KIVER EMBANKMENTS. 489 



are raised by the solid material transported by their currents, 

 their velocity is diminished, they deposit gravel and sand at con- 

 stantly higher and higher points, and so at last elevate, in the 

 middle and lower part of their com-se, the beds they had pre- 

 viously scooped out.* The raising of the channels is compen- 

 sated in part by the simultaneous elevation of their banks and 

 the flats adjoining them, from the deposit of the finer particles 

 of earth and vegetable mould brought down from the mountains, 

 without wliich elevation the low grounds bordering all rivers 

 would be, as in many cases they in fact are, mere morasses. 

 All arrangements which tend to obstruct this process of raising 



running water. Thus plains, more or less steeply inclined, are formed, in 

 which the river is constantly changing its bed, according to the perpetually 

 varying force and direction of its currents, modified as they are by ever- 

 fluctuating conditions. Thus the Po is said to have long inclined to move its 

 channel southwards, at certain points, in consequence of the mechanical force 

 of its northern aflHuents. A diversion of these tributaries from their present 

 beds, so that they should enter the main stream at other points and in differ- 

 ent directions, might modify the whole course of that great river. But the 

 mechanical force of the tributary is not the only element of its influence on 

 the course of the principal stream. The deposits it lodges in the bed of the 

 latter, acting as simple obstructions or causes of diversion, are not less im- 

 portant agents of change. 



* The distance to which a new obstruction to the flow of a river, whether 

 by a dam or by a deposit in its channel, wiU retard its current, or, in popular 

 phrase, " set back the water," is a problem of more diflScult practical solution 

 than almost any other in hydraulics. The elements — such as straightness or 

 crookedness of channel, character of bottom and banks, volume and previous 

 velocity of current, mass of water far above the obstruction, extraordinary 

 drought or humidity of seasons, relative extent to which the river may be 

 affected by the precipitation in its own basin, and by supplies received through 

 subterranean channels from sources so distant as to be exposed to very differ- 

 ent meteorological influences, effects of clearing and other improvements al- 

 ways going on in new countries — are all extremely diflBcult, and some of them 

 impossible, to be known and measured. In the American States, very nu- 

 merous water-mills have been erected within a few years, and there is scarcely 

 a stream in the settled portion of the country which has not several mill-dams 

 upon it. When a dam is raised — a process which the gradual diminution of 

 the summer currents renders frequently necessary — or when a new dam ia 

 built, it often happens that the meadows above are flowed, or that the retarda- 

 tion of the stream extends back to the dam next above. This leads to fre- 

 quent law-suits. From the great uncertainty of the facts, the testimony ia 

 more conflicting in these than in any other class of cases, and the obstinacy 

 with which "water causes" are disputed has become proverbial. 

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