Info

Stream Velocity and Particle Size

Cobbles

6.4 cm —

r

Dokklar

/

0 2 cm --

J

Sand

I

----

----

---

7,"

06 c

m

1

_SMt_

0.0004 ci

m

1

Clay

0 100 200 300 400 500 600 700 800 Stream velocity (cm/s)

0 100 200 300 400 500 600 700 800 Stream velocity (cm/s)

Figure 9.5 Particles rub, scrape, and grind against one another in a streambed, which can create potholes.

Figure 9.5 Particles rub, scrape, and grind against one another in a streambed, which can create potholes.

Stream Load

The material that a stream carries is known as stream load. Stream load is carried in three ways.

Materials in suspension Suspension is the method of transport for all particles small enough to be held up by the turbulence of a stream's moving water. Particles such as silt, clay, and sand are part of a stream's suspended load. The amount of material in suspension varies with the volume and velocity of the stream water. Rapidly moving water carries larger particles in suspension than slowly moving water.

Bed load Sediment that is too large or heavy to be held up by turbulent water is transported by streams in another manner. A stream's bed load consists of sand, pebbles, and cobbles that the stream's water can roll or push along the bed of the stream. The faster the water moves, the larger the particles it can carry. As the particles move, they rub against one another or the solid rock of the stream-bed, which can erode the surface of the streambed, as shown in Figure 9.5.

Materials in solution Solution is the method of transport for materials that are dissolved in a stream's water. When water runs through or over rocks with soluble minerals, it dissolves small amounts of the minerals and carries them away in the solution. Groundwater adds the majority of the dissolved load to streams. The amount of dissolved material that water carries is often expressed in parts per million (ppm). For example, a measurement of 10 ppm means that there are 10 parts of dissolved material for every 1 million parts of water. The total concentration of materials in solution in streams averages 115-120 ppm, although some streams carry as little dissolved material as 10 ppm. Values greater than 10,000 ppm have been observed for streams draining desert basins.

Figure 9.6

Floods in Focus

Floods have shaped the landscape and affected human lives.

1927 Heavy rains flood the Mississippi River from Illinois to Louisiana leaving more than 600,000 people homeless.

1931 China's Yellow River floods when heavy rain causes the river's large silt deposits to shift and block the channel.

1900

1931 China Floods
1902 In Egypt, the Aswan Dam is built to stabilize the flow of annual flood waters that create the fertile Nile Delta.
1958 Following a flood that claimed almost 2000 lives, Holland begins creating a vast network of dams, dikes, and barriers, shortening its coastline by 700 km.

228 Chapter 9 • Surface Water

(tl)Salvatore Vasapolli/Animals Animals, (bl)Lloyd Cluff/CORBIS, (br)Anthony Cooper/Ecoscene/CORBIS

Stream Carrying Capacity

The ability of a stream to transport material, referred to as its carrying capacity, depends on both the velocity and the amount of water moving in the stream. The channel's slope, depth, and width all affect the speed and direction the water moves within it. A stream's water moves more quickly where there is less friction; consequently, smooth-sided channels with great slope and depth allow water to move the most rapidly. The total volume of moving water also affects a stream's carrying capacity. Discharge, shown in Figure 9.7, is the measure of the volume of stream water that flows past a particular location within a given period of time. Discharge is commonly expressed in cubic meters per second (m3/s). The following formula is used to calculate the discharge of a stream.

discharge = average width X average depth X average velocity (m3/s) (m) (m) (m/s)

The largest river in North America, the Mississippi River, has a huge average discharge of about 17,000 m3/s. The Amazon River, the largest river in the world, has a discharge of about ten times that amount. The discharge from the Amazon River over a two-hour period would supply New York City's water needs for an entire year!

As a stream's discharge increases, its capacity also increases. Both water velocity and volume increase during times of heavy precipitation, rapid melting of snow, and flooding. In addition to increasing a stream's carrying capacity, these conditions heighten a stream's ability to erode the land over which it passes. As a result of an increase in erosional power, a streambed can widen and deepen, adding to the stream's carrying capacity. Streams shape the landscape both during periods of normal flow and during floods, as highlighted in Figure 9.6.

Average

depth Average / ^^ velocity /

1

Average

width

■ Figure 9.7 Stream discharge is

the product of a stream's average width,

average depth, and the velocity of the

water.

