Reconquest of the Water 



regular reed and water-lily association occupies the 

 lake shore.* 



The rapidity of the process of filling up a shallow lake 

 as sketched above depends on so many varying factors 

 that it is almost impossible to give any estimate at all. 



The depth, direction of, and exposure to winds, and 

 the character of the streams entering and leaving a lake, 

 are all important factors. 



As a rule the process of filling up begins where drift 

 accumulates, but all sorts of things may have an influence. 

 Loose gravel set in motion by the waves, or ice grinding 

 along the shore in winter, may hinder the growth of 

 vegetation. 8 Even ducks and geese entering the water 

 at one particular place seem to prevent the growth of 

 the regular colonisers. 



Our rich alluvials, the hay meadows or " ings " of the 

 first Saxon settlers in Britain, were formed after this 

 manner (see also Chap. XXIII.). Such land may be 

 worth $ to 6 per acre in annual rent, for it is upon 

 it that the richest crops are grown. 



But the importance of the process is at once obvious 

 when one reflects upon the valleys of the Thames, Clyde, 

 Severn, the Wash, the rich alluvials of the Rhine, Danube, 

 or Yangtsekiang, and especially on the land of Egypt. 



Lower Egypt seems to have been once a land of wild 

 fowls and hippopotami, probably very similar to the Sudd 

 country of the Upper Nile to-day. 



When the Nile has left the Albert Nyanza and traversed 

 the steep slope from Wadelai and Nimule to Gondokoro, 

 the country becomes a vast and nearly level plain. 

 Here the water leaves its banks and supplies an enor- 

 mous extent of morasses, lagoons, and marshy land, 



* There are abysms in many lakes where bacteria may live in slimy deposits 

 probably far below such levels. James Murray describes a population of 

 worms, rotifers, and Crustacea at 300 feet in Loch Ness, Geographical Journal ', 

 January 1908. 



133 



