388 LOGGING 
lakes are tributary to one stream driving may proceed long 
after the spring freshets are over. 
Sites for dams should have a narrow channel, high banks, and 
a solid bottom for their foundation. In order to store the greatest 
amount of water thej^ should be built at the foot of a lake, at 
the end of a long stretch of dead water, or at a point where 
the maximum amount of water can be stored with a minimum 
of dam height. 
Storage reservoirs should be large enough to permit log driv- 
ing for a minimum of five or six hours daily and the drainage 
area should furnish enough water to again fill the storage basin 
before the driving period on the following day. 
The required watershed area and the capacity of the storage 
basins for a given stream are dependent on 
(a) The amount of moisture precipitation on the watershed 
especially during the fall and winter months and also the rapidity 
with which it is made available in the spring. Drives are gen- 
erally dependent on flood waters and a rapid run-off is desirable 
because the storage basins will then be refilled in the minimum 
time after each splash. 
A logger usually relies on his judgment as to whether a watershed 
is capable of supplying sufficient flood water for driving purposes. 
He bases his conclusions on flood wood and earth deposits which 
are visible along the stream banks, on a familiarity with similar 
streams, and on a general knowledge of rainfall and floods in the 
vicinity; however, the amount of water available for driving in a 
given watershed is difficult to determine accurately because 
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. 
(6) 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 
