936 CAUSES OF INUNDATIONS. 
Rivers fed by springs and shaded by woods are comparatively 
uniform in volume, in temperature, and in chemical compo- 
sition.* Their banks are little abraded, nor are their courses 
much obstructed 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 
superficial and subterranean waters into the beds of rivers fas- 
ter than those channels can discharge them. The insufticiency 
of the channels is occasioned partly by their narrowness and 
partly by obstructions to their currents, the most frequent of 
than four American townships, would receive 53,777,777 cubic yards of water. 
Of this quantity it would retain, or rather detain, if wooded, 41,000,000 yards; 
if bare, only 11,000,000. The difference of discharge from wooded and un- 
wooded soils is perhaps exaggerated in Col. Torelli’s report, but there is no 
doubt that in very many cases it is great enough to prevent, or to cause, de- 
structive inundations. 
* Dumont gives an interesting extract from the Misopogon of the Emperor 
Julian, showing that, in the fourth century, the Seine—the level of which 
now varies to the extent of thirty feet between extreme high and extreme low 
water mark—was almost wholly exempt from inundations, and flowed with a 
uniform current through the whole year. ‘‘Ego olim eram in hibernis apud 
caram Lutetiam, [sic] enim Galli Parisiorum oppidum appellant, qua insula 
est non magna, in fluvio sita, qui eam omni ex parte cingit. Pontes sublicii 
utrinque ad eam ferunt, rardque fluvius minuitur ac crescit; sed qualis estate, 
talis esse solet hyeme.”—Des Travaux Publics dans leur Rapports avec 0 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 
provinces 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. 
