REMOVAL or OBSTRUCTIONS. 497 



Hemoval of Obst/ntcUons. 



The removal of obstructions in tlie beds of rivers by dredging 

 the bottom or blasting rocks, the washing out of deposits, and the 

 local increase of the depth of water through narrowing the channel 

 by means of spurs or other constructions projecting from the 

 banks, and, finally, the cutting o£E of bends, thus shortening 

 the course of the stream, diminishing the resistance of its shores 

 and bottom and giving the bed a more rapid dechvity, have all 

 been employed not only to facilitate navigation, but as auxiliaries 

 to more effectual modes of preventing inundations. But a bar 

 removed from one point is almost sure to re-form at the same or 

 another, spurs occasion injurious eddies and unforeseen diversions 

 of the current,* and the cutting off of bends, though occasionally 

 effected by nature herself, and sometimes advantageous in tor- 

 rential streams whose banks are secured by soKd walls of stone or 

 other artificial constructions, seldom establishes a permanent chan- 

 nel ; and besides, the increased rapidity of the flow through the 

 new cut often injuriously affects the regime of the river for a 

 considerable distance below.f 



* The introduction of a new system of spurs with parabolic curves has been 

 attended with great advantage in France. — Annates du Genie Givil, Mai, 1863. 



f This practice has sometimes been resorted to on the Mississippi with ad- 

 vantage to navigation, but it is quite another question whether that advantage 

 has not been too dearly purchased by the injury to the banks at lower points. 

 If we suppose a river to have a navigable course of 1,600 miles as measured 

 by its natural channel, with a descent of 800 feet, we shall have a fall of six 

 Inches to the mile. If the length of channel be reduced to 1,200 miles by 

 cutting off bends, the fall is increased to eight inches per mile. The augmen- 

 tation of velocity consequent upon this increase of inclination is not computable 

 without taking into account other elements, such as depth and volume of water, 

 diminution of direct resistance, and the like ; but in almost any supposable case, 

 it would be suificient to produce great effects on the height of floods, the de- 

 posit of sediment in the channel, on the shores, and at the outlet, the erosion 

 of banks and ocher points of much geographical importance. 



The Po, in those parts of its course where the embankments leave a wide 

 space between, often cuts off bends in its channel and straightens its course. 

 These short cuts are called salti, or leaps, and sometimes abridge the distance 

 between their termini by several miles. In 1777, the salto of Cottaro short- 

 ened a distance of 7,000 metres by 5,000, or, in other words, reduced the 

 length of the river by five kilometres, or about three miles, and in 1807 and 

 1810 the two salti of Mezzanone effected a still greater reduction. 



