CHA.KACTERISTICS AND DISTRIBUTION OF LAKES 517 
From this point of view it will be at once evident that rivers are 
frequently older than the present topography of the land-surfaces ; 
they can often cut their way through folds of the crust as rapidly as 
these arise across their course. It is equally evident, on the other 
hand, that lakes must be regarded as but transient features of the 
ever-changing surface of the earth. They come and go, arise and 
become extinguished with the varying cycles of topographic develop- 
ment, and with the climatic changes of the regions in which they are 
located. 
The temperature of the water in lakes varies with the latitude Temperature, 
and with the altitude. It is subject to much variation, depending 
on the depth, the mass of water, and the superficial area of the lake, 
that is, the extent of surface exposed to the sun and sky. The 
that tlieir channels meander through great curves. When a meander is abandoned 
for a cut-ofF, it remams for a time as a crescentic 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-slo23es 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 slojDe, 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 ponded back. No 
cases of this kind are descril^ed, to my knowledge, but they will very likely be 
found ; or we may at least expect them to appear when the northern branches 
of the Indus cut their ways backwards through the innermost range of the 
Himalaya, and gain possession of the drainage of the plateaus beyond ; for then, 
as the high-level waters find a steep outlet to a low-level discharge, they will 
carve out caiions the like of which even Dutton has not seen, and the heavy 
wash of waste will shut in lakes in lateral ravines at many points along the 
lower valleys, 
" In its old age, a river settles down to a quiet, easy, steady-going existence. 
It has overcome the difficulties of its youth, it has corrected the defects that arose 
from a period of too rapid growth, it has adjusted the contentions along the 
boundary-lines of its several members, and has established j)eaceful relations with 
its neighbors : its lakes disaj^pear, and it fiows along channels that meet no 
ascending slope on their way to the sea, 
"Certain accidents to which rivers are subject are responsible for many lakes. 
Accidents of the hot kind, as they may be called for elementary distinction, are 
