able to the Commission to provide for traffic at the earliest date possible, and the bridge was opened to traffic as soon as the pavement of the roadways had been completed, and several weeks before the last of the work, comprising the finishing touches to the structure, had been carried out.
The following pages include a general description of the bridge and approaches, and information concerning the loadings for which the bridge is designed; a description of some of the interesting construction features, a tabulation of the quantities in the structure and of the costs and classification of costs; a summary of the franchise provisions of the Portland Railway, Light and Power Company; and, finally, suggestions for maintenance and operation of the structure, which should come to the attention of the Commission.
GENERAL DESCRIPTION OF BRIDGE
The Columbia River Interstate Bridge and Approaches extends across the valley of the Collunbia River from the city limits of Vancouver, \Vashington, to the city limits of Portland, Oregon, a distance of three and a quarter miles, and includes about 5,000 lineal ft. of steel bridge structures and 12,000 ft. of embankment, in addition to which there is a secondary embankment on the Oregon side about 6,000 ft. long. The bridge and approaches provide a roadway for street traffic and tracks for street cars, and over steel structures a sidewalk. At times of extreme flood, the high water covers the entire three-mile width of valley and extends from Vancouver to the city limits of Portland.
The Columbia River at the bridge site is 3,500 ft. wide, with a maximum depth of 30 ft. at extreme low water and with variation from extreme low to extreme high water of 33 ft. The water rises each spring to about 20 ft. and about once in four years to 25 ft. So far as known it has reached 33 ft. only once. The bed of the river consists of sand, with small amounts of gravel, extending to great depths. At the north shore very compact cemented gravel is found a few feet below low water, dropping ot't' to 100 ft. below at about 700 ft. from the north shore. Over the remainder of the river only sand with some gravel was found to depths of more than 100 ft.
The Oregon Slough forms an important secondary channel, particularly for flood conditions. It is about 1,000 ft. wide, and
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