Aguas Superficiales

Section 9 . 1
Objectives
◗ Describe how surface water can move weathered materials. ◗ Explain how a stream carries its load. ◗ Describe how a floodplain develops.

Surface Water Movement
MAIN Idea Running water is an agent of erosion, carrying sediments in streams and rivers and depositing them downstream.
Real-World Reading Link Have you ever noticed that sometimes a river is muddy butother times it is clear? In floods, rivers can carry greater amounts of materials, which makes them muddy. Under normal conditions, they often carry less sediment, which makes them clearer.

Review Vocabulary
solution: a homogeneous mixture in which the component particles cannot be distinguished

The Water Cycle
Earth’s water supply is recycled in a continuous process called the water cycle,shown in Figure 9.1. Water molecules move continuously through the water cycle following many pathways: they evaporate from a body of water or the surface of Earth, condense into cloud droplets, fall as precipitation back to Earth’s surface, and infiltrate the ground. As part of a continuous cycle, the water molecules eventually evaporate back to the atmosphere, form clouds, fall as precipitation,and the cycle repeats. Understanding the mechanics of the water cycle will help you understand the reasons for variations in the amount of water that is available throughout the world. Often, a water molecule’s pathway involves time spent within a living organism or as part of a snowfield, glacier, lake, or ocean. Although water molecules might follow a number of different pathways, the overallprocess is one of repeated evaporation and condensation powered by the Sun’s energy.
Reading Check Explain What happens once water reaches Earth’s

New Vocabulary
runoff watershed divide suspension bed load discharge flood floodplain

surface?

Figure 9.1 The water cycle, also referred to as the hydrologic cycle, is a never-ending, natural circulation of water through Earth’s systems.Identify the driving force for the water cycle.
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Sun Condensation Evaporation Precipitation

Runoff Interactive Figure To see an animation of the water cycle, visit glencoe.com.

Oceans

Transpiration Land Rivers Groundwater Infiltration

224 Chapter 9 • Surface Water

Runoff
Water flowing downslope along Earth’s surface is called runoff. Runoff might reach a stream, river, or lake, itmight evaporate, or it might accumulate as puddles in small depressions and infiltrate the ground. During and after heavy rains, you can observe these processes in your yard or local park. Water that infiltrates Earth’s surface becomes groundwater. A number of conditions determine whether water on Earth’s surface will infiltrate the ground or become runoff. For water to enter the ground, there mustbe large enough pores or spaces in the soil and rock to accommodate the water’s volume, as in the loose soil illustrated in Figure 9.2. If the pores already contain water, the newly fallen precipitation will either remain in puddles on top of the ground or, if the area has a slope, run downhill. Water standing on the surface of Earth eventually evaporates, flows away, or slowly enters thegroundwater. Soil composition The physical and chemical composition of soil affects its water-holding capacity. Soil consists of decayed organic matter, called humus, and minerals. Humus creates pores in the soil, thereby increasing a soil’s ability to retain water. The minerals in soil have different particle sizes, which are classified as sand, silt, or clay. As you learned in Chapter 7, thepercentages of particles of each size vary from soil to soil. Soil with a high percentage of coarse particles, such as sand, has relatively large pores between its particles that allow water to enter and pass through the soil quickly. In contrast, soil with a high percentage of fine particles, such as clay, clumps together and has few or no spaces between the particles. Small pores restrict both the...