22 GEOLOGY OF SW, IDAHO AND SE. OREGON. Lbull.->17. 
The Malheur River derives by far the greater part of its water from 
its branches, which come in from the northwest and have their sources 
in the forest-covered Strawberry Mountains. Its principal tributaries 
are perennial streams, but in summer all of the water which reaches 
the main channel is used for irrigation along the lower 40 miles of its 
course, and great areas of desirable land still remain uncultivated. 
As in the case of Owyhee, the Malheur demands careful study from 
hydraulic engineers, with the view of storing its winter flow for sum- 
mer use. 
A feature of special geographical interest in the case of the Malheur 
is the fact that it has lost fully one-third of its former drainage basin, 
owing, judged by the observations, to the occurrence of a lava flow 
which dammed its channel in the vicinity of Mule, in Harney County, 
between Stein Mountain and the Crow Creek Hills. It is to this lava 
dam, which obstructs the old river bed for a distance of about 30 
miles, that Harney and Malheur lakes are due. The surface of the 
lava is only about 10 or perhaps 15 feet above the normal level of 
Malheur Lake, but the broad, flat-bottom valley to the west of the 
obstruction permits of the spreading out of the waters which reach it 
until evaporation counterbalances the inflow, and the lakes produced 
have never risen sufficiently to discharge across the lava sheet which 
retains them. 
The portion of the former channel of Malheur River now floored 
with a sheet of recent lava is situated about 6 miles north of Mule, 
and leads from the broad basin of Malheur Lake eastward to Camp 
Creek, at present the extreme southern branch of Malheur River. The 
level-floored valley is about 1 mile wide near Mule, with bold bluffs on 
each side, and the essentially level lava sheet now forming its bottom 
is not broken by a stream channel until a point about 6 miles from 
Camp Creek is reached. Near the terminus of the lava stream its 
surface is rugged and presents numerous ridges like those so common 
in the Snake River Plains. The valley is treeless, but is covered 
with a strong growth of sagebrush. 
The failure of Malheur Lake to rise and overflow the low lava dam 
which confines it is evidence that the climate of the region has been 
arid since the obstruction was formed. The ponding of the waters 
above the lava dam and their failure to rise and cross it caused the 
entire region now drained by Malheur and Harney lakes, about 4,500 
square miles in area, to be removed from the Pacific slope drainage 
and added to the Great Basin. 
The principal streams in the region draining to Malheur and Harney 
lakes are Silvies River, which rises in the forest-covered mountains 
to the north of Burns and Harney, flows south, and empties into Mal- 
heur Lake; and Donnerand Blitzen River, which has its head branches 
on the west slope of Stein Mountain, flows northward, and also enters 
Malheur Lake. Of these two important streams, Silvies River is 
much the larger. Each of these streams is utilized to its full capacity 
