RIVER VALLEYS AND CHANNELS 425 



juvenatioh of the streams, and so they were at first interpreted; but I am 

 persuaded that such was not the ease. The depth of the channels was 

 merely a sign of the great fluctuations of stream volume in a region where 

 heavy rains are chiefly discharged by rapid run-off from an open surface. 

 The rock sills in the channel beds sometimes determined the site of little 

 rapids or low cascades in the trickling streamlets that alone represented 

 the rivers in the dry season. This also at first sight suggested a recent 

 rejuvenation of the old drainage system; for rapids and falls are always 

 to be associated with young rivers, while old rivers are supposed to be 

 normally of gentle and even grade, without any trace of local hurry in 

 cascades, all of which should have been obliterated in the earlier stages of 

 the C3^cle. But in this case, as in the other, further consideration shows 

 that the little rapids of the low-water streamlets are not at all inconsistent 

 with normal old age; for when the channels are occupied by the flooded 

 rivers the small unevenness of the channel bed can have no significant 

 effect in disturbing the even slope of the river surface. The smoothness 

 of the floodplain confirms this; it is without perceptible inequality of 

 slope and shows no change of level where low rock sills occur in the chan- 

 nel bed below. It is therefore the high-water river and the floodplain 

 associated with it which should be taken to represent the normal features 

 of old age; the low-water stage is an abnormal condition of a river, and 

 gives less evident indication of the stage of its development. 



STORM-FLOOD CHANNELS 



Some of the wide open, old valley floors were dissected here and there 

 by branching channels cut in the alluvium to a depth of 10 or 20 feet; 

 and this led us to question whether a slight disturbance in the normal 

 progress of events had not taken place. For example, after a time of 

 more thorough erosion, a climatic change might have occurred during 

 which a deposit of alluvium would accumulate, and this in turn might 

 have lately been followed by a period of more active erosion whereby the 

 accumulated alluvium would be channeled. On the other hand, it seems 

 possible that the occasional heavy downpours which are characteristic of 

 the rainfall on the Yeld may sometimes flood the streams into so violent 

 an activity that they cut deep channels in the alluvium that has been 

 accumulating in the long intervals of less rainfall, and thus the master 

 flood of the century may make a record which is very striking if of 

 recent occurrence, but which is gradually lost as the deep cut channels 

 are silted up. In this case the rock floor beneath the alluvium will be 

 worn down only when and where the deep cut channels are ripped open 

 XXXVII — Bull. Geol. Soc. Am., Vol. 17, 1905 



