Fluent Essays

Your Paper should be three to four text-written pages, plus one page of endnotes and one page that shows a map of the river basin. You may have only a few endnotes, two or three. Your paper should be typed and double-spaced (1.5). The final page (not counted as part of the four pages) should have “endnotes”. The citations of sources used in your paper. The one source you must use is your textbook. See the required reading below. Physical Geography 100 -- River Basin Essay – Fall 2019 (3-4 page paper worth 50 Points) Submit your essay through our class page on Canvas. The deadline is Friday, December 13th, at 11:59 p.m.; however, you may submit your Paper on River Basins any time between Sunday, December 8 th through Friday, December 13th at ll:59 p.m. Absolutely no submissions will be accepted after Friday, December 13 th . Email submissions will not be accepted. I would encourage you to not wait until 11:50 p.m. on December 13 to submit your River Basin Essay in case you have questions or issues. I will not be checking my email after Friday, December 13 th after 10:00 p.m., and I will not be able to address any problems after that time. It is your responsibility to submit your essay in a timely manner. Again, I strongly suggest that you do not wait until the last minute. Style and Format of Your Paper:  Your Paper should be three to four text-written pages, plus one page of endnotes and one page that shows a map of the river basin. You may have only a few endnotes, two or three. Your paper should be typed and double-spaced (1.5).  The final page (not counted as part of the four pages) should have “endnotes”. The citations of sources used in your paper. The one source you must use is your textbook. See the required reading below. You must read the following pages of Christopherson textbook as background information: Pages 409- 415, pages 433-435. You may also use the USGS (United Stated Geologic Survey) for more information: https://www.usgs.gov/special-topic/water-science-school/science/rivers-streams-and-creeks?qt- science_center_objects=0#qt-science_center_objects. You may refer to an additional source of your choice, but this is not required. This is not a major research paper. The purpose is to reflect on the issues surrounding a major River Basin, one that is important to you. You may choose any major global river. In a comprehensive discussion of your selected River Basin, address the following issues Your answers should be in your own words, paragraph format with complete sentences, not a listing or bullet format. Address and expand on these issues in your paper. 1.) Introduction: a. Explain why you selected this river and what it means to you. Have you been “on” the river, rafted in the river, fished in the river, swam in the river . . . ? b. Description of the River Basin (including area impacted by its drainage). What is its source and its mouth (delta)? Provide characteristics of length and volume. 2.) What affects the supply of water in the River? Discuss natural disasters such as flooding, but also the impact of human activities. Discuss both positive and negative human impacts. 3.) How is the river used and by whom? 4.) What political and economic forces do you think are at play in planning for the river and its dependent region? Who are the stakeholders (interested parties) and why are they stakeholders? Stakeholders may be numerous - - residents, indigenous populations, politicians, urban officials. 5.) What are the particular hazards the human population will face with this river system in the 21 st century? Provide details and describe. 6.) Discuss any cultural, religious, spiritual, sacred and mystical aspects to the river you have chosen. https://www.usgs.gov/special-topic/water-science-school/science/rivers-streams-and-creeks?qt-science_center_objects=0#qt-science_center_objects https://www.usgs.gov/special-topic/water-science-school/science/rivers-streams-and-creeks?qt-science_center_objects=0#qt-science_center_objects PLAGIARISM Do not plagiarize. If you use exact words of someone, you must use quotation marks. Cite your source directly after a direct quote, an idea, or statistics. Place the author and year of publication in parenthesis. ENDNOTES: An example of the format of an endnote is below. You must read the selections in your Christopherson text. Place the endnotes in alphabetical order. You should have no more than four or five endnotes. Christopherson, R.W., and Birkeland, G.H., 2018, Geosystems: An Introduction to Physical Geography, tenth edition, Hoboken, NJ: