[357] Notwithstanding this favorable circumstance, the damage done by the inundation of 1840 in the valley of the Rhone was estimated at seventy-two millions of francs.—Champion, Les Inondations en France, iv, p. 124.
Several smaller floods of the Rhone, experienced at a somewhat earlier season of the year in 1846, occasioned a loss of forty-five millions of francs. "What if," says Dumont, "instead of happening in October, that is between harvest and seedtime, they had occurred before the crops were secured? The damage would have been counted by hundreds of millions."—Des Travaux Publics, p. 99, note.
[358] Troy, Étude sur le Reboisement des Montagnes, §§ 6, 7, 21.
[359] For accounts of damage from the bursting of reservoirs, see Vallée, Mémoire sur les Reservoirs d'Alimentation des Canaux, Annales des Ponts et Chaussées, 1833, 1er sémestre, p. 261.
[360] Some geographical writers apply the term bifurcation exclusively to this intercommunication of rivers; others, with more etymological propriety, use it to express the division of great rivers into branches at the head of their deltas. A technical term is wanting to designate the phenomenon mentioned in the text.
[361] Mardigny, Mémoire sur les Inondations de l'Ardèche, p. 13.
[362] In the case of rivers flowing through wide alluvial plains and much inclined to shift their beds, like the Po, the embankments often leave a very wide space between them. The dikes of the Po are sometimes three or four miles apart.—Baumgarten, after Lombardini, Annales des Ponts et Chaussées, 1847, 1er sémestre, p. 149.
[363] It appears from the investigations of Lombardini that the rate of elevation of the bed of the Po has been much exaggerated by earlier writers, and in some parts of its course the change is so slow that its level may be regarded as nearly constant.—Baumgarten, volume before cited, pp. 175, et seqq. See Appendix, [No. 49].
If the western coast of the Adriatic is undergoing a secular depression, as many circumstances concur to prove, the sinking of the plain near the coast may both tend to prevent the deposit of sediment in the river bed by increasing the velocity of its current, and compensate the elevation really produced by deposits, so that no sensible elevation would result, though much gravel and slime might be let fall.
[364] To secure the city of Sacramento in California from the inundations to which it is subject, a dike or levée was built upon the bank of the river and raised to an elevation above that of the highest known floods, and it was connected, below the town, with grounds lying considerably above the river. On one occasion a breach in the dike occurred above the town at a very high stage of the flood. The water poured in behind it, and overflowed the lower part of the city, which remained submerged for some time after the river had retired to its ordinary level, because the dike, which had been built to keep the water out, now kept it in.