Chapter 11 - Water - Surface Water PDF

11. WATER Learning Objectives By the end of this chapter, students should be able to: Describe the processesof the water cycle. Describe drainage basins, watershed protection, and water budget. Describe reasons for water laws, who controls them, and how water is sha...

11. WATER Learning Objectives By the end of this chapter, students should be able to: Describe the processesof the water cycle. Describe drainage basins, watershed protection, and water budget. Describe reasons for water laws, who controls them, and how water is shared in the western U.S. Describe zone of transport, zone of sediment production, zone of deposition, and equilibrium. Describe stream landforms: channel types, alluvial fans, floodplains, natural levees, deltas, entrenched meanders, and ter­ races. Describe the properties required for a good aquifer; define confining layer water table. Describe three major groups of water contamination and three types of remediation. Describekarst topography, how it is created, and the landforms that characterize it. All life on Earth requires water. The hydrosphere (Earth’s water) is an impor­ tant agent of geologic change. Water shapes our planet by depositing min­ erals, aiding lithification, and altering rocks after they are lithified. Water carried by subducted oceanic plates causes flux melting of upper mantle material. Water is among the volatiles in magma and emerges at the surface as steam in volcanoes. Humans rely on suitable water sources for consumption, agri­ culture, power generation, and many other purposes. In pre- industrial civilizations, the power­ Figure 11.1: Example of a Roman aqueduct in Segovia, Spain. ful controlled water resources. As shown in the figures, two thou­ sand year old Roman aqueducts still grace European, Middle Eastern, and North African skylines. Ancient Mayan architecture depicts water imagery such Figure 11.2: Chac mask in Mexico. as frogs, water-lilies, water fowl to illustrate the importance of water in their societies. In the drier lowlands of the Yucatan Peninsula, mask facades of the hooked-nosed rain god, Chac (or Chaac) are prominent on Mayan buildings such as the Kodz Poop (Temple of the Masks, sometimes spelled Coodz Poop) at the ceremonial site of Kabah. To this day government controlled water continues to be an integral part of most modern societies. 306 | WATER 11.1 Water Cycle Figure 11.3: The water cycle. The water cycle is the continuous circulation of water in the Earth’s atmosphere. During circulation, water changes between solid, liquid, and gas (water vapor) and changes location. The processes involved in the water cycle are evapora­ tion, transpiration, condensation, precipitation, and runoff. Evaporation is the process by which a liquid is converted to a gas. Water evaporates when solar energy warms the water sufficiently to excite the water molecules to the point of vaporization. Evaporation occurs from oceans, lakes, and streams and the land surface. Plants contribute significant amounts of water vapor as a byproduct of photosynthesis called transpiration that occurs through the minute pores of plant leaves. The term evapotranspiration refers to these two sources of water entering the atmosphere and is commonly used by geologists. Water vapor is invisible. Condensation is the process of water vapor transitioning to a liquid. Winds carrywater vapor in the atmosphere long distances. When water vapor cools or when air masses of different temperatures mix,water vapor may condense back into droplets of liquid water. These water droplets usually form around a microscopic piece of dust or salt called condensation nuclei. Thesesmall droplets of liquid water suspended in the atmosphere becomevis­ ible as in a cloud. Water droplets inside clouds collide and stick together, growing into largerdroplets. Once the water droplets become big enough, they fall to Earth as rain, snow, hail, or sleet. Once precipitation has reached the Earth’s surface, it can evaporate or flow as runoff into streams, lakes, and eventually back to the oceans. Water in streams and lakes is called surface water. Or water can also infiltrate into the soil and fill the pore spaces in the rock or sediment underground to become groundwater. Groundwater slowly moves through rock and unconsolidated materials. Some groundwater may reach the surface again, where it discharges as springs, streams, lakes, and the ocean. Also, surface water in streams and lakes can infiltrate again to recharge groundwater. Therefore, the surface water and groundwater systems are connected.  