60 



INITIAL CAUSES. 



Kinds of lakes and ponds. — Lakes and ponds have been classified from many 

 points of view, such as form, origin, physiographic development, chemical 

 nature, climatic region, depth, duration, etc. All of these bases have some 

 relation to succession, though this seems least direct in the case of physio- 

 graphic development and manner of origin. The amount and kind of water 

 are the controlling factors in determining when the pioneer stages of a sere 

 can develop, just as the extent and location determine the pioneers and the 

 rate at which they can invade. Depth, as modified by evaporation, filling, 

 and draining, is the critical point upon which invasion turns. Depth, extent, 

 and kind of water are unimportant points to the geographer, however, and his 

 classification can not be expected to reveal basic vegetational values. It 

 does, however, bring but many points which the ecologist must note in con- 

 nection with water and soil relations. The classification proposed by Russell 

 (1895) perhaps serves this purpose best because of its detail. The agencies 

 of lake and pond formation are grouped as follows: 



1. New land depreBsions. 



2. Atmospheric. 



(1) Wind rock baeine. 



(2) Dune ponds. 



3. Aqueous. 



(1) Streams. 



(a) Excavation and change of bed. 



(b) Lateral deltas and valley cones. 



(c) Ponding of tributaries. 



(d) Ox-bows or cut-offs. 



(e) Delta lakes. 



(2) Waves and currents, 

 (a) Bar lakes. 



4. Glacial. 



(1) Damming of laterals by ice. 



(2) Damming by drift. 



(3) Scouring. 



5. Volcanic. 



(1) Damming by lava. 



(2) Damming by ash. 



(3) Crater lakes. 



6. Meteoric? 



7. Earthquakes. 



8. Organic agencies. 



(1) Coral? 



(2) Peat moss. 



(3) Beaver dams. 



(4) River rafts. 



(5) Gas mud-holes. 



(6) Wallows. 

 9. Diastrophic. 



10. Land-slides. 



11. Chemical action. 



12. Interaction of two or more 



Life-history of a lake. — ^While the relation of a lake to its river system 

 seems to have no significance for succession, it is evident that the life-history 

 of a lake shows a direct relation. A lake in a humid region matures by filling 

 with detritus, by the cutting down of its outlet, by the shallowing action of 

 plants, or by the combined influence of these processes. When the depth is 

 reduced to a few meters, pioneer hydrophytes appear. From this point 

 the maturity and depth of the lake and the succession of the vegetation are 

 but different aspects of one complex process. At any time in this process the 

 lake may be rejuvenated by an increase of the water-supply, by the damming 

 of the outlet, or by the sinking of the basin. All of these have the same effect, 

 namely, that of increasing the depth of the water. The vegetation is wholly 

 destroyed if the depth increases more than a few feet in the early stages of the 

 sere, or even a few inches in later grassland and woodland stages. A new sere 

 of the same succession is initiated as soon as the water is again shallowed to a 

 point where submerged hydrophytes can ecize. The significant fact is that the 

 development of the sere in the original and in the rejuvenated lake will be 

 essentially or wholly identical. Physiographically, the two lakes are essen- 

 tially different. As initial areas for succession, they are identical. 



