,rrf 
eussell.] UPGRADING STREAMS. 138 
account of the demands made on its tributaries for purposes of irriga- 
tion, now becomes dry for a period each year. 
The marked exception furnished by the Malade to the prevailing 
character of the neighboring streams, when the influence of irriga- 
tion is not considered, at once suggests that some exceptional circum- 
stance favors it. The reason for this exception is not difficult to 
discover. The Malade is formed by the union of Big Wood and Little 
Wood rivers. Each of these streams has been turned aside in recent 
times by a lava Aoav, and the two caused to unite. By their union 
they form for the region in which they flow an exceptionally strong 
river. The fact that each of the main branches of the Malade has 
been turned aside from what we are justified in assuming would be its 
natural course is at once suggested by an inspection of a map on 
which the drainage of southern Idaho is shown. On leaving the 
mountains they turn abruptly southwestward instead of continuing 
southeastward, as would seem to be their natural course. The reason 
for this in the case of Little Wood River is the presence of a recent 
lava flow in the mountains across the mouth of the valley down which 
the stream flows. In the case of Big Wood River the lava stream 
from Black Butte filled its former channel and caused it to take a new 
course. Had it not been for these " accidents " each of the streams 
would have maintained its independence, and most probably would 
have been "lost" on the plains in the same manner as in the other 
instances cited. 
The region that the two streams just referred to would have crossed 
in order to reach Snake River had the} 7 not been turned aside has as 
yet not been closely examined, and it will be of interest to discover 
whether abandoned channels or alluvial deposits occur there to confirm 
the hypothesis of recent deflections here presented. 
UPGRADING STREAMS. 
In the case of the kC lost rivers' 1 of the Snake River Plains, instances 
are furnished of upgrading in which the process has not advanced far 
enough to permit the streams engaged in the work to cross the flat 
tract they invade and reach larger drainage channels. On the east 
side of the same flat region, however, there are numerous streams 
that have been more successful. Examples of this are furnished by 
the South Fork of Snake River, the Blackfoot, and Portneuf rivers, and 
probably other streams, which have their sources in the mountains of 
southeastern Idaho and adjacent portions of Wyoming and flow north- 
westward. There are no "lost rivers" in this region, although many 
of the smaller streams become dry in summer. The reason for this 
contrast between the streams reaching the Snake River Plains from 
the north and those coming in from the southeast is twofold — the 
former are not onlv smaller as a rule than the latter, but have a far 
