One of the difficulties was this : If obstacles were placed in the way of a free flow of water, the river would by so much at least desert this pass and run through the others; so that their heads had to be closed up a sufficient extent to prevent this. Plans for improving the South west Pass in like manner were submitted by United States army engineers. Construction was begun toward the end of 1903 and com pleted five years later at a cost of about $2,625, 000. The east jetty was extended for 3,000 feet and the west for 3,750 feet in 1909-12. The final depth gained is 35 feet.
At the mouth of the Brazos, west of Gal veston, Tex., an ingenious plan was adopted for avoiding interference in the work by flood tides : A long trestle was built out to deep water above high tide, the mattresses hung un der it by ropes and the stone dropped on them from above to sink them. Instead of being launched from shore and towed, they were car ried on a portable railway running on top of the trestle and let down.
The Columbia River jetty is the most con spicuous example of the single instead of the double dike. It is 4234 miles in length, the long est in the world. The bar at the mouth of the river, ever shifting and sometimes not over 12 feet deep, had half spoiled this superb river for navigation, and was greatly dreaded. But the river has a mean high-water discharge of 60,000 cubic feet per second, a mean tidal ebb of 1,000,000, with tides of 62 feet; and in 1884 a single curved line of brush mattresses with rub ble-stone copings was begun, completed in 1894, to turn the current away from spreading itself on both sides and scour out the channel on one. This was finished in 1894 with a channel 30 feet deep and made the river a highway of the heaviest ocean commerce, with lines to all Pacific lands. The improvement was not
permanent and in 1902 the channel was only 21 feet in depth. In 1903 a north jetty 2% miles long was projected, together with a 2$ mile addition to the south jetty. These additions were completed in 1913.
Others are too numerous for more than brief mention. At Yaquima Bay, Ore., 115 miles south of the Columbia — an estuary 20 miles long discharging into the sea through a narrow, tortuous, shifting channel, and over a sand-bar with 7 feet of water—parallel jetties about half a mile long, one of rubble-stone on a rock bed, one of brush and stone on a sand bed, have doubled the depth of water and made the chan nel calculable. At Galveston, the single jetty was a relative failure, it needing a double one to converge the tides; and in 1896 the govern ment completed it, with sides of 35,000 and 25,000 feet, costing over $8,000,000, and furnish ing 27 feet of water between the island and the mainland. Other notable ones are at the mouth of Saint John's River, Florida, beginning at the sides of the river-mouth and converging to 1,000 feet apart at the crest of the bar. Charles ton's double one has sides of 15,000 feet each.
Consult Corthell, E. H., (The Mississippi
(New York 1881) ; Haupt, L. M.,