Pearson. Email me if you have questions about this assignment. Again, do not wait until the last minute. If you have difficulty, finding sources let me know. Always feel free to contact me. Chapter 14 River Systems 433 because offshore currents remove sediment as quickly as it is deposited. Floods Despite our historical knowledge of flood events and their effects, floodplains continue to be important sites of human activity and settlement. These activities place lives and property at risk during floods, especially in less-developed regions of the world. Bangladesh is per- haps the most persistent example: It is one of the most densely populated countries on Earth, and more than three-fourths of its land area is a floodplain and delta complex—an area the size of Alabama. The historic floods of 1988 and 1998 inundated 60% and 75%, respectively, of the country’s land area, causing extensive crop losses and thousands of fatalities. A f lood is defined as a high water flow that passes over the natural bank along any portion of a stream. As discussed earlier, floods in a drainage basin are strongly connected to precipitation and snowmelt, which are, in turn, connected to weather patterns (Fig- ure 14.32). Floods can result from periods of prolonged rainfall over a broad region, from intense rainfall as- sociated with short-lived thunderstorms, from rapid melting of the snowpack, or from rain-on-snow events that accelerate snowpack melting. Floods vary in mag- nitude and frequency, and their effects depend on many factors. Flood Probability Maintaining extensive historical records of discharge during precipitation events is critical for predicting the behavior of present streams under similar conditions. The U.S. Geological Survey has detailed records of stream discharge at stream-gaging stations since the 1900s, with Recurrence Interval, in Years Probability of occurrence in a Given Year Percent Chance of occurrence in a Given Year 10 1 in 10 10 50 1 in 50 2 100 1 in 100 1 500 1 in 500 0.20 1000 1 in 1000 0.10 table 14.2 Recurrence Interval and Probability of Occurrence for Flood Discharges the most consistent data collected since the 1940s. These relatively short-term historical data form the basis for flood probability estimates. Recurrence Interval Scientists rate flood discharges statistically according to the recurrence interval (or return interval), the estimated time interval between peak discharges of similar size. For example, based on dis- charge data for a particular stream, a “100-year flood” on that stream has a recurrence interval of 100 years and a 1% chance of occurring in any given year (Table 14.2). The use of historical data works well where available; however, urbanization and dam construction can change the magnitude and frequency of flood events on a stream or in a watershed. These statistical estimates are probabilities that events will occur randomly during a specified period; they do not mean that events will occur regularly during that time period. For example, several centuries might pass without a 100-year flood, or a 100-year level of flooding could occur twice in one century. Annual Exceedance Probability Another method for describing floods and precipitation events uses the annual exceedance probability (AEP) to represent the statistical likelihood of occurrence. By this measure, a 100-year flood has a 1% AEP. WoRkitOut 14.4 Recurrence of Rainfall and Flooding News reports about the 2016 West Virginia flooding pictured in Everyday Geosystems (Figure 14.1) described it as being caused by a “thousand-year precipitation event.” 1. What is the percent chance of a rainfall event of this magni- tude occurring in any given year? 2. Could a precipitation event of that magnitude occur again in your lifetime? 3. Does a 1000-year rain event produce a 1000-year flood event? Explain. ▲ Figure 14.32 Flooding from Hurricane Matthew in 2016. Rising floodwater caused by heavy rainfall from Hurricane Matthew inundated portions of North Carolina in October 2016; shown here is the town of Rocky Mount, flooded by the Tar River. [Thomas Babb/The News & Observer via AP.] M14_CHRI7119_10_SE_C14.indd 433 24/11/16 12:10 PM 434 Geosystems Floodplain Risk The flood recurrence interval is useful for floodplain management and hazard assessment. A 10-year flood in- dicates a moderate threat to a floodplain. A 50-year or 100-year flood is of greater consequence, but it is also less likely to occur in a given year. Scientists and developers define and map flood- plains using flood recurrence intervals, for