In plains and large vallies, where there are great rivers, the beds are generally the lowest part of the valley, but the surface of the water is very often higher than the ground adjacent. For example, when a river begins to overflow, the plain will presently be inundated to a considerable breadth, and it will be observed that the borders of the river will be covered the last; which proves that they are higher than the rest of the ground, and that from the banks to a
certain part of the plain, there is an insensible inclination, so that the surface of the water must be higher than the plain when the river is full. This elevation on the banks of rivers proceeds from the deposit of the mud and sand at the time of inundations. The water is commonly very muddy in the great swellings of rivers; when it begins to overflow, it runs very gently over the banks, and by depositing the mud and sand purifies itself as it advances into the plain; so that all the soil which the currents of the river does not carry along, is deposited on the banks, which raises them by degrees above the rest of the plain.
Rivers are always broadest at their mouths; in proportion as we advance in the country, and are more remote from the sea, their breadth diminishes; but what is more remarkable, in the inland parts they flow in a direct line, and in proportion as they approach their mouths the windings of their course increase. I have been informed by M. Fabry, a sensible traveller, who went several times by land into the western part of North America, that travellers, and even the savages, are seldom deceived in the distance they are from the sea if they follow the bank of a large river; when the direction of
the river is straight for 15 or 20 leagues, they know themselves to be a great distance from the coast; but, on the contrary, if the river winds, and often changes its direction, they are certain of not being far from the sea. M. Fabry himself verified this remark in his travels over that unknown and almost uninhabited country. In large rivers there is a considerable eddy along the banks, which is so much the more considerable as the river is less remote from the sea, which may also serve as a guide to judge whether we are at a great or short distance from the mouth; and as the windings of rivers increase in proportion as they approach the sea, it is not surprising that some of them should give way to the water, and be one reason why great rivers generally divide into many arms before they gain the sea.
The motion of the waters in rivers is quite different from that supposed by authors who attempt to give mathematical theories on this subject; the surface of a river in motion is not level when taken from one bank to the other, but according to circumstances the current in the middle is considerably higher or lower than the water close to the banks; when a river swells by a sudden melting of snow, or when
by some other cause its rapidity is augmented, if the direction of the river is straight, the middle of the water where the current is rises, and the river forms a convex curve, of a very sensible elevation. This elevation is sometimes very considerable; M. Hupeau, an able engineer of bridges, once measured the river Avieron, and found the middle was three feet higher than near the bank. This, in fact, must happen every time the water has a very great rapidity; the velocity with which it is carried, diminishing the action of its weight in the middle of the current, so that it has not time to sink to a level with that near shore, and therefore remains higher. On the other hand, near the mouths, it often happens that the water which is near the banks is higher than that of the middle, although the current be ever so rapid. This happens wherever the action of the tides is felt in a river, which in great ones often sensibly extends as far as one or two hundred leagues from the sea; it is also a well known fact that the current of a river preserves its motion in the sea to a considerable distance; there is, in this case, therefore, two contrary motions in a river; the middle, which forms the current, precipitates itself towards the sea,
and the action of the tide forms a counter-current, which causes the water near the banks to ascend, while that in the middle descends, and as then all the water must be carried down by the current in the middle, that of the banks continually descends thereto, and descends so much the more as it is higher, and counteracted with more force by the tide.
There are two kinds of ebbings in rivers; the first above-mentioned is a strong power occasioned by the tide, which not only opposes the natural motion of the river, but even forces a contrary and opposite current. The other arises from an inactive cause, such as a projection of land, an island, &c. This does not commonly occasion a very sensible counter-current, yet it is sufficient to impede the progress of boats and craft, and necessarily produces what is called a dead water, which does not flow like the rest of the river, but whirls about in such a manner that when boats are drawn therein they require great strength to get them out. These dead waters are very perceptible at the arches of bridges in rapid rivers. The velocity of the water increases in proportion as the diameter of the channel through which it passes diminishes, the impelling force being the same;
the velocity of a river, therefore, increases at the passage of a bridge, in an inverse proportion of the breadth of the arches to the whole breadth of the river; the rapidity being very considerable in coming through the arch, it forces the water against the banks, from whence it is reflected with such violence as to form dangerous eddies and whirlpools. In going through the bridge St. Esprit, the men are forced to be careful not to lose the stream, even after they are past the bridge, for if they suffer the boat to go either to the right or left, it might be driven against the shore, or forced into the whirling waters, which would be attended with great danger. When this eddy is very considerable, it forms a kind of small gulph, the middle of which appears hollow and to form a kind of cylindrical cavity, around which the water whirls with rapidity: this appearance of a cylindrical cavity is produced by the centrifugal force, which causes the water to endeavour to remove itself from the centre of the whirlpool. When a great swell of water happens, the watermen know it by a particular motion; they then say the water at the bottom flows quicker than common: this augmentation of rapidity at the bottom, according to them,
always announces a sudden rise of the water. The motion and weight of the upper water communicates this motion to them; for in certain respects we must consider a river as a pillar of water contained in a tube, and the whole channel as a very long canal where every motion must be communicated from one end to the other. Now, independent of the motion of the upper waters, their weight alone might cause the rapidity of the river to increase, and perhaps move it at bottom; for it is known, that by putting many boats at one time into the water, at that instant we increase the rapidity of the under part of the river, as well as retard that of the upper.