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Current velocities were measured on a number of transects across the Columbia River in Washington State, to understand how velocity varies within a channel. The velocity at any point in a river is controlled by a number of factors, including the river's slope or gradient, roughness of the channel bed, turbulence of the flow, depth of the river, etc. Typically, water moves faster away from the bed of the river, where obstacles create drag and turbulence. The highest velocity overall is usually in the deepest part of the channel, just below the surface. Knowing the average velocity in a channel is extremely important when calculating river discharge, the volume of water flowing past in a given amount of time.

The data on velocity versus depth in the channel were acquired at a station below Grand Coulee Dam at a distance of 13 feet from the edge of the river. The depth of the Columbia River at this spot was about 12 feet at the time of measurement. Velocities were measured at various heights in the water column.

These data show a regular decrease of velocity with increasing depth in the river, as might be expected. The data are fairly linear, with a high correlation coefficient.

Other locales show a similar decrease in velocity with depth, though often with much more scatter, and without the fairly monotonic behavior exhibited by this example.

Reference: Savini, J. and Bodhaine, G. L. (1971), Analysis of current meter data at Columbia River gaging stations, Washington and Oregon; USGS Water Supply Paper 1869-F.

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 velocity-depth data for the Columbia River below Grand Coulee Dam Savini and Bodhaine (1971), USGS WSP 1869-F measured 13 feet from the shoreline depth (ft) vel (ft/sec) 0.7 1.55 2.0 1.11 2.6 1.42 3.3 1.39 4.6 1.39 5.9 1.14 7.3 0.91 8.6 0.59 9.9 0.59 10.6 0.41 11.2 0.22

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