514 REMOVAL OF OBSTRUCTIONS. 
Removal of Obstructions. 
The removal of obstructions in the beds of rivers by dredging 
the bottom or blasting rocks, the washing out of deposits and 
locally increasing the depth of water by narrowing the channel 
by means of spurs or other constructions projecting from the 
banks, and, finally, the cutting off of bends and thus shortening 
the course of the stream, diminishing the resistance of its shores 
and bottom and giving the bed a more rapid declivity, 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 unfore- 
seen diversions of the current,* and the cutting off of bends, 
though oecasionally effected by nature herself, and sometimes 
advantageous in torrential streams whose banks are secured by 
solid walls of stone or other artificial constructions, seldom es- 
tablishes a permanent channel, and besides, the increased rapid- 
ity of the flow through the new cut often injuriously affects 
the régime of the river for a considerable distance below.t+ 
* The introduction of a new system of spurs with parabolic curves has been 
attended with great advantage in France.—Annales du Génie Civil, Mai, 1863. 
{+ This practice has sometimes been resorted to on the Mississippi with ad- 
vantage to navigation, but it is quite another question whether that advan- 
tage has not been too dearly purchased by the injury to the hanks 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 
augmentation of velocity consequent upon this increase of inclination is not 
eomputable 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 sufficient to produce great effects on the 
height of floods, the deposit of sediment in the channel, on the shores, and at 
the outlet, the erosion of banks and other points of much geographical import- 
ance. 
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. 
