The manner in which the streams in the valleys are made to aid in their own correction is most interesting. Whereas in the mountains it is usually desirable to decrease the erosive action of the water, in the valleys the contrary is the case, as the deposits in the lowlands are as dangerous to life and property as was their abstraction above. The great desideratum to be attained is to have the mountain streams arrive in the valleys in a purely liquid condition, and to give the valley streams the power to carry to the lakes any material they may be so unfortunate as to have taken in charge. To accomplish this latter purpose, the sinuosities of the streams are often reduced to straight lines, an increase of slope being thus secured. The new channels are made of a cross-section to enable the water to carry on its alluvion and silt. Where great freshets occur it is necessary to guarantee the artificial beds against the enormous increase of the water's destructive action. The usual plan is that of having the cross-section of the stream with a deep depression in the center. This depression is of dimensions to insure a proper flow under ordinary conditions. When the stream becomes swollen it overflows the borders of this depression and spreads over a much larger area until the banks proper are encountered. This sudden increase of cross-section reduces the velocity of the water and consequently its destructive power. When the water of a stream is turned from its old channel into the new one that has been prepared for it, the operation is generally very gradually performed, so as to enable the water to fill up the old bed as much as possible by depositing its silt.