42 Chapter 13 Freshwater After Class Questions KEY

docx

School

University of Pittsburgh *

*We aren’t endorsed by this school

Course

101

Subject

Geography

Date

Oct 30, 2023

Type

docx

Pages

Uploaded by rong168

PHS 101 Chapter 13 Freshwater --- After Class Review Questions --- KEY After Class Review Questions 1. Define runoff and explain how runoff can initiate formation of a stream. Answer: Runoff is excess water flow over land. Streams form as runoff cuts down into the substrate, carving channels. Bloom’s Level: Understanding Learning Objectives Covered: o 13A. characterize drainage networks, lakes, and wetlands, and describe how they form and evolve. 2. What factors determine whether a stream is fresh or salty? What is the difference between a lake and a wetland? Distinguish among swamps, marshes, and bogs. Answer: Whether the water in a lake is fresh or salty depends on if the lake has an outlet stream. If an outlet stream is present, the water remains there for only a relatively short time and most of the dissolved salts it contains are removed via the outlet stream, so the concentration of salt remains very low and the lake water is fresh. If there is no outlet, the only way that water can leave the lake is by evaporation, which removes water molecules but leaves behind salts and causes the lake water to be salty. Lakes are standing bodies of water occurring on land. Wetlands are shallow bodies of water that allow for vegetation to grow above the water’s surface. Swamps are wetlands that contain woody plants. Marshes are wetlands colonized by grasses. Bogs are wetlands that contain decaying vegetation. Bloom’s Level: Remembering Learning Objectives Covered: o 13A. characterize drainage networks, lakes, and wetlands, and describe how they form and evolve. 3. What is a drainage network? Describe the different patterns of drainage networks recognized by geologists. Which does the figure show? Answer: A drainage network consists of tributaries (side channels) draining into a main trunk stream. Drainage networks extend into all corners of a region, providing channels for the removal of runoff. The different types of drainage networks recognized by geologists are based on the shape of the network’s map pattern. A dendritic network, as is shown in the figure, looks like the pattern of branches on a tree, and forms where the substrate consists of flat-lying layers of material with uniform strength. A radial network looks like the spokes on a wheel and forms on the slopes of a cone-shaped hill. A rectangular network has channels that follow joints and intersect one another at right angles. A trellis network develops in a landscape of parallel valleys and ridges as tributaries flow down the valleys into a trunk stream that cuts across the ridges. A parallel 1

network consists of several streams flowing in the same direction down a uniform slope. Bloom’s Level: Understanding Learning Objectives Covered: o 13A. characterize drainage networks, lakes, and wetlands, and describe how they form and evolve. 4. Define stream discharge and explain how it can vary along a stream’s length. How can discharge be affected by climate? Why are some streams permanent and some ephemeral? Answer: Stream discharge refers to the cubic feet-per-second water flow within a stream. In a wet region, discharge increases downstream because each tributary that enters the stream adds more water than is removed by other processes. In a dry region, discharge decreases downstream because water seeps into the ground, evaporates, or is used by humans faster than it can be replaced. Permanent streams flow all year, whereas ephemeral streams flow only during part of the year. Ephemeral streams exist in areas where seasonal precipitation occurs and may vanish entirely during the dry season due to the lack of precipitation. Bloom’s Level: Understanding Learning Objectives Covered: o 13B. describe the nature of stream discharge and factors that control it. 5. Describe how streams erode and carry sediment. Distinguish between competence and capacity. Answer: Streams carry sediments in three forms: (1) dissolved load (ions in solution); (2) suspended load (fine grains that are suspended in the water of the stream); and (3) bed load (large clasts that bounce or roll along the streambed). The competence of a stream is the maximum particle size it carries; the greater the competence of the stream, the larger the particles it can carry. The capacity of a stream is the total quantity of sediment it can carry. Bloom’s Level: Understanding Learning Objectives Covered: o 13C. explain how streams produce distinct landscapes by both erosion and deposition. 6. How does a stream’s gradient vary along its length? What is the difference between a local and an ultimate base level? What factors control the locations of rapids and waterfalls? Answer: A stream will have a steeper gradient at its headwaters and when it is on a steep slope. The gradient should lessen as it flows down into valleys and to the coastal plain. A base level is the lowest elevation that a stream can cut down to. A local base level is one that occurs anywhere upstream of a drainage network’s mouth. The ultimate base level of a drainage network is the level of the standing body of water at the mouth of the trunk stream. Rapids form when water flows over ledges or boulders, when the stream channel 2

