Floodwater

“ The presence of a forest on a watershed tends to give the stream which rises thereon its daily supply of water, whether it rains every day or not. By checking evaporation, the forest swells the volume of sea-going water in this stream, and thereby increases its water-power and makes it more useful as a deep waterway. Forests so regulate stream-flow that if all the watersheds were forested but few floods would occur. Forest-destruction has allowed many a flood to form and foam and to ruin a thousand homes. A deforested hillside may, in a single storm, loose the hoarded soil of a thousand years. ”

“ The river 'came down' from the heads and filled the gully with a roaring flood. The lagoon was again almost level with its banks. The dry water-course on the plain sparkled in the distance, like a mirage—only that it was no mirage. No one who has not seen the extraordinary rapidity with which a dry river out West can be changed into a flooded one, could credit the swiftness of the transformation. ”

“ To produce a flood the rainfall must be heavy and widely distributed, and to produce a flood of exceptional height the duration of the rainfall must be so great that the flood waters of the most distant affluents reach the point considered, simultaneously with those from nearer points. The larger the catchment basin the less probable is it that all the conditions tending to produce a maximum discharge should simultaneously occur. Further, lakes and the river bed itself act as storage reservoirs during the rise of water level and diminish the rate of discharge, or serve as flood moderators. ”

“ A FLOOD.
Many of our larger streams, such as the Mississippi, the Ohio, the Illinois, the Arkansas and the Red River, exhibit at certain seasons the most extensive overflowings of their waters, to which the name of floods is more appropriate than the term freshets, usually applied to the sudden risings of smaller streams. If we consider the vast extent of country through which an inland navigation is afforded by the never-failing supply of water furnished by these wonderful rivers, we cannot suppose them exceeded in magnitude by any other in the known world. ”

“ Stream water comes from the surface of the land or out of its porous underlayers, then flows seaward through its creases and folds, affecting the land and the land's creatures along the way and being affected by them. Thus, as we have already noted in more than one way, the management of land and the management of water are closely intertwined, from the way human use of a flood plain may demand structural interference with a river's old habits, to the way erosive farming in some West Virginia valley may help to make it harder to navigate Swedish newsprint into Alexandria by ship. ”

“ Our people were not slow to take advantage of Nature’s kindness and build up once more the happy homes destroyed by the flood. But now we were brought face to face with a most wonderful state of things. Here we were shut in, surrounded by a vast network of streams, and yet taught by our terrible experience so to dread water, that not even to escape from death itself could our people be induced to swim across one of these little rivers or pass over it in any sort of boat. ”

“ Strong gales are prevalent in these latitudes, and these loosen the roots, so that, if the storms are accompanied by flood, large masses of the plants are set free, and begin drifting over the lagoons. Quantities of earth remain clinging to their drifting roots, and thus they are driven backwards and forwards by the wind. The storms come generally just before the flood, and thus there is plenty of water for the reeds to float in. Eventually they are driven, whole acres at a time, into the channel of the river, and are carried down the stream. ”

“ These floods are succeeded by periods of drought. The flow of water in the streams shrinks away, often leaving their beds almost dry. As a consequence, crops and herds suffer, the mill-wheels are stopped or turn but slowly and feebly, the transportation of merchandise is impeded, and the various industries of life suffer. The forests prevent such a deplorable condition of things. The spongy soil formed by their fallen leaves, accumulated for years, retains the rain which falls upon it as in a great reservoir, and obliges it to flow off gradually instead of with a sudden flood. ”

“ But the people who have taken possession of the rich flood plain of the Mississippi River, who have built their homes there, drained and cultivated the land, and built cities and towns on the areas reclaimed from swamps, recognize the elevation of the river bed as the greatest danger that threatens them. Suppose a flood should come. Even if it does not overflow the levees, it may break through the natural banks and thus overflow the cities and the farm lands to left and right. ”

“ The river, or rather the bed of it, is here of great width, and most frightful depth, lined on all sides with huge masses of black rock of every imaginable form. The flood that roars through them is seen only at intervals; here in a full heavy sheet of green transparent water, falling straight and unbroken; there dashing along a narrow channel, with a violence that makes one dizzy to see and hear. ”

“ It is true that a wall of earth everywhere controls the flood to-day, and that the traveller in his boat does not see, as he would have seen two thousand years ago, the glint of water to the north and south at high tide over tufted grass and drowned banks of mud for miles upon either side of his going. ”

“ When, too, in flood time, the hurrying current seems to respond more sensitively to the shape of the shallows and the banks beneath, there boils up from below a ceaseless succession of irregular circles as if the water there expanded from a centre, marking the verge of its outflow with bubbles and raised lines upon the surface. ”

“ Flood waters disappear often within a distance of a few miles, being absorbed by porous soil, stretches of sand, and sometimes by the underlying bed-rocks. In many cases the rivers as they approach the main stream break up into numerous branches, or spread their waters over vast flats. This is especially the case with the tributaries of the Darling on its left bank, where in seasons of great rains these rivers overspread their banks and flood the flat country for miles around and thus reach the main stream. ”

“ Year after year a hundred thousand levee builders, with forty thousand teams of mules or so, and operating huge dredging-machines, repair the damage of the spring tides, build new levees, raise old ones, and pave the banks with revetment—great Willow-tree carpets or rugs covered with broken stone to keep the flood in bounds by stopping the washing away of the soft river banks and the undermining of the protection. ”

