bussell.] HYDROGRAPHY. 27 
LAKES. 
As previously stated, the portions of Idaho and Oregon shown on 
the accompanying map (PL I) have undergone long erosion, since 
the youngest of the more extensive geological formations occurring 
there were spread out as lacustral sediments or lava flows, and during 
or since these same formations were folded and faulted. As a result 
of such a well-advanced stage in topographical development, the land 
would be expected to be well provided with drainage channels, cas- 
cades and waterfalls should be rare, and all lake basins should have 
been either filled with sediment or drained on account of the cutting 
of channels by their overflowing streams. These expectations are ful- 
filled throughout the greater part of the region referred to, and the 
exceptions are due to special occurrences which are seemingly acci- 
dents, and which have interrupted the normal course of events. The 
so-called accidents referred to are of two classes, namely, lava flows, 
and decrease in precipitation accompanied by an increase in the rate 
of evaporation. Locally conditions dependent on each of these causes 
have acquired control in the processes of stream erosion and denuda- 
tion, and lakes appear in situations where they would otherwise be 
absent. 
There are three localities in the region under consideration where 
lakes are present, all of which are in Oregon. These are on Cow 
Creek, a branch of Jordan Creek, in the eastern part of Malheur 
County, where the Cow Creek lakes are situated; the Harney Valley, 
in Harney County, which is occupied in part by Malheur, Harney, 
and Little Silver lakes; and Alvord Valley, also in Harney County, 
and east of Stein Mountain, where there are about 8 lakes during rainy 
periods, most of wiiich evaporate to dryness during summer seasons. 
COW CREEK LAKES. 
In the valley of Cow Creek, about 12 miles northwest of Jordan 
Valley village, there are several small lakes, with markedly irregular 
outlines. These lakes are due to the damming of the waters of Cow 
Creek by recent lava flows. As will be described later, not less than 
four small volcanic craters of recent origin, which it is convenient to 
term the Jordan Craters, occur in what was previous to their formation 
a well-drained valley. The craters referred to are situated approxi- 
mately on a north-south line, a little to the west of the former course 
of Cow Creek, and from each crater have been poured out what may 
be truthfully stated to be immense lava flows, in comparison with the 
size of the craters from which they came. Each of these flows acted 
as a dam to the waters of Cow Creek and led to the formation of one or 
more lakes. The most characteristic example of the lakes formed in 
this way is the most northerly in the series, which is held in check 
by the last lava flow from the Jordan Craters. This lava sheet has an 
