Soil and Stream-flow 57 



tend to be gathered together in streams, and thus about 

 one- third of our rainfall runs away. In humid areas 

 small streams converge to form larger ones, and flow 

 onward to the seas. In arid regions they tend to 

 spread out in sheet floods, and to disappear in the sands. 



In a state of nature little rain water runs over the 

 surface of the ground, apart from streams. It mainly 

 descends into the soil. How much the soil can hold 

 depends upon its composition. Dried soils have a 

 capacity for taking up and holding water about as fol- 

 lows: sharp sand 25%, loam 50%, clay 60%, garden 

 mould 90% and humus 1 80% of their dry weight. Water 

 descends most rapidly through sand and stands longest 

 upon the surface of pure clay. Thick vegetation with 

 abundant leaf fall, and humus in the soil tend to hinder 

 run-off of storm waters, and to prolong their passage 

 through the soil. Thus the excess of rainfall is gradually 

 fed into the streams by springs and seepage. Under 

 natural conditions streams are usually clear, and their 

 flow is fairly uniform. 



Unwise clearing of the land and negligent cultivation 

 of the soil facilitate the run-off of the water before the 

 storm is well spent, promote excessive erosion and 

 render the streams turbid and their volume abnormally 

 fluctuating. Little water enters the soil and hence the 

 springs dry up, and the brooks, also, as the seepage of 

 ground water ceases. Two great evils immediately 

 befall the creatures that live in the streams and pools: 

 ( i ) There is wholesale direct extermination of them with 

 the restriction of their habitat at low water. (2) There 

 occurs smothering of them under deposits of sediment 

 brought down in time of floods, with indirect injury to 

 organisms not smothered, due to the damage to their 

 foraging grounds. 



The waters of normal streams are derived mainly 

 from seepage, maintained by the store of water accumu- 