example, by delineating the “100-year floodplain.” Using these maps, scientists and engineers can develop the most effective flood-management strategy. Restrictive zoning using these floodplain designations helps determine degrees of risk across the floodplain and can help avoid potential flood damage. However, restrictive zoning based on flood hazard mapping is not always enforced. Flood Protection In the United States, floods cause an average of about $6 billion in annual losses. The catastrophic floods along the Mississippi River and its tributaries in 1993 and 2011 produced damage that exceeded $30 billion in each oc- currence. Flood protection, when in place, generally takes the form of dams (discussed in Chapter 8) and arti- ficial levee construction along river channels. Usually, the term levee connotes an element of human construction, and these engineered features are common across the United States and throughout the world. Artificial levees are earthen embankments, often built on top of natural levees. They run parallel to the channel (rather than across it, like a dam) and increase the capacity in the channel by adding to the height of the banks (Figure 14.33). For efficient use of time and mate- rials, channels are often straightened during levee con- struction. Levees are intended to hold floods within the channel, but not prevent them completely. Eventually, given severe enough conditions, an artificial levee will be overtopped or damaged in a flood. When overtopping (known as levee breaching) or levee failure occurs, exten- sive flood damage and erosion can result downstream. (b) Sheep graze on the slopes of an artificial levee along the Sacramento River in California. Note that the agricultural fields are lower in elevation than the river, caused by subsidence of the Sacramento River delta. (a) A natural levee. Natural levee (c) The MIssissippi River flows over part of an intentional breach in the Bird's Point levee in Missouri in 2011. During the winter floods of early 2016, 11 levees were breached nearby as the Mississippi crested to near-record levels in the U.S. Midwest. georeport 14.2 America’s levees By several estimates, over 100,000 miles of artificial levees exist along rivers and streams in the United States, the vast majority of them privately owned. The U.S. population living in areas protected by levees is estimated to be in the tens of millions; some major urban areas with levee systems are New Orleans, Sacramento, Dallas–Fort Worth, St. Louis, and Washington, D.C. In fact, over 30 major cities in America are lo- cated on floodplains. Currently, no national policy exists concerning the safety of levees (see http://www.leveesafety.org/docs/NCLS-Recom- mendation-Report_012009_DRAFT.pdf). ▲Figure 14.33 Natural and artificial levees. [(b) California Department of Water Resources. (c) Scott Olsen/Getty Images News.] M14_CHRI7119_10_SE_C14.indd 434 24/11/16 12:11 PM http://www.leveesafety.org/docs/NCLS-Recom-mendation-Report_012009_DRAFT.pdf http://www.leveesafety.org/docs/NCLS-Recom-mendation-Report_012009_DRAFT.pdf In 2011, Americans spent $42 million on fishing-related activities. Streams in Montana, Missouri, Michigan, Utah, and Wisconsin are of high enough quality that they are designated “blue ribbon fisheries” based on sustainability criteria such as water quality and quantity, accessibility, and the specific species present. [Karl Weatherly/Getty Images.] After days of heavy rain, the Seine River reached its highest flood stage in over 30 years in Paris, France, in June 2016. High water closed rail lines, the Metro system, numerous tourist attractions, and all boat traffic through the city. [Joel Saget/AFP/Getty Images.] A proposed series of dams on the free-flowing Nu/Salween River system in Southeast Asia would relocate some 60,000 people in China. The dams would also block the movement of sediment that replenishes farmlands along the river’s floodplain and delta. [Bradley Mayhew/Getty Images.] A Texas Department of Safety boat patrols the U.S.–Mexico border along the Rio Grande in Texas for drug trafficking and human smuggling activities. The border follows the center of the river and was surveyed and permanently established to avoid disputes related to channel changes. [Polaris/Newscom.] 