WATER | 307 One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=al-do-HGuIk Video 11.1: Water cycle. If you are using an offline version of this text, access this YouTube video via the QR code. Take this quiz to check your comprehension of this section. If you are using an offline version of this text, access the quiz for section 11.1 via the QR code. An interactive H5P element has been excluded from this version of the text. You can view it online here: https://pressbooks.lib.vt.edu/introearthscience/?p=765#h5p-74 11.2 Water Basins and Budgets The basic unit of division of the landscape is the drainage basin, also known as a catchment or watershed. It is the area of land that captures precipitation and con- tributes runoff to a stream or stream segment. Drainage divides are local topographic high points that separate one drainage basin from another. Water that falls on one side of the divide goes to one stream, and water that falls on the other side of the divide goes to a different stream. Each stream, tributary and streamlet has its own drainage basin. In areas with flatter topography, drainage divides are not as easily identified but they still exist. Figure 11.4: Map view of a drainage basin with main trunk streams and many tributaries with drainage divide in dashed red line. 308 | WATER The headwater is where the stream begins. Smaller tribu­ tary streams combine downhill to make the larger trunk of the stream. The mouth is where the stream finally reaches its end. The mouth of most streams is at the ocean. How- ever, a rare number of streams do not flow to the ocean, but rather end in a closed basin (or endorheic basin) where the only outlet is evaporation. Most streams in the Great Basin of Western North America end in endorheic basins. For example, in Salt Lake County, Utah, Little Cot- tonwood Creek and the Jordan River flow into the endorheic Great Salt Lake where the water evaporates. Figure 11.5: Oblique view of the drainage basin and divide of the Latorita River, Romania. Figure 11.6: Major drainage basins color coded to match the related ocean. Closed basins (or endorheic basins) are shown in gray. Perennial streams flow all year round. Perennial streams occur in humid or temperate climates where there is sufficient rainfall and low evaporation rates. Water levels rise and fall with the seasons, depending on the discharge. Ephemeral streams flow only during rain events or the wet season. In arid climates, like Utah, many streams are ephemeral. These streams occur in dry climates with low amounts of rainfall and high evaporation rates. Their channels are often dry washes or arroyos for much of the year and their sudden flow causes flash floods. Along Utah’s Wasatch Front, the urban area extending north to south from Brigham City to Provo, there are several water-sheds that are designated as “watershed protection areas” that limit the type of use allowed in those drainages in order to protect culinary water. Dogs and swimming are limited in those watersheds because of the possibility of contamination by harmful bacteria and substances to the drinking supply of Salt Lake City and surrounding municipalities.  WATER | 309 Water in the water cycle is very much like money in a personal budget. Income includes precipitation and stream and groundwater inflow. Expenses include groundwater withdrawal, evaporation, and stream and groundwater outflow. If the expenses outweigh the income, the water budget is not balanced. In this case, water is removed from savings, i.e. water storage, if available. Reservoirs, snow, ice, soil moisture, and aquifers all serve as storage in a water budget. In dry regions, the water is critical for sustaining human activities. Understanding and managing the water budget is an ongoing political and social challenge. Hydrologists create groundwater budgets within any designated area, but they are generally made for watershed (basin) boundaries, because groundwater and surface water are easier to account for within these boundaries. Water budgets can be created for state, county, or aquifer extent boundaries as well. The groundwater budget is an essential component of the hydrologic model; hydrologists use measured data with a conceptual workflow of the model to better understand the water system. Take this quiz to check your comprehension of this section. If you are using an offline version of this text, access the quiz for section 11.2 via the QR code. An interactive H5P element has been excluded from this version of the text. You can view it online here: https://pressbooks.lib.vt.edu/introearthscience/?p=765#h5p-75 310 | WATER 11.3 Water Use and Distribution Figure 11.7: Agricultural water use in the United States by state. In the United States, 1,344 billionliters (355 billion gallons) of ground and surface water are used each day, of which 288 billionliters (76 billion gallons) are fresh groundwater. The state of California uses 16% of national groundwater. Utah is the second driest state in the United States. Nevada, having a mean statewide precipitation of 31 cm (12.2 inches) per year, is the driest. Utah also has the second highest per capita rate of total domestic water use of 632.16 liters (167 gallons) per person per day. With the combination of relatively high demand and limited quantity, Utah is at risk for water budget deficits.  