238

RIVER AND INLAND NAVIGATION.

Any secondary streams intercepting the line of the towing-path must be bridgedover in such a way as to otfer no obstacle to the tow-line.

Worts for the Preservation of the Bants and of the Bed of Rivers,

28. These are of two descriptions; sometimes they are intended to preserve theneighbouring lands from the corrosions of the stream ; sometimes for the purpose ofmaintaining the latter in its proper direction, and at a regular distance from thetowing-paths. The dredging works for the purpose of clearing the alluvial depositsmay be included in this category.

In rivers which run with great velocity, such as the Rhine , the Rhone, and theDordogne , these works are of the very highest national importance. In some casesthe rivers run between the territories of nations at war with each other, as in thecase of the Rhine ; at others they traverse consecutively several states, as the Danube or the Po. The lower Rhine , whose floods formerly spread themselves over nearlythe whole width of the valley in which it flows, has been gradually confined withinits present bed by a series of embankments about 10 feet wide on the crown, whichare disposed occasionally in several nearly parallel lines; so that if one embankmentgave way, its ruin would only affect the portion of land it immediately protected.The banks of the Loire , between Orleans and Angers, answer the same purpose;although, from the fact of their being only single, some serious devastations have re-sulted from their rupture.

The overflowing of the freshets of such rivers as the Rhine , the Rhone, and the Loire in France , and of the Arno and Po in Italy , have produced the following result; viz.that the zones in the immediate proximity to the river are generally more elevatedthan those which are farther removed and upon the extreme edge of the valley.This is to be accounted for by the fact that the great floods deposit near to theirbanks the heaviest and most voluminous matters they hold in suspension, and thatthey are far less loaded when they spread out over the plain. From this causemarshes are eventually formed; because the rain and spring water, being unable toflow into the river channels, remain upon the surface of the land.

29. One of the cheapest, and at the same time it is one of the most effectual,methods of protecting the lied of a river, is by means of planting aquatic trees,such as the willow, &c., on both its banks. The roots of these trees spread veryrapidly, and in themselves form the first defence of the bed of the river, as also ofthe banks. At the same time, inasmuch as they retain the mud which may be in sus-pension, they serve also gradually to raise those parts where they are planted. It is,however, necessary to execute such plantations on both sides at once; otherwise thedefence of one side will but serve to increase the force with which the waters attackthe other.

Rectification of A very remarkable work of this description, which, it may be added, has beenthe Midouze. attended with signal success, was that executed for the purpose of rectifying andregulating the bed of the Midouze, between the Port de Marsan and its embouchurein the Adour, for a distance between 25 and 26 miles. The width of the pass createdvaried between 71 feet and 91 feet; the variations in the width being equally, ornearly equally, divided into four parts; and the limits of the new bed of the riverwere comprised between two concentric curves.

The original bed of the river was very wide and irregular, being formed in themidst of moveable sand-banks, through which the waters found a channel shiftingalmost at the caprice of every wind. The waters of the Midouze, even in theirnormal state, carried down great quantities of mud and sand. The system adopted