14a 14b 14d 14c RIVER SYSTEMS IMPACT HUMANS • Humans use rivers for recreation and have farmed fertile floodplain soils for centuries. • Flooding affects human settlements on floodplains and deltas. HUMANS IMPACT RIVER SYSTEMS • Dams and diversions alter river flows and sediment loads, affecting river ecosystems and habitat. River restoration efforts include dam removal to restore ecosystems and threatened species. • Urbanization, deforestation, and other human activities in water- sheds alter runoff, peak flows, and sediment loads in streams. • Levee construction affects floodplain ecosystems; levee failures cause destructive flooding. ISSUES FOR THE 21ST CENTURY • Increasing population will intensify human settlement on floodplains and deltas worldwide, especially in developing countries, making more people vulnerable to flood impacts. • Stream restoration will continue, including dam decommissioning and removal, flow restoration, vegetation reestablishment, and restoration of stream geomorphology. • Global climate change may intensify storm systems, including hurricanes, increasing runoff and flooding in affected regions. Rising sea level will make delta areas more vulnerable to flooding. QUESTIONS TO CONSIDER 1. How do human activities affect river systems? Try to think of both negative and positive impacts. 2. What hazards will human populations on floodplains and deltas face during the 21st century? TheHumandenominator 14 Rivers, Floodplains, and Deltas M14_CHRI7119_10_SE_C14.indd 435 24/11/16 12:11 PM M isso uri R . Mississip p i R . Oh io R. Gulf of Mexico M isso uri R . Mississip p i R . Oh io R. 0 250 500 KILOMETERS0 250 500 MILES0 250 500 KILOMETERS0 250 500 MILES ▲Figure GN 14.1 Map of the Mississippi River basin. 409 GEOSYSTEMSnow The Disappearing Delta Before modern engineering of the chan- nel, the Mississippi River carried over 400 million metric tons of sedi- ment annually to its mouth. River deposits built from this sediment now underlie most of coastal Louisiana. Today, the flow carries less than half its previous sediment load. This decline, combined with land subsidence and sea-level rise, means that the delta region is shrinking in size each year. The tremendous weight of sediment deposition at the Mississip- pi’s mouth has caused the entire delta region to lower as sediments become compacted, a process that is worsened by human activities such as oil and gas extraction. In the past, additions of sediment bal- anced this subsidence, allowing the delta to build. With the onset of human activities such as upstream dam construction, the delta is now subsiding without sediment replenishment. Compounding the problem is the maze of excavated canals through the delta for shipping and oil and gas exploration. As the land surface sinks, these canals allow seawater to flow inland, changing the salinity of inland waters. Freshwater wetlands whose roots help stabilize the land surface during floods are now declining. This makes the delta more vulnerable to flooding from hurricane storm surge, another factor hastening the delta’s demise. Finally, sea-level rise threatens coastal land and wetlands, most of which are less than 1 m (3.2 ft) above sea level. With continued local sea-level rise, lands not protected by levee embankments and other structures that prevent flooding will con- tinue to submerge. In this chapter, we examine the natural pro- cesses by which rivers erode, transport, and de- posit sediment, forming landforms such as deltas. 1. Why are engineers trying to keep the Mississippi River in its present channel? 2. What three factors are causing the Mississippi delta to disappear? Changes on the Mississippi River Delta T he immense Mississippi River basin drains 41% of the continental United States (Figure GN 14.1). From its head- waters in Lake Itasca, Minnesota, the Missis- sippi’s main stem flows southward, collecting water and sediment over hundreds of miles. As the river nears the Gulf of Mexico, the flow energy diminishes and the river depos- its its sediment load. This area of deposition forms the delta, the low-lying plain at the river’s end. Like most rivers, the Mississippi continu- ously changes its channel, seeking the short- est and most efficient course to the ocean. In southern Louisiana, the Mississippi’s chan- nel has—over thousands of years—shifted course across an area encompassing thou- sands of square miles. Throughout this time span, floods caused the river to abandon pre- vious channels and carve new ones. The Mis- sissippi River attained its present position about 500 years ago and began building the delta we see today (Figure GN 14.2). Engineering the River Channel Since about 1950, engineers have worked to keep the Mississippi River in its present channel, a feat accomplished by dams, floodgates, and artificial levees (earthen embankments designed to prevent channel overflow). The U.S. Army Corps of Engineers built the Old River Control Structure in 1963 to block the Mississippi River from shifting westward toward the Atchafalaya River, which takes a steeper, shorter route to the Gulf of Mexico. Such a shift would cause the river to bypass two major U.S. ports, Baton Rouge and New Orleans, with negative eco- nomic consequences. Despite such measures, the Atchafalaya delta is growing even as the rest of the Mississippi’s delta disappears. 