Learning Objectives

Each statement represents the primary learning objective for the corresponding major heading within the chapter. After you complete the chapter, you should be able to:

6.1 Explain the role of mass movement in the development of valleys and discuss the factors that trigger and influence mass-movement processes.

6.2 List the hydrosphere’s major reservoirs and describe the different paths that water takes through the hydrologic cycle.

6.3 Describe the nature of drainage basins and river systems.

6.4 Discuss streamflow and the factors that cause it to change.

6.5 Summarize the ways that streams erode, transport, and deposit sediment.

6.6 Contrast bedrock and alluvial stream channels. Distinguish between two types of alluvial channels.

6.7 Contrast narrow V-shaped valleys, broad valleys with floodplains, and valleys that display incised meanders or stream terraces.

6.8 Discuss the formation of deltas and natural levees.

6.9 Contrast regional floods and flash floods. Describe some common flood control measures.

6.10 Discuss the importance of groundwater and describe its distribution and movement.

6.11 Compare and contrast wells, artesian systems, and springs.

6.12 List and discuss three important environmental problems associated with groundwater.

6.13 Explain the formation of caverns and the development of karst topography.

Concept Checks 

6.1 How does water affect mass movement? Describe the significance of the angle of repose. How might a forest fire influence mass movement? Link earthquakes and landslides.

6.2 Describe or sketch the movement of water through the hydrologic cycle. Once precipitation has fallen on land, what paths might the water take? What is meant by the term evapotranspiration? Over the oceans, evaporation exceeds precipitation, yet sea level does not drop. Explain this.

6.3 List several factors that cause infiltration and runoff to vary from place to place and from time to time.Draw a simple sketch of a drainage basin and a divide. Label each. What are the three main parts (zones) of a river system?

6.4 Contrast laminar flow and turbulent flow. Summarize the factors that influence flow velocity. What typically happens to channel width, channel depth, flow velocity, and discharge between the headwaters and the mouth of a stream? Briefly explain why these changes occur.

6.5 List two ways in which streams erode their channels. In what three ways does a stream transport its load? Which part of the load moves most slowly? Contrast capacity and competence.

6.6 Are bedrock channels more likely to be found near the head or the mouth of a stream?Describe or sketch the evolution of a meander, including how an oxbow lake forms. Describe a situation that might cause a stream channel to become braided.

6.7 Define base level and distinguish between ultimate base level and temporary (local) base level. Provide an example of each. Explain why V-shaped valleys often exhibit rapids and waterfalls. Relate the formation of incised meanders and stream terraces to changes in base level.

6.8 What feature may form where a stream enters the relatively still waters of a lake, an inland sea, or an ocean? What are distributaries, and why do they form? Briefly describe the formation of a natural levee. How is this feature related to back swamps and yazoo tributaries?

6.9 Contrast regional floods and flash floods. Describe three basic flood-control strategies. What is meant by a nonstructural approach to flood control?

6.10 About what percentage of freshwater is groundwater? How does this change if glacial ice is excluded? What are two geologic roles for groundwater? When it rains, what factors influence the amount of water that soaks into the ground? Distinguish between porosity and permeability. Contrast aquifers and aquitards.

6.11 Relate drawdown to cone of depression. In Figure 6.32, two wells were drilled down to the same level. Why was one successful and the other not? Sketch a simple cross section of an artesian system with a flowing artesian well. Label aquitards, the aquifer, and the pressure surface. Describe the circumstances that created the springs in Figure 6.32.

6.12 Describe the problem associated with pumping groundwater for irrigation in parts of the High Plains. What happened in the San Joaquin Valley as a result of excessive groundwater pumping? Which aquifer would be most effective in purifying polluted groundwater: one consisting of coarse gravel, sand, or cavernous limestone? Explain. 

6.13 How does groundwater create caverns? How do stalactites and stalagmites form? Describe two ways in which sinkholes form.

Concepts in Review

6.1 Mass Movement on Slopes: The Work of Gravity

Explain the role of mass movement in the development of valleys and discuss the factors that trigger and influence mass-movement processes.

Key Terms: mass movement, trigger, angle of repose

*After weathering breaks apart rock, gravity moves the debris downslope, in a process called mass movement. Sometimes this occurs rapidly as a landslide, and at other times the movement is slower. Landslides are a significant geologic hazard, taking many lives and destroying property every year.

*Mass movement serves an important role in landscape development. It widens stream-cut valleys and helps tear down mountains thrust up by internal processes.

*An event that initiates a mass movement process is referred to as a trigger. The addition of water, oversteepening of the slope, removal of vegetation, and shaking due to an earthquake are four important examples. Not all landslides are triggered by one of these four processes, but many are.

6.2 The Hydrologic Cycle 

List the hydrosphere’s major reservoirs and describe the different paths that water takes through the hydrologic cycle.

Key Terms: hydrologic cycle, evaporation, infiltration, runoff, transpiration, evapotranspiration

*Water moves through the hydrosphere’s many reservoirs by evaporating, condensing into clouds, and falling as precipitation in the hydrologic cycle. Once it reaches the ground, rain can either soak in, evaporate, be returned to the atmosphere by plant transpiration, or run off. Running water is the most important agent sculpting Earth’s varied landscapes.

 6.3 Running Water

Describe the nature of drainage basins and river systems.

Key Terms: stream, river, drainage basin, watershed, divide

*The land area that contributes water to a stream is its drainage basin. Drainage basins are separated by imaginary lines called divides.

*As a generalization, river systems tend to erode at the upstream end, transport sediment through the middle section, and deposit sediment at the downstream end.

6.4 Streamflow Characteristics

Discuss streamflow and the factors that cause it to change.

