destroyed 72 villages, and 100,000 people per ished. During the Spanish wars the people ‘oluntarily admitted the sea to protect their cities from the attacks of the Spaniards.