• 1988 Monsoon rains in Bangladesh flood two-thirds of the country, affecting 45 million people

'If 2005 Category 5 Hurricane Katrina slams into Louisiana, Mississippi, and Alabama, devastating New Orleans.

• 1988 Monsoon rains in Bangladesh flood two-thirds of the country, affecting 45 million people

1974 The United Kingdom begins building the Thames Barrier to protect London from rising tide levels as the city sinks and sea levels rise.

% 1996 Volcanic eruptions in Iceland release meltwater from under the Vatnajokull glacier that washes away power lines, major roads, and bridges.

Interactive Time Line To learn more about these discoveries and others, visit btti _ glencoe.com. ScieiK^pn lino

1974 The United Kingdom begins building the Thames Barrier to protect London from rising tide levels as the city sinks and sea levels rise.

% 1996 Volcanic eruptions in Iceland release meltwater from under the Vatnajokull glacier that washes away power lines, major roads, and bridges.

Interactive Time Line To learn more about these discoveries and others, visit btti _ glencoe.com. ScieiK^pn lino

Section 1 • Surface Water Movement 229

Jerry Grayson/Helifilms Australia PTY Ltd/Getty Images

Figure 9.8 When rivers overflow their banks, the floodwater deposits sediment. Over time, sediment accumulates along the edges of a river, resulting in natural levees.

Flood plain Sediment deposited Natural levees

Figure 9.8 When rivers overflow their banks, the floodwater deposits sediment. Over time, sediment accumulates along the edges of a river, resulting in natural levees.

Flood plain Sediment deposited Natural levees

Floodplains And Natural Levees

Figure 9.9 This flood was caused by heavy rainfall upstream. Notice the farm fields that have been covered in floodwater.

Analyze What long-term effects might this flood have on the crops grown in this area?

Figure 9.9 This flood was caused by heavy rainfall upstream. Notice the farm fields that have been covered in floodwater.

Analyze What long-term effects might this flood have on the crops grown in this area?

230 Chapter 9 • Surface Water

Barrie Rokeach/Getty Images

Floods

The amount of water being transported in a particular stream at any given time varies with weather conditions. Sometimes, more water pours into a stream than the banks of the stream channel can hold. A flood occurs when water spills over the sides of a stream's banks onto the adjacent land. The broad, flat area that extends out from a stream's bank and is covered by excess water during times of flooding is known as the stream's floodplain.

Floodwater carries along with it a great amount of sediment eroded from Earth's surface and the sides of the stream channel. As floodwater recedes and its volume and speed decrease, the water drops its sediment load onto the stream's floodplain. After repeated floods over time, sediments deposited by floods tend to accumulate along the banks of the stream. These develop into continuous ridges along the sides of a river, called natural levees, as shown in Figure 9.8. Floodplains develop highly fertile soils as more sediment is deposited with each subsequent flood. The fertile soils of floodplains make some of the best croplands in the world.

^p Reading Check Describe what happens when floodwaters recede.

Flood stages Floods are a natural occurrence. After a rain event or snowmelt, it takes time for runoff water to reach the streams. As water enters the streams, the water level continues to rise and might reach its highest point, called its crest, days after precipitation ends. When the water level in a stream rises higher than its banks, the river is said to be at flood stage. The resulting flooding might occur over localized areas or across large regions. The flooding of a small area is known as an upstream flood.

Heavy accumulation of excess water from large regional drainage systems results in downstream floods. Such floods occur during or after long-lasting, intense storms or spring thaws of large snowpacks. The tremendous volume of water involved in a downstream flood can result in extensive damage. The effects of flooding on the landscape are shown in Figure 9.9.

230 Chapter 9 • Surface Water

Barrie Rokeach/Getty Images

Figure 9.10 Gaging stations, like this one, can send data to meteorologic stations. There, scientists can process the information and alert the public to potential floods.

Flood Monitoring and Warning Systems

^ In order to provide warnings for people at risk, government agencies, such as the National Weather Service, monitor potential flood conditions. Earth-orbiting weather satellites photograph Earth and collect and transmit information about weather conditions, storms, and streams. In addition, the U.S. Geological Survey (USGS) has established approximately 7300 gaging stations in the United States to provide a continuous record of the water level in each stream as shown in Figure 9.10. These gaging systems often transmit data to satellites and telephone lines where the information is then sent to the local monitoring office.

In areas that are prone to severe flooding, warning systems are the first step in implementing emergency management plans. Flood warnings and emergency plans often allow people to safely evacuate an area in advance of a flood. ^

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