RIVER EMBANKMENTS. 5OL 
Tn the cases of diversion of streams aboye mentioned, impor- 
tant geographical changes have been directly produced by 
those operations. By the rarer process of draining glacier 
lakes, natural eruptions of water, which would have occasioned 
not less important changes in the face of the earth, have been 
prevented by human agency. 
River Embankments. 
The most obyious and doubtless earliest method of prevent- 
ing the escape of river-waters from their natural channels, and 
the overflow of fields and towns by their spread, is that of 
raised embankments along their course.* The necessity of such 
embankments 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. When a river is embanked at a given point, and, con- 
* 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 channel. 
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 observable in 
Ecypt (see FIGARI Bry, Studi Scientifict sul? Hgitto, 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 milee—HUMPHREYS AND 
Axpsort, feport, 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 oif 
here and there, as it approaches the sea, a system of lateral streams resembling 
the fan-shaped discharge of a torrent. 
