Transportation on Water 381 



to all land cultivation in the vicinity made necessary protective 

 and preventive measures. The construction of dikes, called 

 levees, helped to some extent in keeping the river within its 

 banks ; but also gave rise to new difficulties. A river restrained 

 by dikes is usually sluggish, and if its load of suspended ma- 

 terial is large, it deposits this in considerable quantities, thus 

 gradually raising the channel of the river above the level of 

 the surrounding country. A break in the levees is then worse 

 than a gentle flooding of the land. 



The Mississippi carries along on the floor of its channel 

 enough material to cover a square mile nineteen feet deep 

 every year. It carries enough suspended matter to increase 

 the depth of the square mile by twenty-four feet. It would 



FIG. 135. Jetties to force the Mississippi River to keep its channel clean and 

 prevent dangerous floods. 



be a gigantic, perhaps impossible, task to remove this by dredg- 

 ing. The engineers associated with the Inland Waterways 

 Commission attempted to solve the problem by making the 

 river clear its own channel. The diagram (Fig. 135) shows 

 how this is done. The section is transverse, and the river is 

 supposed to flow away from the observer. Jetties, large dams 

 of loose rock, are built out from both sides of the river at inter- 

 vals of about thirteen hundred feet. At low water the river 

 flows at the level, b, b, and not at c, c, where it would be too 

 widespread and sluggish. By confining the stream to the 

 narrower channel, its usual low water depth is avoided and 

 its flow increased and accelerated enough to clean the floor, d, d. 

 At high water the whole channel a-a, is occupied. For the 

 guidance of navigators the location of the jetties is shown on 

 charts. 

 Future of river commerce. The inland waterways of the 