WATER | 311 Figure 11.8: Trends in water use by source. 11.3.1 Surface Water Distribution Fresh water is a precious resource and should not be taken for granted, especially in dry climates. Surface water makes up only 1.2% of the fresh water available on the planet, and 69% of that surface water is trapped in ground ice and permafrost. Stream water accounts for only 0.006% of all freshwater and lakes contain only 0.26% of the world’s fresh water. Global circulation patterns are the most important factor in distributing surface water through precipitation. Due to the Coriolis effect and the uneven heating of the Earth, air rises near the equator and near latitudes 60° north and south. Air sinks at the poles and latitudes 30° north and south (see chapter 13). Land masses near rising air are more prone to humid and wet climates. Land masses near sinking air, which inhibits precipitation, are prone to dry conditions. Prevailing winds, ocean circulation patterns such as the Gulf Stream’s effects on eastern North America, rain shadows (the dry leeward sides of mountains), and even the proximity of bodies of water can affect local climate patterns. When this moist air collides with the nearby mountains causing it to rise and cool, the moisture may fall out as snow or rain on nearby areas in a phenome­ non known as “lake-effect precipitation.” 312 | WATER Figure 11.9: Distribution of precipitation in the United States. The 100th Meridian is approximately where the average precipitation transitions from relatively wet to dry. (Source: U.S. Geological Survey) In the United States, the 100th meridian roughly marks the boundary between the humid and arid parts of the country. Growing crops west of the 100th meridian requires irrigation. In the west, surface water is stored in reservoirs and mountain snowpacks, then strategically released through a system of canals during times of high water use. Some of the driest parts of the western United States are in the Basin and Range Province. The Basin and Range has mul­ tiple mountain ranges that are oriented north to south. Most of the basin valleys in the Basin and Range are dry, receiving less than 30 cm (12 inches) of precipitation per year. However, some of the mountain ranges can receive more than 1.52 m (60 inches) of water as snow or snow-water-equivalent. The snow-water equivalent is the amount of water that would result if the snow were melted, as the snowpack is generally much thicker than the equivalent amount of water that it would produce.  WATER | 313 11.3.2 Groundwater Distribution Water source Water volume (cubic miles) Fresh water (%) Total water (%) Oceans, seas, and bays 321,000,000 — 96.5% Ica caps, glaciers, and permanent snow 5,773,000 68.7% 1.74% Groundwater (total) 5,614,000 — 1.69% Groundwater (fresh) 2,526,000 30.1% 0.76% Groundwater (saline) 3,088,000 — 0.93% Soil moisture 3,959 0.05% 0.001% Ground ice and permafrost 71,970 0.86% 0.022% Lakes (total) 42,320 — 0.013% Lakes (fresh) 21,830 0.26% 0.007% Lakes (saline) 20,490 — 0.006% Atmosphere 3,095 0.04% 0.001% Swamp water 2,752 0.03% 0.0008% Rivers 509 0.006% 0.0002% Biological water 269 0.003% 0.0001% Table 11.1: Groundwater distribution. Source: Igor Shiklomanov's chapter "World fresh water resources" in Peter H. Gleick (editor), 1993, Water in Crisis: A Guide to the World's Fresh Water Resources (Oxford University Press, New York). Groundwater makes up 30.1% of the fresh water on the planet, making it the most abundant reservoir of fresh water acces­ sible to most humans. The majority of freshwater, 68.7%, is stored in glaciers and ice caps as ice. As the glaciers and ice caps melt due to global warming, this fresh water is lost as it flows into the oceans. Take this quiz to check your comprehension of this section. If you are using an offline version of this text, access the quiz for section 11.3 via the QR code. An interactive H5P element has been excluded from this version of the text. You can view it online here: https://pressbooks.lib.vt.edu/introearthscience/?p=765#h5p-76 11.4 Water Law Federal and state governments have put laws in place to ensure the fair and equitable use of water. In the United States, the states are tasked with creating a fair and legal system for sharing water. 