narrows, or when the gradient changes. Waterfalls form when the gradient becomes so steep that water free-falls above the stream bed. Bloom’s Level: Understanding Learning Objectives Covered: o 13C. explain how streams produce distinct landscapes by both erosion and deposition. 7. How does a braided stream differ from a meandering stream, and what factors determine which forms at a location? Describe how meanders form and evolve. Answer: Braided streams form when rapidly flowing streams during a flood slow down after the flood. The coarse sediment that the stream was carrying during the flood settles out and chokes the channel with numerous elongated gravel bars and sandbars, and the stream divides into many intertwining strands much like strands of hair in a braid. Meandering streams form where streams flow over a landscape with a very gentle gradient, causing the stream channel to wind back and forth in a series of snake-like curves called meanders. Gradient, proximity to mountains, and sediment supply and size determine which type of stream forms. Since the location of the strongest current in a stream tends to wander, it is sometimes nearer the middle of the channel and sometimes nearer the banks. When the fastest current runs along the bank, it begins to cut the bank away since faster-moving water erodes sediments more rapidly. As a result, the channel starts to curve, and the fastest current preferentially flows along the outer arc of the curve. Over time, the curve migrates sideways and grows more pronounced until it becomes a meander. As more time passes, the shape of a meander evolves. On the outside edge of a meander, erosion continues to erode the channel wall, forming a cut bank. On the inside edge, water slows down and deposits its sediments, forming a wedge-shaped deposit called a point bar. With continued erosion, a meander may curve so much that the cut bank at the upstream end of the meander starts to meet the cut bank at its downstream end, leaving a meander neck (a narrow strip of land separating the parts of the meander). When erosion cuts through a meander neck, a cutoff forms and the channels at the upstream and downstream ends of the meander rejoin to form a new, straighter channel. The cut off meander is called an oxbow lake if it remains filled with water, or an abandoned meander if it dries out. Bloom’s Level: Understanding Learning Objectives Covered: o 13C. explain how streams produce distinct landscapes by both erosion and deposition. 8. How does a drainage network evolve over time, and how does it change when stream rejuvenation takes place? What is stream piracy? What causes a drainage reversal? Answer: Downcutting and headward erosion cause the land to slope toward the stream channel, forming tributaries and establishing a drainage network. Drainage networks may change shape over time. When stream rejuvenation takes place, a stream flowing over the 3

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

land surface starts to downcut once again. The rejuvenated stream will carve a deeper channel along its current course, so a meandering stream will create incised meanders. Stream piracy describes the process by which one stream “captures” the water of another. Stream piracy occurs when headward erosion by one stream cuts through a drainage divide and it intersects the course of another stream. The stream that cut through the divide causes the water of the captured stream to start flowing down the pirate stream, so the channel of the captured stream downstream of the point of capture dries up. Drainage reversal is when the flow of a stream changes direction. Tectonic uplift or rifting may cause drainage reversals. Bloom’s Level: Understanding Learning Objectives Covered: o 13C. explain how streams produce distinct landscapes by both erosion and deposition. 9. How do deltas grow, and how do they differ from alluvial fans? Answer: A delta develops when a stream enters a standing body of water, such as a lake or the sea, and deposits a wedge of sediment. At the mouth of a stream, its water spreads out over a wider area and its flow slows, causing its sediment load to be deposited. Deltas grow if the rate of deposition is higher than the rate at which sediment is taken away by waves or currents in the body of water that the delta is deposited in. An alluvial fan forms when a stream exits a narrow canyon onto a plain in a dry environment, rather than into a body of water. Bloom’s Level: Remembering Learning Objectives Covered: o 13C. explain how streams produce distinct landscapes by both erosion and deposition. 10. What is the difference between a slow-onset flood and a flash flood? What phenomena can cause flooding, and how do people try to prevent flooding? Answer: Slow-onset floods take days to develop, last for weeks, and involve the trunk stream of a major drainage network, whereas flash floods happen very quickly, last for only minutes to hours, and generally affect a small area. Slow-onset floods occur during spring thaws as snow melts, during the sustained rains of a distinct wet season, or when a system of storms sits over a broad region for a long time. In all these situations, the ground becomes saturated and cannot absorb any more water, so runoff from a broad drainage area eventually flows into the trunk stream, whose discharge becomes too high to be contained in its channel. Flash floods happen when so much water falls on an area at once that it doesn’t have enough time to sink into the ground and so it becomes runoff instead. This occurs during particularly intense rainfall, when dams collapse, or when artificial levees fail. People have tried to control flooding with artificial barriers like dams, levees, and floodwalls, but these have had limited success. Flood control may also involve restoration of wetland areas along rivers or prohibiting construction within 4

