Surface Water Movement

I MA N 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 but other 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.

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 overall process is one of repeated evaporation and condensation powered by the Sun's energy.

^p Reading Check Explain What happens once water reaches Earth's 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.

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

Sun r

Condensation

Evaporation

Oceans

Transpiration

Precipitation

Runoff

Land

Rivers

Infiltration

Groundwater

Groundwater

Runoff

Water flowing downslope along Earth's surface is called runoff. Runoff might reach a stream, river, or lake, it might 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 must be 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 the groundwater.

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, the percentages 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 amount of water that can enter the ground and the ease of movement of water through the soil.

Rate of precipitation Light, gentle precipitation can infiltrate dry ground. However, the rate of precipitation might temporarily exceed the rate of infiltration. For example, during heavy precipitation, water falls too quickly to infiltrate the ground and becomes runoff. Thus, a gentle, long-lasting rainfall is more beneficial to plants and causes less erosion by runoff than a torrential downpour. If you have a garden, remember that more water will enter the ground if you water your plants slowly and gently.

Sand grains

Vocabulary

Academic Vocabulary

Accommodate to hold without crowding or inconvenience The teacher said she could accommodate three more students in her classroom

Figure 9.2 Soil that has open surface pores allows water to infiltrate. The particle size that makes up a soil helps determine the pore space of the soil.

Sand grains

Runoff Grain Size

Large grain size

Figure 9.2 Soil that has open surface pores allows water to infiltrate. The particle size that makes up a soil helps determine the pore space of the soil.

Runoff Grain Size

Fine grain size

Mixed grain size

Large grain size

Fine grain size

Mixed grain size

Figure 9.3 Vegetation can slow the rate of runoff of surface water. Raindrops are slowed when they strike the leaves of trees or blades of grass, and they trickle down slowly.

Figure 9.3 Vegetation can slow the rate of runoff of surface water. Raindrops are slowed when they strike the leaves of trees or blades of grass, and they trickle down slowly.

Water Movement Down Hill

Vegetation Soils that contain grasses or other vegetation allow more water to enter the ground than do soils with no vegetation. Precipitation falling on vegetation slowly flows down leaves and branches and eventually drops gently to the ground, where the plants' root systems help maintain the pore space needed to hold water, as shown in Figure 9.3. In contrast, precipitation falls with far more force onto barren land. In such areas, soil particles clump together and form dense aggregates with little space between them. The force of falling rain can then push the soil clumps together, thereby closing pores and allowing less water to enter.

Slope The slope of a land area plays a significant role in determining the ability of water to enter the ground. Water from precipitation falling on slopes flows to areas of lower elevation. The steeper the slope, the faster the water flows. There is also greater potential for erosion on steep slopes. In areas with steep slopes, much of the precipitation is carried away as runoff.

Stream Systems

Precipitation that does not enter the ground usually runs off the surface quickly. Some surface water flows in thin sheets and eventually collects in small channels, which are the physical areas where streams flow. As the amount of runoff increases, the channels widen, deepen, and become longer. Although these small channels often dry up after precipitation stops, the channels fill with water each time it rains and become larger and longer.

Tributaries All streams flow downslope to lower elevations. However, the path of a stream can vary considerably, depending on the slope and the type of material through which the stream flows. Some streams flow into lakes, while others flow directly into the ocean. Rivers that flow into other streams are called tributaries. For example, as shown in Figure 9.4, the Missouri River is a tributary of the Mississippi River.

Whose Land The Rivers Divide

Continental Divide

Drainage basin of the Mississippi River

r it

Continental Divide i

Mississippi Delta

Watersheds and divides All of the land area whose water drains into a stream system is called the system's watershed. Watersheds can be relatively small or extremely large in area. A divide is a high land area that separates one watershed from another. In a watershed, the water flows away from the divide, as this is the high point of the watershed.

Each tributary in a stream system has its own watershed and divides, but they are all part of the larger stream system to which the tributary belongs. The watershed of the Mississippi River, shown in Figure 9.4, is the largest in North America.

^P Reading Check Describe what a divide is and what role it plays in a watershed.

Figure 9.4 The watershed of the Mississippi River includes many stream systems, including the Mississippi, Missouri and Ohio Rivers. The Continental Divide marks the western boundary of the watershed. Identify what portion of the continental United States eventually drains into the Mississippi River.

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