28 GEOLOGY OF SW. IDAHO AND SE. OREGON. [bill. 217. 
area of about 50 square miles and entered the channel which Cow 
Creek had eroded through a previous lava sheet. The recent lava 
flowed up the old canyon for about a mile and now terminates with 
a low, irregular slope. The lava forms a dam, above which a lake 
about 4 miles long has been caused to form. The lake extends into 
lateral valleys so as to have an irregular outline, but on an average 
is perhaps half a mile wide. During high-water stages it overflows 
across basalt and supplies other lakes to the south. In summer it is 
lowered by evaporation, leaving broad areas exposed about its eastern 
and northern borders. These lands are covered with wild grass, and 
form natural meadows from which several hundred tons of hay are 
cut each year. The lake has never been known to eva{>orate to dry- 
ness, and is only slightly alkaline. Its water, as is common with 
many playa lakes (that is. lakes which evaporate each summer so as to 
leave large portions, or as is most common, their entire beds, exposed 
as mud plains or playas), is always turbid with fine silt in suspension. 
South of the lake just described, which is the largest of the series 
of which it is a member, there are seven or eight other similar water 
bodies and several small ponds. The irregularities of these lakes and 
ponds and their indefinite or rather variable number are due to the fact 
that the water fluctuates in volume, and as the surface of the lava, 
which is in part submerged, is irregular, marked changes in the out- 
lines and in the number of lakelets to be seen occur between the rainy 
and -dry seasons. The more southern of the Cow Creek lakes, like the 
one at the north, fluctuates greatly in volume, and in summer leaves 
broad natural meadows exposed on its eastern border, from which 
large quantities of naturally irrigated wild grass are cut each year. 
•Cow Creek, south of the lakes in which it is ponded, flows through an 
irregular, ill-defined channel, and near Jordan Creek its waters are 
utilized for irrigation. 
About the Cow Creek lakes the creek which supplies them flows 
through a rather deep canyon which is joined by well-defined branches, 
thus demonstrating that the process of stream adjustment and of ero- 
sion was well advanced before the Jordan Craters sent out lava which 
invaded the medial portion of the valley it entered, and buried its 
previous stream-sculptured surface. 
The Cow Creek lakes furnish a fresh and unmodified illustration of 
the effects of lava flows on drainage, and present typical examples of 
lava-dammed lakes. Other lakes of the same character occur about 
the Diamond Craters, at the west base of Stein Mountain, in Harney 
County, but are much less extensive, and in fact swamps instead of 
open water mark the presence of obstructions to the escape of the 
surface waters. The Bowden Crater, as will be described later, also 
sent out a broad lava sheet, which obstructed the drainage and caused 
a lake to form, but in this instance the outflowing stream has cut a 
drainage channel, and the lake, which once existed, has been drained. 
