The Great Terrace of the Columbia. — Russell. 365 
series of similar terraces which occur along many of the rivers 
of eastern Washington. There is a similar great gravel 
terrace in the canyon of Snake river, another along Spokane 
river, and other in the valleys of the Okanogan, Methow. 
Yakima, etc. These several terraces have many features in 
common, but as they have not been adequately studied, more 
information concerning them is necessary before their histories 
can be interpreted with confidence. All the streams referred 
to, however, flow to the Columbia and converge toward the 
depressed region in east-central Washington. In all cases 
thus far observed, also, the terraces are conspicuous on the 
upper course of the rivers where their valleys or canyons are 
narrow, and disappear lower down the river where their 
valleys become broad. For example, on following the 
Columbia southward from the mouth of Chelan river, the 
valley soon broadens and one looks in vain for the great 
gravel terrace so conspicuous farther up stream. This, 
termination of the great terrace of the Columbia when traced 
down stream is one of the reasons for believing in the former 
existence of a lake in the basin-like portion of south-central 
Washington. This water body was named lake Lewis by 
Symons, but full and unquestionable evidence of its actual 
existence can not be said to have been discovered. With 
lake Lewis filling the valley of the Columbia below where 
the great terrace terminates, we are enabled to give a natural 
interpretation of the great gravel deposit in the valley of the 
upper Columbia. That is, the Columbia discharged into lake 
Lewis, and built a delta of coarse material, being, as I am in- 
clined to believe, overloaded with debris like many other 
rivers in the same general region, on account of the changes 
in conditions accompanying the Glacial epoch. With the 
building of the delta the river channel, up stream from it, be- 
came deeply filled with gravel, and after the waters of lake 
Lewis were withdrawn, the Columbia excavated its present 
inner channel through the gravel, partially filling its old rock- 
cut valley. 
This is in outline my interpretation of the history of the 
great terrace of the Columbia, but as only a portion of the 
field has been studied and this in connection with recon- 
npissance work, I hold my conclusions lightly and am willing 
to change them if additional observations demand it. 
