Floodplain

“ The stream is indeed rolling yards high—a red, turbid flood coursing along some fifty feet below, in the bottom of its bed—rearing its mighty masses up in great hissing, crashing waves, rolling over tree-trunks and all kinds of driftwood, with here and there a drowned bullock, whose branching horns and ghastly staring eyes leap weirdly into view, immediately to be drawn in and sucked under by the flood. ”

“ 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. ”

“ And thus as the erosion goes on, so are the beds of the rivers, each season, deepening and leaving remains of their flood plains above. In our times, of course, these formations are going on more slowly and by lesser differences of level, but still the beds of rivers in mountainous districts are, where unobstructed by solid rock, generally deepening, and eating their way more and more into the mountain sides. Consequently they leave remains of their flood plains higher and higher above their beds. ”

“ They have now become broad, shallow stream beds, in which muddy water trickles in slender currents during the dry seasons, while when it rains there are freshets, and roaring muddy torrents come tearing down, bringing disaster and destruction everywhere. Moreover, these floods and freshets, which diversify the general dryness, wash away from the mountain sides, and either wash away or cover in the valleys, the rich fertile soil which it took tens of thousands of years for Nature to form; and it is lost forever, and until the forests grow again it can not be replaced. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ The spring floods find their way, through openings in these higher banks, into the hollow centres of the islands, carrying with them a vast quantity of drift timber, which, being left there, becomes water-soaked, and, finally, firmly impacted in the mud. The young willows, which spring up rapidly, contribute much towards raising the borders of the stream, by intercepting the drift sand which the wind sweeps from the margin of the shallow ponds as they dry up in summer. ”

“ The chief rivers possess all the attributes essential to greatness—width—depth—clearness—rapidity—in one word power. And some of them have long courses—rising in the central heights, and winding round many a huge projection, against which in flood we have seen them dashing like the sea. Highland droughts are not of long duration; the supplies are seldom withheld at once by all the tributaries; and one wild night among the mountains converts a calm into a commotion—the many-murmuring voice into one roar. In flood they are terrible to look at ”

“ The roaring and wailing of the turbid flood now seemed to be repeating in cruel mockery the despairing cries of all the drowning people who were ever the prey of the water-fiends that draw downward in whirlpools to depths where twilight passes into darkness, and take the form of the long waving weeds that look so innocent, but whose grasp is deadly, or guide the current that utters never a sound as it seizes its victim and bears him into an unfathomed gulf under the pitiless rock. ”

“ 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. ”

“ As far inland as the eye can reach there is nothing but flood to be seen, the currents opening out and racing away in every direction. At some distance away may be seen a flooded settlement, the water washing the windows of fifteen or twenty abandoned cottages. On a huge mound some five or six feet out of the outflow, is a group of disconsolate horses and mules who have taken refuge from the rising flood, and other hungry-looking brutes wander over the levee. ”

“ 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. ”

“ 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 shade trees that stood in the streets so trim and beautiful are all bedraggled and bent, their branches festooned with floating wreckage and all manner of offensive things, their leaves sodden, their trunks caked with mud. The streets are seas of yellow ooze. Garden fences and hedges are twisted or torn away. Reeking heaps of indescribable refuse lie moldering where there were smooth lawns and bright flower beds. The houses that stand are all smeared with the dirt that shows the height of the flood. ”

“ When the rains fall upon the open, unwooded country, unless it is of a quite level character, they flow off at once into the beds of the neighboring streams, and pour their united flood into the larger rivers, swelling their volume rapidly to such an extent that their waters can not be confined within their banks, but break out and overspread the adjacent lands, carrying destruction oftentimes to the growing crops, covering fertile fields with masses of gravel and rubbish of various sorts, interfering with manufacturing interests, and often destroying life itself. ”

“ Later, what with failure of rainfall and decrease of slope, there is perhaps a return to the early imperfection of drainage, and the number of side streams diminishes as branches fall from a dying tree. The flood-plains of maturity are carried down to the sea, and at last the river settles down to an old age of well-earned rest with gentle flow and light load, little work remaining to be done. ”

“ When it rains too much, many rivers rise and overflow their banks. The farmer's crops are destroyed, his cattle drowned, and his buildings washed away. We can lessen the danger from these floods, which are very bad in such river basins as those of the Ohio and Mississippi, by building reservoirs in the highlands where the rivers take their start. If when summer comes these rivers are too shallow for safe navigation, the reservoirs can be opened and the streams supplied with this stored water. ”

“ The river boundary in summer flood,
The swirling waters as they rush and roar,
Mock at the wearied limbs that reach their banks,
And can no more, although the foe is on them!
Numbers die here; numbers plunge in—and drown. ”

“ 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. ”

“ The curtailment of the trunk, by its immersion in the river, quite destroys the fair proportions of the tree, and thus makes us sensible of a regularity and propriety in the usual forms of nature. The flood of the present season—though it never amounts to a freshet on our quiet stream—has encroached farther upon the land than any previous one for at least a score of years. ”

“ 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. ”

“ The flatness of the land, in conjunction with the heavy rainfall and the melting mountain snows, often brought floods in early spring. Even before the forests were cut away the rivers frequently overflowed their banks, spreading desolation and destruction over the lowlands; and with the conversion of more and more land to agriculture, the threat of floods proportionately increased, so that the farmer came to look with apprehension at lowering clouds and the steady fall of the rain. ”

“ Even when the rare and sudden flood does come, it has its own way very much as before, for any ordinary channel must be inadequate to contain it. The proper method is to have broad paved or metalled streets with an almost imperceptible slope from one end to the other, and also from the centre to the sides, so that the water may rapidly run off without the necessity for any drain whatever. ”