0 15 30 KILOMETERS0 15 30 MILES Mississippi River delta Old River Control Structure Atchafalaya River delta Atchafalaya River Gulf of Mexico M is s is s ip p i R i v e r ▲Figure GN 14.2 Mississippi River landscape, southern Louisiana. Inset photo shows the Old River Control Auxilliary Structure. NASA/USGS; Inset photo by Bobbé Christopherson. Mobile Field Trip https://goo.gl/bpcQAU Mississippi River Delta M14_CHRI7119_10_SE_C14.indd 409 06/12/16 1:19 AM https://goo.gl/bpcQAU 410 Geosystems Earth’s rivers and waterways form vast arterial net-works that drain the continents. Even though this volume is only 0.003% of all freshwater, the work per- formed by this energetic flow makes it an important natural agent of landmass denudation. Rivers shape the landscape by removing the products of weathering, mass movement, and erosion and transporting them downstream. Remember from Chapter 8 that hydrology is the sci- ence of water at and below Earth’s surface. Processes that are related expressly to streams and rivers are termed fluvial (from the Latin fluvius, meaning “river”). The terms river and stream share some overlap in usage. Spe- cifically, the term river is applied to the trunk or main stream of the network of tributaries forming a river sys- tem. Stream is a more general term for water flowing in a channel and is not necessarily related to size. Fluvial systems, like all natural systems, have characteristic pro- cesses and produce recognizable landforms. The ongoing interaction between erosion, transpor- tation, and deposition in a river system produces fluvial landscapes. Erosion in fluvial systems is the process by which water dislodges, dissolves, or removes weath- ered surface material. This material is then transported to new locations, where it is laid down in the process of deposition. Running water is an important erosional force; in fact, in desert landscapes it is the most signifi- cant agent of erosion even though precipitation events are infrequent. We discuss fluvial processes in arid land- scapes in Chapter 15. Rivers also serve society in many ways. They provide us with essential water supplies; dilute, and transport wastes; provide critical cooling water for industry; and form critical transportation networks. Throughout his- tory, civilizations have settled along rivers to farm the fer- tile soils formed by river deposits. These areas continue to be important sites of human activity and settlement, plac- ing lives and property at risk during floods (Figure 14.1). Drainage Basins Streams, which come together to form river systems, lie within drainage basins, the portions of landscape from which they receive their water. Every stream has its own drainage basin, or watershed, ranging in size from tiny to vast. A major drainage basin system is made up of many smaller drainage basins, each of which gathers and delivers its runoff and sediment to a larger basin, even- tually concentrating the volume into the main stream. Figure 14.2 illustrates the drainage basin of the Amazon River, from headwaters to the river’s mouth (where the river meets the ocean). The Amazon carries millions of tons of sediment through the drainage basin, which is as large as the Australian continent. Drainage Divides In any drainage basin, water initially moves downslope as overland flow, which takes two forms: It can move as A flooding river carries not only water but also sediment and debris. When a river overflows its banks into human develop- ments, the flow can pick up vehicles and knock houses off their foundations. As the floodwaters recede, debris such as trees come to rest and sediment is deposited over most surfaces, including the interiors of houses. In June 2016, flooding in West Virginia caused extensive damage, 23 fatalities, and left residents cleaning up a land- scape of mud. everydaygeosystems What kind of damage occurs during a river flood? ◀Figure 14.1 The aftermath of flooding along the Elk River, Clendenin, West Virginia, in June 2016. [Ty Wright/Getty Images.] M14_CHRI7119_10_SE_C14.indd 410 