CLIMATE AND VEGETATION. 13 
with the stream of the lower portion of the valley, and then turns 
abruptly westward to Beaver Creek through a narrow canyon that 
interrupts inconspicuously the continuity of the limestone ridge. 
This upper part of the valley has been reduced somewhat below the 
level of the lower part, but not to an extent appreciable in a general 
view. It is an example of stream diversion, a minor tributary of the 
Beaver having cut through the limestone ridge and diverted to itself 
the waters of the upper portion of a valley that formerly drained 
southwestward along the entire eastern base of the limestone ridge. 
The diversion of the drainage from the upper valley has weakened 
the stream that occupies the lower portion, and its forceless character 
is indicated by the string of small ponds along its present headwaters. 
The streams, therefore, are by no means permanent, independent 
units of drainage, but are most delicately adjusted and may in their 
development encroach upon one another's drainage areas. 
CLIMATE AND VEGETATION. 
Owing to the high latitude of the area, 65° to 66° north, there are 
great differences in the characteristics of winter and summer. The 
annual range of temperature is great. At Tanana, for the period 
from August, 1901, to December, 1902, the temperature varied from 
76° F. below zero in January to 79° F. above zero in August. The 
intense cold of winter is not accompanied by excessive snowfall, but 
the water circulation is reduced to a minimum and the long continu- 
ance of such conditions has resulted in freezing to great depths a 
large part of the superficial deposits. But even during the winter, 
when the larger streams are covered with ice up to 6 feet thick, a 
considerable amount of water is in circulation, and it frequently 
breaks through the ice, causing overflows. Many small streams thus 
form thick accumulations of ice that may remain throughout much of 
the summer. The ice begins to go out of the Yukon at Tanana at 
dates varying in different years from about May 10 to May 15, and 
a few days later the river is clear. Mush ice begins to run at dates 
varying from October 15 to 25, and in from one to two weeks later 
the river is generally closed to navigation. The Tanana at the mouth 
of Baker Creek froze October 20, 1905, and the ice went out May 6, 
1906. The summers are characterized essentially by conditions like 
those in temperate latitudes, except that the sun is above the horizon 
a much larger portion of a summer day than in more temperate 
regions and that the season itself is short. The summer advances 
rapidly from the time of the break-up of the ice, the days become li«»i 
to a degree comparable with those of regions much farther south, and 
generally no killing frosts occur till about the first of September. The 
mean annual precipitation in the interior of Alaska, including 1><>i!i 
