The Physics
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Speed of a River

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Serway, Raymond A., Faughn, Jerry S., Moes, Clement J. College Physics Sixth Edition. Pacific Grove, California: Thomson Learning, 2003: 71. "The boat in Figure 3.17 is heading due north a it crosses a wide river with a velocity of 10.0 km/h relative to the water. The river has a uniform velocity of 5.00 km/h due east. Determine the velocity of the boat with respect to an observer on the riverbank." 1.4 m/s
Glossary. Manhattan Island Foundation. "The state of a tidal current when its speed is near zero, especially the moment when a reversing current changes direction and its speed is zero." ~ 0 m/s
Swift, Ellsworth. New River Gorge NR-Geology. National Park Service, 5 June 2006. "Speed also varies along the stream channel, being fastest where the channel is narrowest and the gradient steepest, and it changes with time, being fastest at flood stage. Speed probably varies from about 3 ½ to 7 miles per hour." 1.6 to 3.1 m/s
Dunn, Jimmy, The Nile River, Tour Egypt, June 5, 2006. "The Nile flowed from south to north at an average speed of about four knots during inundation season." 2.1 m/s

The speed of a river varies from close to 0 m/s to 3.1 m/s (7 mph). Factors that affect the speed of a river include the slope gradient, the roughness of the channel, and tides. Rivers tend to flow from a higher elevation to a lower elevation. The gradient is the drop of the elevation of a river. Therefore, the rivers speed is at its maximum at the headwaters (high gradient, high energy) and at its minimum at the base level (no gradient, lowest energy). An incoming tide can reverse a river and cause it to flow against the gradient -- uphill!

Erosion and speed have a direct relationship. As speed increases erosion increases as well. With the process of erosion, it is found that fast water carries smaller rocks and sand downstream. Sand and gravel are deposited in slower waters. Just as speed affects erosion, erosion affects speed. The speed of a river affects the shape of the river itself. At a high gradient, the speed of a river is at its maximum therefore the shape of the river is very narrow. It stretches out as the slope gradient decreases.

In situations where people use water as an energy source the speed of the river is an important factor. Kinetic energy is proportional to velocity squared. So if the speed of a river was 1 m/s and, the energy density was 500 W/m2, if the speed was doubled to 2 m/s the energy density would quadruple to 2000 W/m2.

Figure 1.1

Figure 1.2

Nervana Gaballa -- 2006