STREAM FLOW
A slow stream flows at 0.25 to 0.5 meter per second (1
to 2 kilometers per hour), whereas a steep, flooding
stream may race along at about 7 meters per second (25
kilometers per hour). Three factors control current velocity:
(1) the gradient of the stream; (2) the discharge;
and (3) the shape and roughness of the channel.
236 CHAPTER 14 STREAMS AND LAKES
Ice caps and
glaciers
1.8%
Precipitation
108
Atmosphere 0.001%
Underground
water 0.63% Lakes
and
rivers
0.01% Ocean
97.5%
Evaporation
455
Precipitation
409
Evaporation
62
Runoff
46
Figure 14–1 The hydrologic cycle shows that water circulates constantly among the sea,
the atmosphere, and the land. Numbers indicate thousands of cubic kilometers of water
transferred each year. Percentages show proportions of total global water in different portions
of the Earth’s surface.
Gradient
Gradient is the steepness of a stream. The lower
Mississippi River has a shallow gradient and drops only
10 centimeters per kilometer of stream length. In contrast,
a tumbling mountain stream may drop 40 meters or
more per kilometer. Obviously, if all other factors are
equal, a stream flows more rapidly down a steep channel
than a gradual one.
STREAM FLOWA slow stream flows at 0.25 to 0.5 meter per second (1to 2 kilometers per hour), whereas a steep, floodingstream may race along at about 7 meters per second (25kilometers per hour). Three factors control current velocity:(1) the gradient of the stream; (2) the discharge;and (3) the shape and roughness of the channel.236 CHAPTER 14 STREAMS AND LAKESIce caps andglaciers1.8%Precipitation108Atmosphere 0.001%Undergroundwater 0.63% Lakesandrivers0.01% Ocean97.5%Evaporation455Precipitation409Evaporation62Runoff46Figure 14–1 The hydrologic cycle shows that water circulates constantly among the sea,the atmosphere, and the land. Numbers indicate thousands of cubic kilometers of watertransferred each year. Percentages show proportions of total global water in different portionsof the Earth’s surface.GradientGradient is the steepness of a stream. The lowerMississippi River has a shallow gradient and drops only10 centimeters per kilometer of stream length. In contrast,a tumbling mountain stream may drop 40 meters ormore per kilometer. Obviously, if all other factors areequal, a stream flows more rapidly down a steep channelthan a gradual one.
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