canyon, with precipitous walls of black
columnar basalt 1000 to 3000 ft. in height. The finest portion is below
the Rock Island Rapids. In this part of its course, along the Cascade
range in the Great Plain and at its passage of the range westward,
rapids and cascades particularly obstruct the imperfectly opened bed. In
the lower Columbia, navigation is first interrupted 160 m. from the
mouth at the Cascades, a narrow gorge across the Cascade range 4.5 m.
long, where the river falls 24 ft. in 2500; the rapids are evaded by a
canal constructed (1878-1896) by the Federal government, and by a
portage railway (1890-1891). Fifty-three miles above this are the
Dalles, a series of falls, rapids and rock obstructions extending some
12 m. and ending at Celilo, 115m. below Wallula, with a fall of 20 ft.
There are also impediments just below the mouth of the Snake; others in
the lower course of this river below Riparia; and almost continuous
obstructions in the Columbia above Priests Rapids. The commerce of the
Columbia is very important, especially that from Portland, Vancouver,
Astoria, and other outlets of the Willamette valley and the lower
Columbia. The grain region of the Great Plain, the bottom-land orchards
and grain field on the plateaus of the Snake, have not since 1880 been
dependent upon the water navigation for freighting, but in their
interest costly attempts have been made to open the river below the
Snake uninterruptedly to commerce.

The Columbia is one of the greatest salmon streams of the world (see
OREGON). The tonnage of deep-sea vessels in and out over the bar at the
river's mouth from 1890-1899 was 9,423,637 tons. From 1872-1899 the
United States government expended for improvement of the Snake and
Columbia $6,925,649. The mouth of the latter is the only deep-water
harbour between San Francisco and Cape Flattery (700 m.), and the only
fresh water harbour of the Pacific coast. To facilitate its entrance,
which, owing to bars, tides, winds, and the great discharge of the
river, has always been difficult, a great jetty has been constructed
(1885-1895, later enlarged) to scour the bars. It was about 4.5 miles
long, and in 1903 work was begun to make it 2.5 miles longer. The tides
are perceptible 150 m. above the mouth (mean tide at Astoria _c._ 6.2
ft.), the average tidal flow at the mouth being about 1,000,000 cub. ft.
per second; while the fresh water outflow is from 90,000 to 300,000 cub.
ft. according to the