314 | WATER 11.4.1 Water Rights Because of the limited supply of water, especially in the western United States, states disperse a system of legal water rights defined as a claim to a portion or all of a water source, such as a spring, stream, well, or lake. Federal law mandates that states control water rights, with the special exception of federally reserved water rights, such as those associated with national parks and Native American tribes, and navigation servitude that maintains navigable water bodies. Each state in the United States has a different way to disperse and manage water rights. A person, entity, company, or organization, must have a water right to legally extract or use surface or groundwater in their state. Water rights in some western states are dictated by the concept of prior appropriation, or “first in time, first in right,” where the person with the oldest water right gets priority water use during times when there is not enough water to fulfill every water right. The Colorado River and its tributaries pass through a desert region, including seven states (Wyoming, Colorado, Utah, New Mexico, Arizona, Nevada, California), Native American reservations, and Mexico. As the western United States became more populated and while California was becoming a key agricultural producer, the states along the Colorado River real­ ized that the river was important to sustaining life in the West. To guarantee certain perceived water rights, these western states recognized that a water budget was necessary for the Colorado River Basin. Thus was enacted the Colorado River Compact in 1922 to ensure that each state got a fair share of the river water. The Compact granted each state a specific volume of water based on the total measured flow at the time. However, in 1922, the flow of the river was higher than its long-term average flow, consequently, more water was allocated to each state than is typically available in the river. Over the next several decades, lawmakers have made many other agreements and modifications regarding the Colorado River Compact, including those agreements that brought about the Hoover Dam (formerly Boulder Dam), and Glen Canyon Dam,and a treaty between the American and Mexican governments. Collectively, the agreements are referred to as “The Law of the River” by the United States Bureau of Reclamation. Despite adjustments to the Colorado River Compact, many believe that the Colorado River is still over-allocated, as the Colorado River flow no longer reaches the Pacific Ocean, its original terminus (base level). Dams along the Colorado River have caused water to divert and evaporate, creating serious water budget concerns in the Colorado River Basin. Predicted drought associated with global warm­ ing is causing additional concerns about over-allocating the Colorado River flow in the future. The Law of the River highlights the complex and prolonged nature of interstate water rights agreements, as well as the importance of water. One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=MZrKW-Q9X8E Video 11.2: The Colorado River Compact of 1992. If you are using an offline version of this text, access this YouTube video via the QR code.  WATER | 315 The Snake Valley straddles the border of Utah and Nevada withmore of the irrigable land area lying on the Utah side of the border.In 1989, the Southern Nevada Water Authority (SNWA) submitted applications for water rights to pipe up to 191,189,707 cu m (155,000 ac-ft) of water per year (an acre-foot of water is one acre covered with water one foot deep) from Spring, Snake, Delamar, Dry Lake, and Cave valleys to southern Nevada, mostly for Las Vegas. Nevada and Utah have attempted a comprehensive agreement, but negotiations have not yet been settled. Complete this interactive activity to check your understanding. If you are using an offline version of this text, access this interactive activity via the QR code, or by visiting https://www.arcgis.com/apps/MapJournal/index.html?appid=79199afd183e459596e6e21315159354. NPR story on Snake Valley One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.npr.org/player/embed/10953190/10956967# If you are using an offline version of this text, access this NPR story via the QR code, or by visiting https://www.npr.org/player/embed/10953190/10956967#. SNWA History Dean Baker Story One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=eCZ8KLrmUXo Video 11.3: Transporting Snake Valley water to satisfy a thirsty Las Vegas. If you are using an offline version of this text, access this YouTube video via the QR code. 316 | WATER 11.4.2 Water Quality and Protection Two major federal laws that protect water quality in the United States are the Clean Water Act and the Safe Drinking Water Act. The Clean Water Act, an amendment of the Federal Water Pollution Control Act, protects navigable waters from dumping and point-source pollution. The Safe Drinking Water Act ensures that water that is provided by public water sup­ pliers, like cities and towns, is safe to drink. The U.S. Environmental Protection Agency Superfund program ensures the cleanup of hazardous contamination, and can be applied to situations of surface water and groundwater contamination. It is part of the Comprehensive Environmen­ tal Response, Compensation, and Liability Act of 1980. Under