146 PHYSIOGRAPHY 



Conditions necessary for the existence of lakes. The condi- 

 tions necessary for the existence of lakes are (1) depressions without 

 outlets, and (2) a sufficient supply of water. In moist regions most 

 natural basins contain lakes, while the basins of arid regions are 

 often lakeless. 1 Lake water comes from rain, melting snow and 

 ice, springs, and rivers. Since springs and rivers depend on rain 

 and snow, the source of lake water may be said to be rainfall and 

 snowfall, or atmospheric precipitation. 



Changes now taking place in lakes. Various changes are now 

 taking place in all lakes, and they throw light both on the past and 

 the future of these bodies of water. 



Their basins are being filled all the time, and in various ways. 

 1. Streams and other surface waters which flow into lakes carry 

 in gravel, sand, or mud. Deltas are often built in lakes, and in 

 rare cases they have been extended quite across a narrow lake, 



Fig. 150. Sketch showing the position of the delta on which the village of 

 Interlaken, Switzerland, stands. The two lakes were formerly one, and 

 the lowland on which the village is situated is a delta built across the 

 middle of the original lake, separating it into two parts. The area of the 

 delta-land is shaded with dots. 



separating it into two parts, as at Interlaken, Switzerland (Fig. 150). 

 Large deltas occur at the ends of some of the Finger Lakes of New 

 York, as shown in Fig. 2, PI. XXXVIII, p. 133. Deltas built 



1 There are intermittent lakes, in the basins of which water is not always 

 present. 