24/11/16 12:10 PM Chapter 14 River Systems 411 0 200 400 KILOMETERS0 200 400 MILES Amazon River basin Mouth of Amazon River Amazon River PACIFIC OCEAN ATLANTIC OCEAN A n d e s M o u n t a i n s Elevation in m (ft) 250 (820) 0 (0) 750 (2460) 1500 (4920) 3000 (9840) 4500 (14,760) ▲Figure 14.2 Amazon River drainage basin and mouth. [NASA SRTM image by Jesse Allen, University of Maryland, Global Land Cover Facility; stream data World Wildlife Fund, HydroSHEDS project (see http:// hydrosheds.cr.usgs.gov/).] Interfluves Drainage divide Drainage basin Drainage basin Drainage divide Drainage divide Drainage divide Valley Valley Rill Gully Shee tflow ▶Figure 14.3 Drainage divides. A drainage divide separates drainage basins. georeport 14.1 Locating the source of the Amazon Over the past several centuries, scientists and explorers have designated at least six different sources as the true beginning of the Ama- zon River. In the 1970s, southwest Peru’s Apurímac River was deemed the longest tributary stream, and in 2000, Lake Ticlla Cocha on the slopes of Mount Mismi was named as the Apurimac's source. Then in 2014, a team of kayakers used GPS tracking data and satellite images to determine that the Mantaro River, also in southwest Peru, is the longest upstream extension of the Amazon River. However, the new claim remains under debate. sheetflow, a thin film spread over the ground surface, and it can concentrate in rills, small-scale grooves in the land- scape made by the downslope move- ment of water. Rills may develop into deeper gullies and then into stream channels leading to the valley floor. The high ground that separates one valley from another and directs sheetflow is called an interfluve (Figure 14.3). Ridges act as drainage divides that define the catchment, or water-receiving, area of every drain- age basin; such ridges are the dividing lines that control into which basin the surface runoff drains. M14_CHRI7119_10_SE_C14.indd 411 24/11/16 12:10 PM http://hydrosheds.cr.usgs.gov/).] 412 Geosystems A special class of drainage divides, continental divides, separate drainage basins that empty into dif- ferent bodies of water surrounding a continent (Figure 14.4). For North America, these bodies are the Pacific Ocean, the Gulf of Mexico, the Atlantic Ocean, Hudson Bay, and the Arctic Ocean. These divides form water- resource regions and provide a spatial framework for water-management planning. In North America, the con- tinental divide separating the Pacific and Gulf/Atlantic basins runs the length of the Rocky Mountains, reaching its highest point in Colorado at the summit of Gray’s Peak at 4352 m (14,278 ft) elevation (Figure 14.5). As discussed in Geosystems Now, the great Mississippi–Missouri–Ohio River system drains 41% of the continental United States. Within this basin, rain- fall in northern Pennsylvania feeds hundreds of small streams that flow into the Allegheny River. At the same time, rainfall in western Pennsylvania feeds hundreds of streams that flow into the Monongahela River. The two rivers then join at Pittsburgh to form the Ohio River. The Ohio connects with the Mis- sissippi River, which eventually flows to the Gulf of Mexico. Each contributing tributary, large or small, adds its discharge and sedi- ment load to the larger river. In our example, sediment weathered and eroded in Pennsyl- vania is transported thousands of kilometers and accumulates on the floor of the Gulf of Mexico, where it forms the Mississippi River delta. Internal Drainage The ultimate outlet for most drainage ba- sins is the ocean. In some regions, however, stream drainage does not reach the ocean. Instead, the water leaves the drainage basin by means of evaporation or subsurface gravi- tational flow. Such basins are described as having internal drainage. Regions of inter- nal drainage occur in Asia, Africa, Australia, Mexico, and the western United States in Nevada and Utah (discussed in Chapter 15). An example within this region is the Hum- boldt River, which flows westward across Nevada and eventually disappears into the Humboldt Sink as a result of evaporation and seepage losses to groundwater. The area surrounding Utah’s Great Salt Lake, out- let for many streams draining the Wasatch Mountains, also exemplifies internal drain- age, since its only outlet is evaporation. In- ternal drainage is also a characteristic of the Dead Sea region in the Middle East and the region around the Aral Sea and Caspian Sea in Asia (Figure 14.6). Drainage Basins as Open Systems Drainage basins are open systems. Inputs include pre- cipitation