this act, state governments and the U.S. Environmental Pro­ tection Agency can use the superfund to pay for remediation of a contaminated site and then file a lawsuit against the polluter to recoup the costs. Or to avoid being sued, the polluter that caused the contamination may take direct action or provide funds to remediatethe contamination. Take this quiz to check your comprehension of this section. If you are using an offline version of this text, access the quiz for section 11.4 via the QR code. An interactive H5P element has been excluded from this version of the text. You can view it online here: https://pressbooks.lib.vt.edu/introearthscience/?p=765#h5p-77 11.5 Surface Water Geologically, a stream is a body of flowing surface water confined to a channel. Terms such as river, creek and brook are social terms not used in geology. Streams erode and transport sediments, making them the most important agents of the earth’s surface, along with wave action (see chapter 12) in eroding and transporting sediments. They create much of the surface topography and are an important water resource. Several factors cause streams to erode and transport sediment, but the two main factors are stream–channel gradient and velocity. Stream–channel gradient is the slope of thestream usually expressed in meters per kilometer or feet per mile. A steeper channel gradient promotes erosion. When tectonic forces elevate a mountain, the stream gradi­ ent increases, causing themountain stream to erode downward and deepen its channel eventually forming a val­ ley. Stream–channel velocity is the speed at which channel water flows. Factors affecting channel velocityinclude channel gradient which decreases downstream, discharge and channel size which increaseas tributaries coalesce, and channel roughness which decreases as sediment lining the channel walls decreases in size thus reducing friction. The combined effect of these factors is that channel velocity actually increases from mountain brooks to the mouth of the stream. 11.5.1 Discharge Stream size is measured in terms of discharge, the volume of water flowing past a point in the stream over a defined time interval. Volume is commonly measured in cubic units (length x width x depth), shown as feet3 (ft3) or meter3 (m3). There- fore, the units of discharge are cubic feet per second (ft3/sec or cfs). Therefore, the units of discharge are cubic meters per second, (m³/s or cms, or cubic feet per second (ft³/sec or cfs). Stream discharge increases downstream. Smaller streams  WATER | 317 have less discharge than larger streams. For example, the Mississippi River is the largest river in North America, with an average flow of about 16,990.11 cms (600,000 cfs). For comparison, the average discharge of the Jordan River at Utah Lake is about 16.25 cms (574 cfs) and for the annual discharge of the Amazon River, (the world’s largest river), annual dis­ charge is about 175,565 cms (6,200,000 cfs). Discharge can be expressed by the following equation: Q=VA Q = discharge cms (or ft3/sec), A = cross-sectional area of the stream channel [width times average depth] as m2 (or in2 or ft2), V = average channel velocity m/s (or ft/sec) At a given location along the stream, velocity varies with stream width, shape, and depth within the stream channel as well. When the stream channel narrows but discharge remains constant, the same volume of water must flows through a narrower space causing the velocity to increase, similar to putting a thumb over the end of a backyard water hose. In addition, during rain storms or heavy snow melt, runoff increases, which increases stream dis­ charge and velocity. When the stream channel curves, the highest velocity will be on the outside of the bend. When the stream channel is straight and uniformly deep, the highest velocity is in the channel center at the top of the water where it is the farthest from frictional contact with the stream channel bottom and sides. In hydrology, the thalweg of a river is the line drawn that shows its natural progression and deepest channel, as is shown in the diagram. Figure 11.10: Thalweg of a river. In a river bend, the fastest moving water is on the outside of the bend, near the cutbank. Stream velocity is higher on the outside bend and the water surface which is farthest from the friction of the stream bed. Longer arrows indicate faster velocity (Earle 2015). 