14 ON THE RECLAMATION 



only by slow degrees. The discharge of other districts, again, 

 may be much influenced by their geological formation and the 

 absorbing power of the underlying strata. 



Agricultural improvements, also, exert an important influence 

 on the drainage of land, large tracts of sheep grazing pasture, 

 for example, being less absorbent than an equal area of well- 

 drained arable land. Again, all rivers which flow from lakes 

 may be said to have natural reservoirs at their source for the 

 storage of surplus water, which tends to check the floods in 

 the low country. But this, again, is not always the case, for 

 it has been found at the Tay, which flows from Loch Tay a 

 sheet of water 14 miles long and three-quarters of a mile 

 broad that in heavy westerly gales the wind acting on so long 

 a reach heaps up the water at the outlet, and, if combined with 

 heavy rainfall, greatly increases the flood in the river. It thus 

 appears that there are so many local circumstances which affect 

 the frequency and severity of floods in different localities, that 

 no rule generally applicable can be stated regarding them. 

 What may be termed the ordinary floods of rivers generally 

 occur with heavy rain and melting snow, for then the bed of 

 the river has to discharge a compound flood made up of what 

 is falling in the form of rain and what is draining off the land 

 as melted snow. But extraordinary floods, affecting a limited 

 area, may occur at any period in connection with thunderstorms, 

 and it is the fact, that some of the most remarkable floods, such 

 as that in the northern counties of Scotland in 1829, occurred in 

 August, and were due, not to a sudden thunderstorm, but to 

 the continuance of heavy rainfall over a large district of 

 country, and in the instance referred to, 3| inches of rain fell 