and the minerals and rocks of the regional geology. Energy and materials are redistributed as the stream constantly adjusts to its landscape. System out- puts of water and sediment disperse through the mouth of the stream or river into a lake, another stream or river, or the ocean. Change that occurs in any portion of a drainage basin can affect the entire system. For example, the building of a dam not only affects the immediate stream envi- ronment around the structure, but can also change the movement of water and sediment for hundreds of miles downstream. Natural processes such as floods can also push river systems to thresholds, where banks collapse or channels change course. Throughout changing condi- tions, a river system constantly strives for equilibrium among the interacting variables of discharge, chan- nel steepness, channel shape, and sediment load, all of which are discussed in the chapter ahead. (a) Loveland Pass, Colorado, lies along the continental divide between the Pacific and Gulf/Atlantic drainage basins. (b) A backpacker approaches the continental divide at Cutbank Pass, Glacier National Park, Montana. ▲Figure 14.4 The U.S. Continental Divide, Colorado and Montana. [(a) Erika Nusser/Alamy. (b) Design Pics Inc./Alamy.] M14_CHRI7119_10_SE_C14.indd 412 24/11/16 12:10 PM Chapter 14 River Systems 413 No rth A tla nti c dr ain ag e Pacific drainage Pacific drainage Arctic drainage Arctic drainage Hudson Bay drainage Gulf/Atlantic drainage Atlantic drainage 6 0 °N170°E 180° 170°W 160°W 150°W 140°W 110°W 90°W 60°W 50°W 40°W 30°W 20°W 70°E 60 °N 50 °N 40 °N 30° N 20°N 20°N 30°N 40°N 50°N 70°N 80°N 120°W130°W 80°W Ar cti c Ci rcl e Tropic of Cancer Bering Sea Gulf of Alaska Labrador Sea Hudson Bay Baffin Bay Beaufort Sea Gulf of Mexico ARCTIC OCEAN ATLANTIC OCEAN ARCTIC OCEAN PACIFIC OCEAN C A L IF O R N I A NELSON LABRADOR–NEWFOUNDLAN D TEN NE SS EE L O W E R M IS S IS S IP P I U PP ER C OL OR AD OInternal Drainage GREAT BASIN COLUMBIA FRASER PACIFIC COASTAL YUKON MACKENZIE PEACE– ATHABASCA KEEWATIN CHURCHILL SASKATCHEWAN ASSINIBOINE– RED NORTHERN QUEBEC GREAT LAKES ST. LAWRENCE NORTH SLOPE– GASPÉ MARITIME COASTAL NORTH ATLANTIC UPPER MISSISSIPPI MISSOURI OHIO SOUTH ATLANTIC– GULF ARKANSAS–WHITE–RED TEXAS–GULFRIO GRANDE LOWER COLORADO ARCTIC COAST AND ISLANDS 0 250 500 KILOMETERS0 250 500 MILES DRAINAGE BASIN DISCHARGE CANADA: Hudson Bay 682,000 (553) Atlantic 670,000 (544) Pacific 602,000 (488) Arctic 440,000 (356) Gulf of Mexico 105 (0.9) UNITED STATES: Gulf/Atlantic 718 (886,000) Pacific 334 (412,000) Atlantic 293 (361,000) millions acre-feet per year (millions m3 per year) millions m3 per year (millions acre-feet per year) Continental divides ◀Figure 14.5 Drainage basins and continental divides, North America. Continental divides (red lines) separate the major drainage basins that empty through the United States into the Pacific Ocean, Atlantic Ocean, and Gulf of Mexico, and to the north, through Canada into Hudson Bay and the Arctic Ocean. Subdividing these major drainage basins are major river basins. [After U.S. Geological Survey; The National Atlas of Canada, 1985, “Energy, Mines, and Resources Canada”; and Environment Canada, Currents of Change— Inquiry on Federal Water Policy—Final Report 1986.] ◀Figure 14.6 Utah’s Great Salt Lake, part of an interior drainage system. [Delphotos/ Alamy.] M14_CHRI7119_10_SE_C14.indd 413 24/11/16 12:10 PM 414 Geosystems number and length of channels in a given area reflect the landscape’s regional geology and topography. For exam- ple, landscapes with underlying materials that are easily erodible will have a higher drainage density than land- scapes of more resistant rock. The drainage pattern is the arrangement of channels in an area. Distinctive patterns can develop based on a combination of factors, including • regional topography and slope inclination, • variations in rock resistance, • climate and hydrology, and • structural controls imposed by the underlying rocks. Consequently, the drainage pattern of any land area on Earth is a remarkable visual summary of every geologic and climatic characteristic of that region. A familiar pattern is dendritic drainage (Figure 14.7a), a treelike pattern (from the Greek word dendron, meaning “tree”) similar to that of many natural systems, such as capillaries in the human circulatory system or the veins in tree leaves. Energy expenditure in the mov- ing of water and sediment through this drainage system is efficient because the total length of the branches is mini- mized. In landscapes with steep