DEPOSITS AT THE MOUTH OF THE COLUMBIA. 667 
trees, which are calculated to entangle or stop the floating leaves and 
branches of the river, and secure a considerable part of its detritus ; that 
the vegetation of the shores is contributing to the accumulations; that 
the roots, also, of the trees raise the surface by adding their own bulk 
to that of the soil; that the plain, back from the shores, lies for some 
time under water during freshets, which water exerts a pressure upon 
the material below, and may render it more compact than the allu- 
vium of the shores. 
Deposits at the mouth of the Columbia.—The Columbia, for the last 
fifteen miles of its course, averages four miles in width. Extensive 
submerged flats occupy nearly the whole breadth, leaving a channel 
along either shore. From the capes, which are six miles apart, there 
stretch out large sand-banks, the southern five miles long, and nearly 
two wide, extending a little north of east ; and the northern four miles 
long, and a mile wide, stretching nearly to the south. Between the ex- 
tremities of the two, there is a passage a mile wide, which divides and 
turns both to the right and left, around a large sand-patch, lying at the 
middle of the mouth. ‘The whole surface of the submerged alluvial 
flats is about forty square miles in area. 
The shores of the river are gaining little from these depositions ex- 
cept about the south cape, and, especially, just outside of the southern 
breaker, between Killimook Head and the river. The sea-beach south 
of the river is about sixteen miles in length. From a chart of the 
region by Mr. Drayton, and from his observations, we ascertain that 
back of the beach there are three drift ridges of sand. They are a 
little broken and uneven, but have a general parallelism with the 
beach, and in consequence of their positions, a stream of water, rising 
near Killimook Head, flows first far to the north, back of the third 
ridge, and then returns between the second and third, to Killimook 
Head, where it enters the sea. The /irst of the ridges is a fourth of 
a mile from the sea, and ten to fifteen feet high. The second, three- 
fourths of a mile back of the first, and twenty to twenty-five feet 
high ; the ¢herd, one and one-fourth miles from the second, and fifteen 
or eighteen feet above the sea. The prairie plain extends for a mile 
back of these ridges, and then becomes densely wooded, and rises forty 
feet above its former level, which is the greatest height between the 
sea-beach referred to, and Young’s Bay, an indentation in the south 
shore of the Columbia, just within the south cape. The whole point 
between the beach and Young’s Bay, is, apparently, the result of 
river and marine action combined. ‘The soil of the inner portion is a 