designated land areas adjacent to a channel. Bloom’s Level: Remembering Learning Objectives Covered: o 13D. discuss the nature and causes of ﬂooding and how society can protect against ﬂood damage. 11. What is the annual probability of a flood, and how is it related to the recurrence interval? Answer: The recurrence interval of a flood is the average number of years between successive floods with a given discharge. For example, if a flood with a given discharge happens once in 100 years on average, then it is given a recurrence interval of 100 years and is called a “100-year-flood.” The annual probability of flooding is the chance that a flood of a certain size will happen during a given year and is the reciprocal of the recurrence interval: Annual probability=1Recurrenceinterval For example, the annual probability of the 100-year flood described above is 1⁄100, which can also be written as 0.01 or 1%. Bloom’s Level: Remembering Learning Objectives Covered: o 13D. discuss the nature and causes of ﬂooding and how society can protect against ﬂood damage. 12. What is groundwater, and where does it reside? How do porosity and permeability differ? Using these terms, contrast an aquifer with an aquitard. Answer: Precipitation that falls and infiltrates the subsurface is groundwater. This is a reservoir for freshwater, underground. Porosity refers to the total volume of open space within a material, specified as a percentage. Permeability refers to the ability of a material to allow fluids to pass through an interconnected network of pores. An aquifer is a sediment or rock with high permeability and porosity, and an aquitard is a sediment or rock with relatively low permeability, regardless of porosity. Bloom’s Level: Remembering Learning Objectives Covered: o 13E. define groundwater, explain where it comes from and where it occurs, and describe factors that control its ﬂow. o 13F. distinguish aquitards from aquifers, define the water table, and discuss factors that affect water-table depth. 13. What is the water table, and what factors affect its level? What factors affect the rate and flow direction of the groundwater? Why did the stream in the diagram dry up? Answer: The water table is the underground boundary between the unsaturated zone near the ground surface and the saturated zone below. The unsaturated zone is the region in which water only partially fills pores, whereas in the saturated zone all pore spaces are filled with water. The level of the water table depends on the rainfall of the region: the 5

wetter it is, the closer the water table is to the ground surface. The slope of the water table and the permeability of the material that the groundwater is flowing through influences the speed of groundwater flow. Groundwater flows faster through high-permeability rocks and in regions where the water table has a steep slope. The rate of groundwater flow is much slower than that of surface streams: Groundwater flows between 5 to 500 m per year, whereas surface streams can reach flow rates of up to 30 km per hour. The shape of the water table also roughly follows the shape of the overlying land. Groundwater flows from areas of high pressure and elevation to areas of low pressure and elevation. In the diagram, the water table is lowered because the region is dry. Any water that was in the stream or that falls on the streambed will seep into the ground until it reaches the water table. This is common in times of drought. Bloom’s Level: Understanding Learning Objectives Covered: o 13F. distinguish aquitards from aquifers, define the water table, and discuss factors that affect water-table depth. 14. How is an artesian well different from an ordinary well? Answer: An artesian well penetrates a confined aquifer in which water is under enough pressure to rise on its own to a level above the surface of the aquifer. In some artesian wells the pressure is enough to push the water in the well to the ground surface, forming a flowing artesian well. The base of an ordinary well lies below the water table in an unconfined aquifer, so water simply seeps from the aquifer into the well and fills it to the level of the water table. The water in an ordinary well needs to be extracted using artificial means, such as a pump or a simple bucket. Bloom’s Level: Remembering Learning Objectives Covered: o 13G. contrast the ways in which groundwater reaches the surface through wells and through springs. 15. Why do natural springs form? Explain why hot springs form and why geysers erupt. Answer: Natural springs form at locations where there are outlets for groundwater to flow or seep onto the ground surface. Hot springs form where groundwater can rise back to the surface by a pathway that is fast enough that it doesn’t lose the heat that it gains at depth (because of the geothermal gradient, temperatures rise above 30°C at a depth of several kilometers underground in most places). This occurs in places where faults or fractures provide a high-permeability conduit for rising deep water. Hot springs also form in regions of igneous activity, where magma-heated rock produces hot groundwater relatively close to the Earth’s surface. Geysers also form in regions of igneous activity, where there is a network of irregular fractures in very hot rock. Groundwater in these fractures is superheated by the rock. Because the boiling point of water increases with increasing pressure, hot groundwater at depth can remain in liquid form even if its temperature rises above the boiling point of water at the Earth’s surface. When such 6