“ Any works in the river must be strong enough to withstand the enormous body of water of the flood stage. The locks in the chutes or side channels within the highwater levees will be inundated. The movement of the water must cause scour; and weirs, locks and abutments leading thereto would be threatened with imminent destruction.
The ratio of sediment carried by the river to the stream discharge is large. Much of this sediment is rolled along the bed of the river in waves transverse to the direction of stream flow. ”

“ If the country is flat a river gradually raises the level on each side, the water which overflows during floods being retarded by reeds, bushes, trees, and a thousand other obstacles, gradually deposits the solid matter which it contains, and thus raising the surface, becomes at length suspended, as it were, above the general level. When this elevation has reached a certain point, the river during some flood bursts its banks, and deserting its old bed takes a new course along the lowest accessible level. ”

“ All rivers in densely populated countries should [441] have their flood waters stored in capacious reservoirs, with proper sluices, in the main or adjacent subsidiary valleys, so that during the dry seasons there may be always an ample supply of good water for domestic and agricultural purposes, irrigation, and navigation. The reservoirs will also be advantageous in preventing the too frequent inundations and consequent devastation caused by floods. ”

“ Floods resulting from heavy rains are likely to be greatest where the slopes above the drainage lines are on the whole greatest, for this is the condition under which the water is most quickly gathered into the drainage channels. The most disastrous floods, humanly speaking, are those which affect wide-bottomed valleys, where the flats are settled. In such cases a relatively slight rise may flood very extensive areas. In such valleys the most disastrous floods are generally in the spring, when the waters from the melting snows of the preceding winter are being discharged. ”

“ When at its height, it undergoes little change for some days, and after this subsides as slowly as it rose. A flood generally lasts from four to six weeks, though it sometimes extends to two months. The American naturalist, Audubon, has given a striking account of the rush of waters overspreading the land when once this mighty river has begun to overflow its banks:—
“No sooner has the water reached the upper part of the banks, than it rushes out, and overspreads the whole of the neighbouring swamps, presenting an ocean overgrown with stupendous forest trees. ”

“ The greater also would be the probability of the stream, on the cessation of the flood, not returning to its original channel, which might have become obliterated or filled up, but of its flowing along some new course, it may be miles away from its former channel. Even when not flooded so as to overflow their banks, rivers along which a larger body of water flowed than there does at present, would, so long as they were not confined within deep valleys, have a tendency to wander over a much wider tract of country than that now occupied by their valleys. ”

“ When the stream is in flood and comes roaring down the rocky gorge bearing along with it a vast amount of mud, gravel, and - 169 - stones, one can easily see how the friction of the transported material must wear down the hard bed and sides of the channel, and how this process repeated month after month and year after year, must aid the decomposing forces in scooping out a deep ravine. From the cascade the ascent of the stream becomes steeper and the run of water is consequently more rapid. ”

“ In many cases the natural fall can be increased by a dam or weir thrown across the stream. The engineer will also examine to what extent the head will vary in different seasons, and whether it is necessary to sacrifice part of the fall and give a steep slope to the tail race to prevent the motor being drowned by backwater in floods. Streams fed from lakes which form natural reservoirs or fed from glaciers are less variable than streams depending directly on rainfall, and are therefore advantageous for water-power purposes. ”

“ These waters accumulate, and, overflowing the banks, spread out over the lowlands, resulting in the formation of a well-defined flood plain. This is a periodical occurrence with mature streams, and it occurs more or less regularly with all that are not torrent-like in character. The effect of the overflow is to destroy or to place at a disadvantage those plants of the flood plain that are not hydrophytes. At the same time, a thin layer of fresh silt is deposited upon the valley floor of sand or alluvium. Flooding is most frequent and of longest duration near the banks of the stream. ”

“ How ghastly it is, you would have realized if you could have gone with the writer into the flooded districts of Ohio and Indiana, traveling from point to point in automobiles and motor boats, penetrating to the heart of the flood in boats even before the waters receded, and afterwards on foot. The upper floors of houses not torn from their foundations look all right, but it fairly makes you sick to see the waves of turbid water lapping at second floor sills, with tangled tree branches and broken furniture floating about. ”

“ For instance, a heavy rain in the watershed in the Seine, unless very prolonged, causes less difference in the flow of the river, say at Paris, than might at first have been expected, because the height of the flood in the nearer affluents has passed down the river before that from the more distant ones has arrived. The highest floods are when the rain in the districts drained by the various affluents happen to be so timed that the different floods coincide in their arrival at Paris. ”

“ When there is a gradual rise lasting for a considerable time—and this is most likely to cause a high flood—there is no rise of the flood-level downstream of the embanked reach, except such as is due to the increase in the discharge of the stream consequent on the absorption and evaporation being less than before, owing to the reduced area of flooding in the embanked reach. ”

“ Such a quick rise will cause one body of flood-water to extend for forty or fifty miles in succession, with a width varying from a quarter of a mile to a mile; but it stays sometimes for six weeks, or even two months, upon the ground. And those floods come down with an alarming power and velocity—bridges which have stood for a century are washed away, and districts where floods were previously unknown have became liable to their sudden periodical inundations. The land being wholly in meadow, suffers very heavily from the destruction of its hay. ”

“ But besides the stones and gravel and mud carried down by a river, we must also consider the fate of the remains of plants and animals that are swept along by it, -56- especially in flood-time. In any ordinary flood trees and shrubs, and the smaller animals like mice and moles and rabbits, are drowned by the flood. In greater floods birds and even large animals are drowned, and their remains are buried in the sediment. If they are quite covered over they may perhaps be preserved, and their bones may last for an indefinite period. ”