THE WORK OF RUNNING WATER. 109 



mature stage its basin has the roughest topography which it will have 

 at any time during that cycle of erosion. At that stage, therefore, 

 road construction is relatively difficult. If the relief be great, roads 

 must follow the valleys, or the crests of the ridges between them, if 

 they would avoid heavy grades. In such regions roads are usually 

 few and crooked. 



The stage of development of valleys has an influence on the navi- 

 gability of their streams. Streams well advanced in life are much 

 more readily navigable than young ones, because their grades are 

 lower and their volumes of water greater. Old streams, on the other 

 hand, are sometimes depositing sand or silt along their lower courses 

 to such an extent as to interfere with navigation. 



At certain stages of their development the power of streams is 

 more easily utilized than at others. Young streams, depending as 

 they do for their supply on the rainfall of a limited area, are likely 

 to be fitful in their flow, and therefore unrehable as a source of power. 

 This is especially true where the precipitation is unequally distributed, 

 and where the slopes are steep and free from forests. Because of 

 their great volume, old and large streams, though sluggish, have great 

 power, but it is less easily controlled. Where streams are large enough 

 to be navigable industrial considerations often prevent the utilization 

 of their power, the streams being more serviceable as highways than 

 as sources of power. Other things being equal, it follows that streams 

 are most available for water-power when they are large enough to have 

 a moderately steady flow, and not so large as to be beyond ready con- 

 trol, or to be valuable for purposes of navigation. 



Streams are subject to more disastrous floods in some stages of 

 their development than in others. Floods resulting from heavy rains 

 are likely to be greatest where the slopes above the drainage lines are 

 on the whole greatest, for this is the condition under which the water 

 is most quickly gathered into the drainage channels. The most disas- 

 trous floods, humanly speaking, are those which affect wide-bottomed 

 valleys, where the flats are settled. In such cases a relatively slight 

 rise may flood verj^ extensive areas. In such valleys the most disas- 

 trous floods are generally in the spring, when the waters from the melt- 

 ing snows of the preceding winter are being discharged.^ Many other 



* For disastrous floods of the lower Mississippi, see Johnson, Bull. Geol. Soc. Am., 

 Vol. II, pp. 20-25. For effect of precipitation and forests on floods, see Russell's Mete- 

 orology, pp. 198-217, and Vermeule, Report on Water Supply, Geol. Surv. of N. J. 