Key Terms: laminar flow, turbulent flow, gradient discharge, longitudinal profile

*The flow of water in a stream may be laminar or turbulent. A stream’s flow velocity is influenced by the channel’s gradient; the size, shape, and roughness of the channel; and the stream’s discharge.

*A cross-sectional view of a stream from head to mouth is a longitudinal profile. Usually the gradient and roughness of the stream channel decrease going downstream, whereas the size of the channel, stream discharge, and flow velocity increase in the downstream direction.

QUESTION: Sketch a typical longitudinal profile. Where does most erosion happen? Where is sediment transport the dominant process?

6.5 The Work of Running Water

Summarize the ways that streams erode, transport, and deposit sediment.

Key Terms: pothole, dissolved load, suspended load, bed load, capacity, competence, sorting, alluvium

*Streams erode when turbulent water lifts loose particles from the streambed. The focused “drilling” of the stream armed with swirling particles also creates potholes in solid rock.

*Streams transport their load of sediment dissolved in the water, in suspension, and along the bottom (bed) of the channel.

*A stream’s ability to transport solid particles is described using two criteria. Capacity refers to how much sediment a stream is transporting, and competence refers to the particle sizes the stream is capable of moving.

*Streams deposit sediment when velocity slows and competence is reduced. This results in sorting, the process by which like-size particles are deposited together.

6.6 Stream Channels

Contrast bedrock and alluvial stream channels. Distinguish between two types of alluvial channels.

Key Terms: meander, cut bank, point bar, cutoff, oxbow lake, braided channel

*Bedrock channels are cut into solid rock and are most common in headwaters areas where gradients are steep. Rapids and waterfalls are common features.

*Alluvial channels are dominated by streamflow through alluvium previously deposited by the stream. A floodplain usually covers the valley floor, with the river meandering, or moving through braided channels.

*Meanders change shape through erosion at the cut bank (the outer edge of the meander) and deposition of sediment on point bars (the inside of a meander). A meander may become cut off and form an oxbow lake.

QUESTION: The town of Carter Lake is the only portion of the state of Iowa that lies on the west side of the Missouri River. It is bounded on the north by its namesake, Carter Lake, on the south by the Missouri River, and on the east and west by Nebraska. After examining the accompanying map, prepare a hypothesis that explains how this unusual situation could have developed. 

6.7 Shaping Stream Valleys

Contrast narrow V-shaped valleys, broad valleys with floodplains, and valleys that display incised meanders or stream terraces.

Key Terms: stream valley,base level, floodplain, incised meander, stream terrace

*A stream valley includes the channel itself, the adjacent floodplain, and the relatively steep valley wall. Streams erode downward until they approach base level, the lowest point to which a stream can erode its channel. A river flowing toward the ocean (the ultimate base level) may encounter several local base levels along its route. These could be lakes or resistant rock layers that retard downcutting by the stream.

*A stream valley is widened through the meandering action of the stream, which erodes the valley walls and widens the floodplain. If base level were to drop or if the land were uplifted, a meandering stream might start downcutting and develop incised meanders. Streams underlain by deep alluvium are likely to develop terraces.

QUESTION: Meanders are associated with a river that is eroding from side to side, whereas narrow canyons are associated with rivers that are vigorously downcutting. The river in this image is confined to a narrow canyon but is also meandering. Explain. 

6.11 Wells, Artesian Systems, and Springs

Compare and contrast wells, artesian systems, and springs.

Key Terms: well, drawdown, cone of depression, artesian, confined aquifer, spring, perched water table

*Wells, which are openings bored into the zone of saturation, withdraw groundwater and may create roughly conical depressions in the water table known as cones of depression.Artesian wells tap into inclined aquifers bounded above and below by aquitards. For a system to qualify as artesian, the water in the well must be under sufficient pressure that it can rise above the top of the confined aquifer. 

*Artesian wells may be flowing or nonflowing, depending on whether the pressure surface is above or below the ground surface.

*Springs occur where the water table intersects the land surface and a natural flow of groundwater results. They may be due to the intersection of a perched water table and the ground surface.

6.12 Environmental Problems Related to Groundwater

List and discuss three important environmental problems associated with groundwater.

*Groundwater can be “mined” by being extracted at a rate that is greater than the rate of recharge. When groundwater is treated as a nonrenewable resource, as it is in parts of the High Plains aquifer, the water table drops.

*The extraction of groundwater can cause pore space to decrease in volume and the grains of loose Earth materials to pack more closely together. This overall compaction of sediment volume results in subsidence of the land surface.

*Contamination of groundwater with sewage, highway salt, fertilizer, or industrial chemicals is another issue of critical concern. Once groundwater is contaminated, the problem is very difficult to solve, requiring expensive remediation or abandonment of the aquifer.

6.13 The Geologic Work of Groundwater

Explain the formation of caverns and the development of karst topography.

Key Terms: cavern, speleothem, stalactite, stalagmite, flowstone, karst topography, sinkhole (sink)

*Groundwater dissolves rock, in particular limestone, leaving behind spaces in the rock. Caverns form at the zone of saturation, but later dropping of the water table may leave them open and dry—and available for people to explore.

*Dripstone is rock deposited by dripping of water containing dissolved calcium carbonate inside caverns. Features made of dripstone include stalactites, stalagmites, and columns.

*Karst topography develops in limestone regions and exhibits irregular terrain punctuated with many depressions called sinkholes. Some sinkholes form when the cavern roofs collapse.

QUESTION: Identify the three cavern deposits labeled in this photograph. (Miroslav Krob/age Fotostock) The speleothems are as follows. A, a tapering feature that extends from the roof of the cavern. B, a feature that extends upward from the cavern floor C, a tube shape feature that extends down from the cavern roof and connects with the floor.