26 



AQUEOUS AGENCIES. 



lands of the river-swamp. As the bed of the river continues to rise 

 by deposit, the levees must be constantly elevated in proportion ; but 

 the river-swamp, being deprived of its share of deposit, does not rise. 

 Thus, under the combined effect of human and river agencies contend- 

 ing for mastery, an ever-increasing embankment is formed, until finally 

 the river runs in an aqueduct elevated far above the surrounding plain. 

 This is very remarkably the case with the river Po, which is said to run 

 in a channel that has been thus elevated above the tops of the houses 

 in the town of Ferrara. Fig. 17 is an ideal cross-section of a river and 



Fig. 17. 



flood-plain, left at first to the action of natural causes for a time, but 

 afterward interfered with by the construction of artificial levees. The 

 dotted strata show the work of Nature, and the undotted the work of 

 man. It is easy to see that the destructive effects of overflow from 

 accidental crevasses become greater and greater with the elevation. 

 The Po has thus several times broken through its levees and deserted 

 its bed, destroying several villages. The best examples of rivers suc- 

 cessfully leveed are those of Italy and Holland. The Mississippi has 

 never been successfully leveed ; but, if it should be, it would commence 

 to build up a similar aqueduct, until the whole bed of the river would 

 finally rise above the level of the river-swamp.* 



6. — Deltas. 



Deltas are portions of land situated at the mouths of rivers, and 

 reclaimed from the sea by their agency. Over the flat surface of the 

 delta the river runs by inverse ramification, and empties by many 

 mouths. They are usually of irregular triangular form, the apex of 

 the triangle pointing up the stream. The delta of the Nile (Fig. 18) 

 is perhaps the best example of the typical form. As seen in the figure, 

 at the head of the delta the river divides into branches, and communi- 

 cates with the sea by many mouths. The area of land thus made va- 

 ries with the size of the river, the proportion of sediment in its waters, 

 and the time it has been making sedimentary accumulations. The 



* It is probable that the effect of levees in raising the river-bed has been greatly ex- 

 aggerated. Recent observations on the Po seem to show that the elevation is confined 

 to the upper reaches of the flood-plain region, being prevented in the lower course by 

 the increased velocity of the current produced by levees. 



