FLOATING AND RAFTING 349 



are visible along the stream banks, on a familiarity with similar 

 streams, and on a general knowledge of rainfall and floods in 

 the vicinity. The amount of water available for driving in a 

 given watershed usually cannot be accurately determined since 

 specific records from which to draw conclusions are seldom 

 available. 



Evaporation may play an important part in influencing the 

 water supply during the summer season by taking moisture 

 both from the soil and from the surface of the storage reservoirs. 

 The water supply for early spring driving is not greatly affected 

 by evaporation, but shallow reservoirs that store water for sum- 

 mer driving have a high rate of evaporation and it is sometimes 

 impossible to collect a head of water. 



(b) The quantity of water required in a given time to carry 

 logs down stream between storage reservoirs. On small streams 

 where large quantities of water are not available or where the 

 banks are low and the water leaves the main channel it may 

 not be possible to drive logs more than a few miles at most 

 before the force of the water is spent. In such cases frequent 

 storage basins are required. 



(c) The length of time for which flood water must be available. 

 If artificial freshets are required only for a short time in the 

 spring when the streams are fed from snow water a smaller stor- 

 age area may be used than when water must be available for 

 several months. 



DAMS 



Dams for logging purposes are usually built of round timber 

 secured close to the dam site. 



It is necessary to construct a dam on solid bottom or bed- 

 rock because if this is not done water will work underneath the 

 sills and ultimately cause the structure to go out. 



There are three types of timber dams used for logging pur- 

 poses: (i) the crib or pier dam; (2) the rafter or self-loading 

 dam; (3) the pile dam. 



Concrete dams of large size are occasionally used by lumber 

 companies, but they are built by engineers, and loggers are 

 seldom concerned in their construction. 



