CHARACTERISTICS AND DISTRIBUTION OF LAKES 637 
northern and western borders of the great plain, but in the glacial 
period, when its present course was obstructed by ice-streams that 
descended from the mountains on the north and west, it Mas forced to 
cut across the plain through a series of deep canons in the lava. The 
valleys formed by the river and its tributaries at that time, now nearly 
dry, are known as " coulees,"" and the main valley as the " Grand 
Coulee." The latter is broken at one place by the cliffs of a former 
cataract, that must have greatly exceeded Niagara in height and 
breadth. Rapids and cataracts form depressions at their bases, where 
excavation is accelerated by the friction of the sand and stones moved 
by the swift current on their bottom and sides. If the stream channel 
in which such inequalities have been produced be abandoned as a line 
of drainage, the basins are transformed into lakes retained in part by 
the barrier formed by the load deposited by the stream waters when 
the current slackened, some distance below the falls and rapids. Two 
such lakes exist in the Grand Coulee, each about one mile long by 
half a mile broad, and of considerable depth. ^ When fractures of the 
earth's crust occur, the edges of the broken strata on one side are 
sometimes elevated and those on the opposite side depressed. In some 
cases lake-basins are produced at these faults, but in others, numerous 
examples of which are seen in the courses of the Columbia and 
Yackima Rivers, the edges of the fault blocks have been upheaved so 
slowly across the streams that the waters have maintained their course 
and cut a channel through the obstructions as they were elevated, 
thus preventing the formation of lakes. 
The drainage of one of the " coulees has been obstructed by 
immense sand-dunes formed by drifting sand, which frequently travels 
across the country for a score of miles in the direction of the prevail- 
ing winds, and the dam so formed retains the waters of Moses Lake.'-^ 
Below the dam are several springs, which serve to keep the waters 
of the lake fresh, and, fed by lake-waters percolating through the 
obstruction, combine to form Alkali Creek, which in winter sometimes 
has sufficient volume to reach the Columbia, but in summer suffers 
from evaporation and terminates in a series of alkaline pools. 
Lake Chelan, in the Cascade range, draining to the Columbia by 
a river about 2 miles long, is a narrow, river-like sheet of water 
with windings extending westward from the Columbia 70 miles into 
the mountains, bordered on either hand by a continuous series of 
rugged peaks that rise from 5000 to over 7000 feet above its surface. 
The deep, narrow, trench-like valley, now partially filled with water, 
continues beyond the head of the lake for a distance of about 25 
^ See I. C. Russell, " Geological Reconnaissance in Central Washington," Bull. 
U.S. Geol. Surv., No. 108, p. 90, 1893. 
2 See I. C. Russell, oj;. cit., p. 90. 
