For flood purposes alone it would be necessary to provide reservoirs of sufficient capacity to contain the run-off waters resulting from the largest storms. With such provisions it would be necessary to entirely empty the reservoir as soon as possible after a storm had passed and leave its full capacity available for the next storm. It is therefore better, wherever possible, to provide a reservoir capacity considerably larger than that represented by the run-off from the heaviest storms, so that water may be stored for use as power or domestic supply. With such provision it is necessary merely to draw from the reservoir water to a depth equivalent to the stream run-off in the drainage area above. The second plan for prevention of flood damages involves provisions for letting the flood water out rapidly by removing obstructions to its flow by straightening and deepening the channels and providing long embankments, dikes, or levees which rise above the ordinary river level to a height exceeding that of the stream during its highest floods. This plan is most generally followed in the case of large rivers like * * * * * U. S. GEOLOGICAL SURVEY WATER-SUPPLY PAPER NO. 92 PL. VI [Illustration: _A._ DEVASTATION IN HEBREW QUARTER, PATERSON, N. J.] [Illustration: _B._ A COMMON EXAMPLE OF FLOOD DAMAGE.] U. S. GEOLOGICAL SURVEY WATER-SUPPLY PAPER NO. 92 PL. VII [Illustration: _A._ INUNDATED LANDS AT PASSAIC, N. J.] [Illustration: _B._ UNDAMAGED BRIDGE ACROSS PASSAIC RIVER AFTER PARTIAL SUBSIDENCE OF FLOOD.] * * * * * the Mississippi, where the contributing area is enormous and the conservation of the waters would be impracticable even if the nature of the country would admit of the construction of reservoirs. In Switzerland, where the torrents occasioned by the rapidly melting snows are especially destructive, the flood waters are confined by a series of parallel dikes on each side of the river, which have the effect of dividing the flow into several parallel streams. As the main river channel fills and overflows the inner dikes, the overflow water collects into the first series of parallel channels, and when a height is reached at which the second dikes are overflowed the water collects into the third, and so on. This gives an enormous carrying capacity, the limit of which is approached slowly, and therefore abundant opportunity is afforded for preparation upon the part of the riparian owner.