DEPOSIT. 51 



In the case of a lake in an arid region, evaporation is the chief factor in 

 shallowing, though filling by detritus plays a part. The cutting down of the 

 outlet is of little or no importance, owing to the reduction in volume. The 

 shallowing effect of vegetation can be felt only in relative youth, as the 

 increasing salinity destroys the plant population. Hence, the development of 

 the usual water sere ceases long before the death of the lake, in such bodies of 

 water as Great Salt Lake for example. Consequently there is no correspond- 

 ence between the life-history of the lake and the development of the vegetation. 

 Instead of a drying lake-bed, densely clothed with plants, the salt-incrusted 

 bottom is entirely devoid of vegetation. In an arid climate it is only after 

 many years that the salt crust is sufficiently destroyed by solution and removal 

 to permit the appearance of pioneer halophytes. If a striking increase of 

 rainfall or the accession of new streams should rejuvenate the lake, the initia- 

 tion of a sere would depend largely upon the freshness of the water. If vege- 

 tation does appear its development will be determined by whether the water 

 remains fresh or is rendered saline by evaporation. 



In the case of periodic ponds, playa lakes, etc , the drainage is usually such 

 as to prevent the accumulation of solutes. The fife of the pond is often short, 

 and hydrophytic vegetation may fail to develop altogether, the sere beginning 

 on the exposed, rapidly drying bottom. Ponds which last for several years 

 permit more complete expression of the water sere. Salt ponds supplied by 

 springs, such as the Salt Basin at Lincoln, are unique in behavior. They pos- 

 sess no phanerogamic vegetation at all, except when greatly diluted by fresh- 

 water streams. Ordinarily they dry up, forming a salt crust, as in the lakes 

 of arid regions. The periodic ponds of the Great Plains also deposit alkali 

 when they evaporate, though this is often not so great in amount as to prevent 

 the invasion of halophytes. The latter initiate a short succession which ter- 

 minates in the climatic grassland. 



Drainage. — ^Drainage by cutting down the outlet of the lake or pond plays 

 some part in shallowing most lakes of humid regions. It is usually subordinate 

 to filling, and in effect is indistinguishable from it. As an initial cause of new 

 areas for succession it is most evident where natural barriers which produce 

 ponding are suddenly removed, and especially where it is resorted to inten- 

 tionally by man. The effect of drainage upon the course of development is 

 determined by the degree to which the water is removed. If open water with 

 a depth not greater than 12 meters is left, the normal water succession is 

 initiated. Later stages are initial at the respective depths less than this, 

 until a point is reached where drainage completely removes the surface water. 

 This permits the soil to dry more or less rapidly and to become quickly covered 

 with a growth of mesophytic ruderals or subruderals. This is typically the 

 case in areas drained artificially. 



The effect of drainage is perhaps most striking when the level of the lake is 

 reduced to a point where islands and peninsulas appear. The rate of lowering 

 determines the water conditions at which invasion becomes possible, and hence 

 the presence and length of hydrophytic stages. In the behavior of a river- ' 

 plain after flooding, draining differentiates the area into ponds and mud- 

 flats, in which the water sere will appear in a variety of stages. 



