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.® 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 {£5 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 Nimulé 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, arid 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 
