232 CAUSES OF INUNDATIONS. 



Their banks are little abraded, nor are their courses mucli ob* 

 structed by fallen timber, or by earth and gravel washed down 

 from the highlands. Their channels are subject only to slow and 

 gradual changes, and they carry down to the lakes and the sea no 

 accumulation of sand or silt to fill up their outlets, and, by raising 

 their beds, to force them to spread over the low grounds near 

 their mouth.* 



Causes of Inundations. 



The immediate cause of river inundations is the flow of super- 

 ficial and subterranean waters into the beds of rivers faster than 

 those channels can discharge them. The insufficiency of the 

 channels is occasioned partly by their narrowness and partly by 

 obstructions to their currents, the most frequent of which is the 

 deposit of sand, gravel and pebbles in their beds by torrential trib- 

 utaries during the floods.f 



ram Lutetiam, [sic] enim Galli Parisiorum oppidum appellant, quae insula est 

 non magna, in fluvio sita, qui eam omni ex parte cingit. Pontes sublicii 

 utrinque ad eam f erunt, rarbque fluvius minuitur ac crescit ; sed qualis sestate, 

 talis esse solet hyeme." — Des Travaux Publics dans leur Rapports avec V Agri- 

 culture, p. 361, note. 



As Julian was six years in Gaul, and his principal residence was at Paris, his 

 testimony as to the habitual condition of the Seine, at a period when the prov- 

 inces where its sources originate were well wooded, is very valuable. 



* Forest rivers seldom if ever form large sedimentary deposits at their points 

 of discharge into lakes or larger streams, such accumulations beginning, or at 

 least advancing far more rapidly, after the valleys are cleared. 



f The extent of the overflow and the violence of the current in river-flooda 

 are much affected by the amount of sedimentary matter let fall in their chan- 

 nels by their affluents, which have usually a swifter flow than the main stream, 

 and, consequently, deposit more or less of their transported material when they 

 join its more slowly-moving waters. Such deposits constitute barriers which 

 at first check the current and raise its level, and, of course, its violence at 

 lower points is augmented, both by increased volume and by the solid material 

 it carries with it, when it acquires force enough to sweep away the obstruc- 

 tion. — RisLER, 8ur V Influence des Forets sur les cours d'eau, in Revue des Eaux 

 et Forets, 10th January, 1870. 



In the flood of 1868 the torrent Illgraben, which had formerly spread its wa- 

 ter and its sediment over the surface of a vast cone of dejection, having been 

 forced, by the injudicious confinement of its current to a single channel, to 

 discharge itself more directly into the Rhone, carried down a quantity of gravel, 

 sand and mud, sufficient to dam that river for a whole hour, and in the same 



