KIVER EMBANKMENTS. 486 



ing it before it accumulates in sufficient mass to form a new bar 

 rier and threaten serious danger.* 



In the cases of diversion of streams above mentioned, import- 

 ant geographical changes have been directly produced by those 

 operations. Bj the rarer process of draining glacier lakes, nat- 

 ural eruptions of water, which would have occasioned not less 

 important changes in the face of the earth, have been prevented 

 by human agency. 



Hiver Enibcmkinents. 



The most obvious and doubtless earliest method of preventing 

 the escape of river-waters from their natural channels, and the 

 overflow of fields and towns by their spread, is ihat of raised 

 embankments along their course.f The necessity of such em- 

 bankments usually arises from the gradual elevation of the bed of 

 running streams, in consequence of the deposit of the earth and 

 gravel they are charged with in high water ; and, as we have 

 seen, this elevation is rapidly accelerated when the highlands 

 around the headwaters of rivers are cleared of their forests. 



* In 1845 a similar lake was formed by the extension of the Vernagt gla- 

 cier. When the ice barrier gave way, 3,000,000 cubic yards of water were 

 discharged in an hour. — Sonklar, Die Oetzthaler Qebirgsgruppe, § 167. 



f Riparian embankments are a real, if not a conscious, imitation of a natural 

 process. The waters of rivers which flow down planes of gentle inclination 

 deposit, in their inundations, the largest proportion of their sediment as soon 

 as, by overflowing their banks, they escape from the swift current of the chan- 

 nel. The immediate borders of such rivers consequently become higher than 

 the grounds lying further from the stream, and constitute, of themselves, a 

 sort of natural dike of Small elevation. In the " intervales," or " bottoms " of 

 the great North American rivers, the alluvial banks are elevated and dry; the 

 flats more remote from the river, lower and swampy. This is generally ob- 

 servable in Egypt (see Figari Bey, Studi Seientifici sulV Egitto, i., p. 87), 

 though less so than in the valley of the Mississippi, where the alluvial banks 

 form natural glacis, descending as you recede from the river, and in some 

 places, as below Cape Girardeau, at the rate of seven feet in the first mile. — 

 HuMPHKETS AND Abbot, Report, pp. 96, 97. 



In fact, rivers, like mountain torrents, often run for a long distance on the 

 summit of a ridge built up by their own deposits. The delta of the Missis- 

 sippi is a regular cone, or rather mountain, of dejection, extending far out 

 into the Gulf of Mexico along the crest of which the river flows, sending oft 

 here and there, as it approaches the sea, a system of lateral streams resem- 

 bling the fan-shaped discharge of a torrent. 



