FLOODS ON ENGLISH RIVERS.
As a flood is the result of an attempt by Nature to make a river-bed carry off more water than it will hold, it is obvious that where the fall is rapid, and the course of the stream has not been interfered with by man, floods will be rare. The tendency in the course of ages is for mountain-fed rivers to hollow out a far wider and deeper channel than even an excessive rainfall can fill. But where the "full-fed river winding slow" flows through wide flat valleys, of fat soil, without rocks or stones, there is no such gradual deepening of the river-bed. On the contrary, it tends to become silted up, or choked with vegetation, as in the extreme case of the rivers of Norfolk. These streams, the common type of the South of England, have been wont to overflow in rainy weather from the remotest ages. All the flat meadows on their banks have been gradually made by the deposits of flood water, to which we owe these beautiful and character- istic features of English landscape. But the flatness of these meadows also gives a clue to the form which their flooding naturally took. The water in the overflow must have been still and shallow, with no current, or the mud would have been laid in heaps and low mounds, as is the flood deposit of the more violent Northern streams, and not in the wide levels which have made the riverside pastures. The same causes which left the river-beds shallow and unscoured also checked the rate at which the water was delivered into the channel. It had to soak or flow slowly and gradually across the flat valleys into the stream, and a river which was unequal to holding between its banks more than the results of two days' heavy rain did not perhaps receive the inflow until double that time had elapsed.
The cause of the change by which slow rivers, where the soil of their basin is of a heavy nature, tend to flood rapidly is to be found in the wonderful energy of English agriculture, which has undermined millions of acres with drain-pipes, and linked these up with a system of ditches which, though not originally dug on any common system, are almost invariably near some little head-water or feeder of a brook that they can discharge into. Such natural flows of water will be found within a few hundred yards at most of every field in the "heavy land" countries, the almost invisible ridges which make the water-
parting being maiked as contour lines on the Ordnance maps. Once connected with these natural drainage lines, the water dis- charged from the field finds its way to the brook, the brook to the river, and the river to the sea, by the line of least resist- ance formed by ages of rain and denudation. This is excel- lent for the upper lands, which by their artificial pipes throw off the water in perhaps a fifth of the time which would have been necessary before the subsoil drainage was begun. But the effects are seen quite as rapidly in the flooding of the flat valleys. The growth of a Southern flood is first seen at the inflows of the tributary streams. The swollen river, though it has not yet overflowed its banks, passes on, its whole body of water moving solidly forward, giving to the side-streams of less force and volume no more chance of "edging in" than a loose crowd in a side-street has of pene- trating into a column of troops marching down a central thoroughfare. So the incoming waters are repelled, and forced back up the tributary, which at once overflows, fills a great elbow with muddy refluent water, and in time, from repeated discouragement and the deposit from the overflow on the bank furthest from the source of the main stream, is "deformed." Instead of entering at an acute angle with the course of the main river, it "sags back," and in time enters the river "up-stream," making matters worse than ever. If, on the other hand, the tributary has a rapid flow, it " hits " the flood current of the main river harder than the latter can resist, and forces a way in on more than equal terms. These two conditions of tribu- taries in flood are seen perfectly at the junction of the two Oxfordshire rivers, the Cherwell and the Windrush, with the Thames. In the former the "sagging back" of the tributary made it necessary at last to cut a fresh channel so that its waters might not touch the Thames up-stream. The Wind- rush, on the other hand, comes into the Thames with con- siderable speed, and enters it at an acute angle and with a merry flow, just above Sir John Grolafre's " New " Bridge of the fifteenth century.
The bed of the Thames is of a very unusual character, which has not been greatly affected either by its canalisation, or by the great quantities of water abstracted from it in the lower parts of its course. Except for a few hundred yards at Clifden Hampden, where it flows over rock, the whole of the river-bed is shifting, loose gravel. Between the gravel the water percolates and flows, and so penetrates into the gravel beds on either side far beyond the banks. Thus there is an " underground " stream in the Thames, the level of which rises and falls. In order to fill the Thames, and before this can be done, the "underground Thames," below and parallel with the banks of its channel, must also rise and fill, which probably delays the flooding. In rivers where the bottom is mud and clay the water spills over the banks as though poured into an overfilled conduit. The elasticity of the Thames bed possibly accounts for the general height of the alluvial meadows on its banks. As a rule these are now well above flood mark and lie high and dry, low, flooded ground being quite the exception by its banks. The flood deposits could only have lain on these meadows in times of very exceptional rain, when the "underground Thames" first filled and then the whole river suddenly overflowed them. The enormous antiquity in geological time of the Thames as a river would give ample duration for the formation of the meadows, even by occasional floods.
Floods such as those which have recently filled the river channels of the North with sound and fury differ from those in the South as the blasting of a quarry does from the melting of a snow-drift. They are not gradual accretions of water along the course of the river. They begin at the top where the river leaves the mountain, and come rushing down full-grown. The higher and more rocky the mountain the swifter and strongel. the flood. The face of the waters changes, like the face of an angry man. The dark flush of the surface deepens as the flood mounts, the noise of many waters grows. From beneath them the rolling and clashing of the boulders sound, until the added weight of water smothers the grinding of the rocks in the river's roar. In the whole scene romance and beauty are replaced by violent natural forces unre- strained. The Northern river in flood on its upper waters is the only natural commotion, except the breakers in a storm upon the cliffs of our coast, which in this temperate country
ever presents the .menace of certain and inevitable death. Even the mere nameless beginnings of these floods have the strength of the infant Hercules, where they are born upon the very brows of the mountains. They cut crevices and fissures in the rock as if with saws of adamant. Lower, yet where their course even in flood could be leapt across on foot, they use the boulders as chisels and cut deep into the solid rock, until where the channel widens the blows of each mass as it is slung against the sides by the rush of waters descending leaves a clean-cut cavity, as if from the stroke of a mason's hammer. But the very speed of these floods soon empties the river-bed. Twenty-four hours, at a speed of five miles an hour, would leave a hundred and twenty miles of river-bed dry; and as a rule half that time sees the river at its normal level once more.
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