STREAM FLOW

STREAM FLOW

a vast subterranean reservoir known as ground wa- ter . Although surface water is more conspicuous, 60

A slow stream flows at 0.25 to 0.5 meter per second (1 times more water is stored as ground water than in

to 2 kilometers per hour), whereas a steep, flooding all streams, lakes, and wetlands combined. Ground

stream may race along at about 7 meters per second (25 water also seeps through bedrock and soil toward

kilometers per hour). Three factors control current ve- the sea, although it flows much more slowly than

locity: (1) the gradient of the stream; (2) the discharge; surface water.

and (3) the shape and roughness of the channel.

Stream Erosion 237

Gradient Channel Shape and Roughness Gradient is the steepness of a stream. The lower

Friction between flowing water and the stream channel Mississippi River has a shallow gradient and drops only

slows current velocity. Consequently, water flows more

10 centimeters per kilometer of stream length. In con- slowly near the banks than near the center of a stream. trast, a tumbling mountain stream may drop 40 meters or

If you paddle a canoe down a straight stream channel, more per kilometer. Obviously, if all other factors are

you move faster when you stay away from the banks. equal, a stream flows more rapidly down a steep channel

The total friction depends on both the shape of a than a gradual one.

stream channel and its roughness. If streams of equal cross-sectional area are compared, a semicircular chan-

Discharge nel has the least surface in contact with the water and therefore imposes the least friction. If other factors are

Discharge is the amount of water flowing down a stream. equal, a stream with this shape will flow more rapidly It is expressed as the volume of water flowing past a than one that is either wide and shallow or narrow and point per unit time, usually in cubic meters per second

(m 3 /sec). The largest river in the world is the Amazon,

deep.

3 with a discharge of 150,000 m A rough channel creates more friction than a smooth /sec. In contrast, the one. Boulders in the stream bed increase turbulence and

Mississippi River, the largest in North America, has a discharge of about 17,500 m 3 /sec, approximately one resistance, so a stream flows more slowly through a rough channel than a smooth one (Fig. 14–3).

ninth that of the Amazon.

A stream’s discharge can change dramatically from month to month or even during a single day. For exam-

䊳 14.3 STREAM EROSION

ple, the Selway River, a mountain stream in Idaho, has a

A stream may erode sediment and bedrock from its chan- when mountain snow is melting rapidly. During the dry

discharge of 100 to 130 m 3 /sec during early summer,

nel. When it does so, it carries the sediment and deposits season in late summer, the discharge drops to about 10

it in its bed or flood plain farther downstream, or on a to 15 m 3 /sec (Fig. 14–2). A desert stream may dry up

delta where it enters the sea or a lake. completely during summer but become the site of a flash

STREAM ENERGY: THE ABILITY OF A STREAM flood during a sudden thunderstorm. TO ERODE AND CARRY SEDIMENT Stream velocity increases when discharge increases.

Thus, a stream flows faster during flood, even though its The ability of a stream to erode and carry sediment gradient is unchanged. The velocity of a stream also gen-

depends on its energy. The energy of a stream is erally increases in a downstream direction because trib-

proportional to both velocity and discharge. A rapid, utaries add to the discharge.

high-volume stream is a high-energy stream. It can move

40 Discharge (m 30

Figure 14–2 The hydrograph for the Selway River in the spring

10 15 20 25 1 5 10 15 20 25 1 5 10 15 20 25 1 and summer of 1988 shows that the discharge varied from 125 to

15 m 3 /sec.

238 CHAPTER 14 STREAMSANDLAKES

charge. Thus, a fast, large stream can carry more sedi- ment than a slow, small one.

Because the ability of a stream to erode and carry sediment is proportional to its velocity and discharge, most erosion and sediment transport occur during the few days each year when the stream is flooding. Relatively little erosion and sediment transport occur during the remainder of the year. To see this effect for yourself, look at any stream during low water. It will most likely be clear, indicating little erosion or sediment transport. Look at the same stream when it is flooding. It will probably be muddy and dark, indicating that the stream is eroding its bed and banks and carrying a large load of sediment.