118 SUPPLEMENT. 



meut has no such immature features remaining. The life of most rivers is, 

 however, so long, that few, if any, complete their original tasks undisturbed. 

 Later mountain-growth may repeatedly obstruct their flow ; lakes appear 

 again, and the river is rejuvenated. Lake Lucerne is thus, as Heim has shown, 

 a sign of local rejuvenation in the generally mature Keuss. The head waters 

 of the Missouri have lately advanced from such rejuvenation; visitors to the 

 National Park may see that the Yellowstone has just regained its former 

 steady flow by cutting down a gate through the mountains above Livingston, 

 and so draining the lake that not long ago stood for a time in Paradise valley. 

 The absence of lakes in the Alleghany mountains, that was a matter of sur- 

 prise to Lyell, does not indicate any peculiarity in the growth of the moun- 

 tains, but only that they and their drainage system are very old. 



The disappearance of original and mountain-made lakes is therefore a sign 

 of advancing development in a river. Conversely, the formation of small 

 shallow lakes of quite another character marks adolescence and middle life. 

 During adolescence, Avhen the head-water streams are increasing in number 

 and size, and making rapid conquest of land-waste, the lower trunk-stream 

 Tna,Y be overloaded with silt, and build up its flood-plain so fast that its smaller 

 tributaries cannot keep pace with it : so the lakes are formed on either side of 

 the Eed River of Louisiana, arranged like leaves on a stem; the lower Danube 

 seems to present a similar case. The flood-plains of well-matured streams 

 have so gentle a slope that their channels meander through great curves. 

 When a meander is abandoned for a cut-off, it remains for a time as a cres- 

 centic lake. When rivers get on so far as to form large deltas, lakes often 

 collect in the areas of less sedimentation between the divaricating channels. 

 Deltas that are built on land where the descent of a stream is suddenly 

 lessened and its enclosing valley-slopes disappear, do not often hold lakes on 

 their own surface ; for their slope is, although gentle, rather too steep for that : 

 but they commonly enough form a lake by obstructing the stream in whose 

 valley they are built. Tulare Lake in southern California has been explained 

 by Whitney in this way. 



The contest for drainage area that goes on between streams heading on the 

 opposite slopes of a divide sometimes produces little lakes. The victorious 

 stream forces the divide to migrate slowly away from its steeper slope, and 

 the stream that is thus robbed of its head waters may have its diminished 

 volume clogged by the fan-deltas of side-branches farther down its valley. 

 Heim has explained the lakes of the Engadine in this way. The Maira has, 

 like an Italian brigand, plundered the Inn of two or more of its upper streams 

 and the Inn is consequently ponded back at San Moritz and Silvaplana. On 

 the other hand, the victorious stream may by this sort of conquest so greatly 

 enlarge its volume, and thereby so quickly cut down its upper valley, that its 

 lower course will be flooded with gravel and sand, and its weaker side-streams 