slopes, parallel drainage may occur (Figure 14.7b). In some landscapes, drainage patterns alter their characteristics abruptly in response to slope steepness or rock structure (Figure 14.7c). Other drainage patterns are closely tied to geo- logic structure. Around a volcanic mountain or uplifted dome, a radial drainage pattern results when streams flow off a central large peak. New Zealand’s Mount Rua- pehu, an active volcano on the North Island, shows such a radial drainage pattern (Figure 14.8). In a faulted and (a) Note the drainage channels flowing off the central peak of Mount Ruapehu, which last erupted in 2007. (b) Radial drainage pattern. ◀Figure 14.8 Radial drainage on Mount Ruapehu, North Island, New Zealand. This false-color image of the composite vocano shows vegetation as red, the crater lake as light blue, and rocks as brown. [NASA.] Drainage Patterns A primary feature of any drainage basin is its drainage density, determined by dividing the total length of all stream channels in the basin by the area of the basin. The (a) Dendritic drainage pattern. (c) Dendritic and parallel drainage in response to local geology and relief in central Montana. (b) Parallel drainage pattern. Drainage divide ▲Figure 14.7 Dendritic and parallel drainage patterns. [Bobbé Christopherson.] M14_CHRI7119_10_SE_C14.indd 414 24/11/16 12:10 PM Chapter 14 River Systems 415 (a) A rectangular stream pattern develops in areas with jointed bedrock. (b) A trellis stream pattern develops in areas where the geologic structure is a mix of weak and resistant bedrock (such as in folded landscapes). Ridges of resistant rock Valleys cut in less-resistant rock ▲Figure 14.9 Drainage patterns controlled by geologic structure: rectangular and trellis. S u s q u e h a n n a R iv e r As erosion exposes underlying rock with a different structure, the river cuts through ridges of resistant rock rather than flowing around them. Water gap in the eastern United States and in the folded land- scapes of south-central Utah. Some landscapes display a deranged pattern with no clear geometry and no true stream valley. Examples include the glaciated shield re- gions of Canada, northern Europe, and some parts of the U.S. upper Midwest. Occasionally, drainage patterns occur that seem to be in conflict with the landscape through which they flow. For example, a stream may initially develop a channel in horizontal strata deposited on top of up- lifted, folded structures. As the stream erodes into the older folded rock layers, it keeps the original course, downcutting into the rock in a pattern contrary to the structure of the older layers. Such a stream is a super- posed stream, in which a preexisting channel pattern has been imposed upon older underlying rock struc- tures (Figure 14.10). For example, Wills Creek, presently cutting a water gap through Haystack Mountain at Cum- berland, Maryland, is a superposed stream. A water gap is a notch or opening cut by a river through a mountain range and is often an indication that the river is older than the landscape. ▲Figure 14.10 The Susquehanna River in Pennsylvania, a superposed stream. The Susquehanna River established its course on relatively uniform rock strata that covered more complex geologic structure below. Over time, as the landscape eroded, the river “superposed” its course onto the older structure by cutting through the resistant strata. [Landsat-7, NASA.] WoRkitOut 14.1 Stream Drainage Patterns Choose among dendritic, parallel, radial, rectangular, trellis, and deranged drainage patterns to answer the following questions. 1. Which drainage pattern often occurs in a landscape with a central mountain peak? 2. Which pattern is prominent in the Amazon River basin in Figure 14.2? 3. Which pattern often occurs in landscapes of jointed bedrock? 4. Which pattern occurs in landscapes of folded rock, such as in southern Utah? 5. Which pattern might be found in the Canadian Shield land- scape shown in Figure 12.2? jointed landscape, a rectangular pattern (Figure 14.9a) directs stream courses in patterns of right-angle turns. In dipping or folded topography, the trellis drainage pattern develops, influenced by folded rock structures that vary in resistance to erosion (Figure 14.9b). Paral- lel structures direct the principal streams, while smaller dendritic tributary streams are at work on nearby slopes, joining the main streams at right angles, as in a plant trellis. Such drainage is seen in the nearly par- allel mountain folds of the Ridge and Valley Province M14_CHRI7119_10_SE_C14.indd 415 24/11/16 12:10 PM