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

superheated groundwater begins to rise through a conduit toward the ground surface, the pressure on it decreases until some and then all of the water transforms into steam. The steam quickly rises and pushes all the water and steam above it out from the conduit in a geyser eruption. Bloom’s Level: Understanding Learning Objectives Covered: o 13G. contrast the ways in which groundwater reaches the surface through wells and through springs. 16. Describe the process leading to the formation of caves, speleothems, and features of karst landscapes. Answer: All the features of karst landscapes form when limestone bedrock dissolves in slightly acidic groundwater. Caves form when groundwater dissolves openings underground. When the water table drops below the level of a cave, most of the water drains away and the cave becomes an open space filled with air. Speleothems form where downward-percolating groundwater containing dissolved calcium carbonate drips from the ceiling of the cave. Calcium carbonate slowly precipitates out of the water, producing travertine that builds up into various intricately shaped cave deposits (speleothems). Sinkholes form when the roofs of underground caves collapse. When most of the ground has collapsed into sinkholes, the ridges between them eventually erode away too, forming bridges and spires. Bloom’s Level: Understanding Learning Objectives Covered: o 13H. discuss the origin of cave networks and related karst landscapes and their relationship to groundwater. 17. How have humans overused and abused surface water supplies? Answer: Humans have polluted waterways, constructed dams, overused water commercially (sometimes consuming the entire volume of a river’s water), and increased runoff and sediment load through urbanization and agriculture. Fertilizer and sewage runoff into lakes can cause eutrophication, which decreases available oxygen and leads to the death of organisms in the lake ecosystem. Lakes can also dry up if the rivers that feed them are overused or dammed. Bloom’s Level: Understanding Learning Objectives Covered: o 13I. identify sustainability and environmental issues that pertain to freshwater resources. 18. Is groundwater a renewable or nonrenewable resource? Describe ways in which human activities affect groundwater. Answer: In the long term, on a time frame of 10,000 years, groundwater can be considered a renewable resource since the hydrologic cycle will eventually refill groundwater reserves. But in the short term, a time frame of 100 to 1,000 years (a human 7

timespan), groundwater in many regions is a nonrenewable resource because it is being used much faster than it can be replenished. Human activities can lower the water table, reverse the direction of groundwater flow, cause saltwater to be drawn into groundwater supplies, cause pore spaces in aquifers to collapse, and pollute groundwater. Bloom’s Level: Understanding Learning Objectives Covered: o 13I. identify sustainability and environmental issues that pertain to freshwater resources. Answers to On Further Thought 19. Records indicate that the height of the Mississippi River above flood stage, for a given amount of discharge, has been rising since 1927, when a system of levees began to block off portions of the floodplain. Why? Answer: The artificial levees of sand and mud, as well as concrete floodwalls, built by the U.S. Army Corps of Engineers starting in 1927, are designed to increase the Mississippi River channel’s volume and to isolate portions of the floodplain from flooding. This means that water is concentrated in the channel instead of being allowed to spread across the entire floodplain during a flood, which raises the height of the river above flood stage for a given discharge when flow increases. Bloom’s Level: Applying Learning Objectives Covered: o 13D. discuss the nature and causes of ﬂooding and how society can protect against ﬂood damage. 20. The population of a desert town in the southwestern United States has been doubling every 10 years. The town has been growing on a flat, gravel-filled basin between two small mountain ranges. Where does the water supply for the town come from? What do you predict will happen to the water table of the area in coming years? Answer: The town likely sits on a dry lake or stream bed, since it is built on a depression between two mountain ranges. Water would have drained into the basin from the slopes of both mountains and then infiltrated into the ground. The town could tap into this groundwater supply, but the water table is probably fairly deep since this is a dry area, so deep wells would be needed. Water might also be transported from distant aquifers hundreds of miles away to bring this resource to a desert town. In coming years, as the population grows, the town will increase its need for water and therefore demands on all water resources will also increase. If the town is relying on local groundwater extraction, the water table will be lowered and the ground surface will experience subsidence. Bloom’s Level: Applying Learning Objectives Covered: o 13F. distinguish aquitards from aquifers, define the water table, and discuss factors that affect water-table depth. 8