EVIDENCES OF CHANGE OF LEVEL 673 
must have been a wonderful network of lakes if, formerly, every river 
was a string of them; and still more wonderful, if it should be found, 
as we believe it will, that the terraces are so far a continuous part of 
a single system, that a river and its branches must have been a single, 
tortuous, many-armed lake. But it is unnecessary to dwell longer 
upon this question. We found no evidence that the terraces were of 
marine origin. 
In this change of level indicated by the terraces, the country from 
beyond the Columbia to the Sacramento region evidently participated 
in different degrees; how far beyond these limits it extended, we 
have as yet no knowledge. The amount of elevation about the 
Cowlitz and the Columbia at Vancouver, was forty or fifty feet; that 
about the Willammet, near the settlement, fifty or sixty feet; that 
upon the Sacramento, shown by the terrace, sixty to seventy feet, or if 
we judge from the greatest height of the alluvial deposits where we 
first made the river, two hundred or two hundred and fifty feet. 
Higher up on the Columbia, at Wallawalla, the height may be one 
hundred and fifty feet; but of this we cannot speak from observation. 
The height on the Umpqua and Shasty rivers does not indicate cor- 
rectly the amount of rise, as they are small streams full of rapids. 
Was this change of level an abrupt one, or was it slow and gradual? 
This seems at first to be a question easily answered. We may best 
understand it, by considering the changes that would take place 
during the elevation of a region of alluvial flats. If a country rise 
abruptly, the river will commence to work itself to a lower level, and 
proceed with rapidity, ending finally in attaining the very gradual 
slope of ordinary rivers, a descent of one to two feet to the mile. At 
the same time, in the seasons of floods, the river would wear into the 
former alluvium, (now its banks,) and widen its surface; and this 
widening would go on every successive freshet, till the river had a 
new lower plain on its borders. The material being easily worn away, 
these results would not require a long time. This effect—the forma- 
tion of a river plain for the river to meander through—we have 
shown, in our remarks on the valleys of the Pacific and those of 
Australia, to be a necessary result of flowing waters. 
But would not the effect be the same during a gradual rise? As 
the country rose slowly, the excavation of the river’s bed and the 
lateral widening during freshets would go on gradually with the same 
results, producing a deeper bed and a new lower flat, both of which 
would change as the change of level progressed. If the lower flat in 
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