CAUSES OF INUNDATIONS. 233 



In accordance witli the usual economy of nature, we should 

 presume that slie had everywhere provided the means of dis- 

 charging, without distm-bance of her general arrangements or ab- 

 normal destruction of her products, the precipitation which she 

 sheds upon the face of the earth. Observation confirms this pre- 

 sumption, at least in the countries to which I confine my inquiries ; 

 for, so far as we know the primitive conditions of the regions 

 brought under human occupation within the historical period, it 

 appears that the overflow of river-banks was much less frequent 

 and destructive than at the present day, or, at least, that rivers 

 rose and fell less suddenly, before man had removed the natural 

 checks to the too rapid drainage of the basins in which their 

 tributaries originate. The aflluents of rivers draining wooded 

 basins generally transport, and of course let fall, Httle or no sedi- 

 ment, and hence in such regions the special obstruction to the 

 currents of watercourses to which I have just alluded does not 

 occur. The banks of the rivers and smaller streams in the Korth 

 American colonies were formerly httle abraded by the currents.* 

 Even now the trees come down almost to the water's edge along 

 the rivers, in the larger forests of the United States, and the sur- 

 face of the streams seems hable to no great change in level or in 

 rapidity of current.f 



great inundation the flow of the Rhine at Thusis was completely arrested for 

 twenty minutes by a similar discharge from the NoUa. See p. 248, post, note. 

 Of course, when the obstructions yielded to the pressure of the accumulated 

 water, the damage to the country below was far greater than it would have 

 been had the currents of the rivers not been thus obstructed. — Marchand, Les 

 Torrents des Alpes, in Bevue des Eaux et Forets, Sept. 1871. 



* In primitive countries, running streams are very generally fringed by 

 groves, for almost every river is, as Pliny, Nat. Hist., v. 10, says of the Upper 

 Nile, an opifex silvarum, or, to use the quaint and picturesque language of 

 Holland's translation, " makes shade of woods as he goeth." 



f A valuable memoir by G. Doni, in the Eivista Forestale for October, 1865, 

 p. 438, is one of the best illustrations I can cite of the influence of forests in 

 regulating and equalizing the flow of running water, and of the comparative 

 action of watercourses which drain wooded valleys and valleys bared of trees, 

 with regard to the erosion of their banks and the transportation of sediment. 



" The Sestajone," remarks this writer, "and the Lima, are two considerable 

 torrents which collect the waters of two great valleys of the Tuscan Apen- 

 nines, and empty them into the Serchio. At the junction of these two tor- 

 rents, from which point the combined current takes the name of Lima, a cu- 



