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IOWA ACADEMY OF SCIENCE Vol. XXIV, 1917 
overloaded glacial streams ; the second by worked-over gravels of 
glacial streams and moraines. It is not always easy to differ- 
entiate the two. On the whole the first maintain a nearly uniform 
level above the present stream; the second holds an absolute 
level upstream, thus gradually approaching the river level. 
The terraces of the Columbia river have already received 
considerable attention, but there has been little attempt at exact 
correlation of the different benches. For instance, what is called 
the Great Terrace displays discrepancies as much as 200 feet 
when careful comparisons are made. Above the mouth of Chelan 
river, the outlet of the lake of the same name, the bench about 
300 feet above the water level appears to be the most important 
one. At least it is the largest and most persistent one for a 
distance of many miles upstream from the point mentioned. It 
probably was formed during the time when the course of the 
Columbia river was dammed by the great glacier which came 
out of the Lake Chelan valley. It seems to represent the level of 
the waters in the vast lake that was thus generated in the Colum- 
bia gorge above, the outlet of which was through the Grand 
Coulee or some other coulee near by. With this high level stage 
it seems most reasonable to associate the principal terraces of 
Okanogan valley. 
Okanogan river flows due southward and enters the Columbia 
river exactly at the sharp elbow of the Big Bend. The coinci- 
dence of the courses of the two streams in a single straight line 
suggests that the present Columbia river in this part of its course 
may be really following an old channel of the Okanogan river. 
Now the present Okanogan is a small stream flowing in a valley 
large out of all proportion to the importance of the watercourse. 
In fact it is a stream tremendously underfitting its valley. The 
latter is a rock-bound gorge sufficiently large to carry the waters 
of the Columbia. Viewed in its larger aspects the Okanogan 
valley extends far to the northward into British Columbia 
where it passes through one arm of Suswap Lake and on into the 
Columbia valley again where the latter passes around the lofty 
Selkirk mountains. 
The Okanogan valley is very straight. It originally was a 
structural depression with a stream-cut channel . Later a glacier 
occupied it down to its mouth, blocked and crossed the Columbia 
gorge, the south side of which it overrode and then extended far 