11.5.2 Runoff versus Infiltration Factors that dictate whether water will infiltrate into the ground or run off over the land include the amount, type, and intensity of precipitation; the type and amount of vegetation cover; the slope of the land; the temperature and aspect of the land; preexisting conditions; and the type of soil in the infiltrated area. High– intensity rain will cause more runoff than the same amount of rain spread out over a longer duration. If the rain falls faster than the soil’s properties allow it to infil­ trate, then the water that cannot infiltrate becomes runoff. Dense vegetation can increase infiltration, as the vegetative cover slows the water particle’s overland flow giving them more time to infiltrate. If a parcel of land has more direct solar radiation or higher seasonal temperatures, there will be less infiltration and runoff, as evapotranspiration rates will be higher. As the land’s slope increases, so does runoff, because the water is more inclined to move downslope than infil­ trate into the ground. Extreme examples are a basin and a cliff, where water infiltrates much quicker into a basin than a cliff that has the same soil properties. Because saturated soil does not have the capacity to take more water, runoff is gen­ erally greater over saturated soil. Clay-rich soil cannot accept infiltration as quickly as gravel-rich soil. 318 | WATER 11.5.3 Drainage Patterns The pattern of tributaries within a region is called drainage pattern. They depend largely on the type of rock beneath, and on structures within that rock (such as folds and faults). The main types of drainage patterns are dendritic, trellis, rec­ tangular, radial, and deranged. Dendritic patterns are the most common and develop in areas where the underlying rock or sediments are uniform in character, mostly flat lying, and can be eroded equally easily in all directions. Examples are alluvial sediments or flat lying sedimentary rocks. Trellis patterns typically develop where sedimentary rocks have been folded or tilted and then eroded to varying degrees depending on their strength. The Appalachian Mountains in eastern United States have many good examples of trellis drainage. Rectangular patterns develop in areas that have very little topography and a system of bedding planes, joints, or faults that form a rectangular network. A radial pattern forms when streams flow away from a central high point such as a mountain top or volcano, with the individual streams typically having dendritic drainage patterns. In places with extensive limestone deposits, streams can disappear into the ground­ water via caves and subterranean drainage and this creates a deranged pattern. Figure 11.11: Various stream drainage patterns. 11.5.4 Fluvial Processes Fluvial processes dictate how a stream behaves and include factors controlling fluvial sediment production, transport, and deposition. Fluvial processes include velocity, slope and gradient, erosion, transportation, deposition, stream equi­ librium, and base level. Streams can be divided into three main zones: the many smaller tributaries in the source area, the main trunk stream in the floodplain and the distributaries at the mouth of the stream. Major stream systems like the Mississippi are composed of many source areas, many tributaries and trunk streams, all coalescing into the one main stream draining the region. The zones of a stream are defined as 1) the zone of sediment production (erosion), 2) the zone of transport, and 3) the zone of deposition. The zone of sediment production is located in the headwaters of the stream. In the zone of sediment trans- port, there is a general balance between erosion of the finer sediment in its channel and transport of sediment across the floodplain. Streams eventually flow into the ocean or end in quiet water with a delta which is a zone of sediment deposi­ tion located at the mouth of a stream. The longitudinal profile of a stream is a plot of the elevation of the stream channel at all points along its course and illustrates the location of the three zones. Zone of Sediment Production The zone of sediment production is located in the headwaters of a stream where rills and gullies erode sediment and contribute to larger tributary streams. These tributaries carry sediment and water further downstream to the main trunk of the stream. Tributaries at the headwaters have the steepest gradient; erosion there produces considerable sediment carried b the stream. Headwater streams tend to be narrow and straight with small or non-existent floodplains adjacent to the channel. Since the zone of sediment production is generally the steepest part of the stream, headwaters are gen­ erally located in relatively high elevations. The Rocky Mountains of Wyoming and Colorado west of the Continental Divide contain much of the headwaters for the Colorado River which then flows from Colorado through Utah and Arizona to Mex­ ico. Headwaters of the Mississippi river system lie east of the Continental Divide in the Rocky Mountains and west of the Appalachian Divide.  WATER | 319 Zone of Sediment Transport Streams transport sediment great distances from the headwaters to the ocean, the ultimate depositional basins. Sediment transportation is directly related to stream gra­ dient and velocity. Faster and steeper streams can trans­ port larger sediment grains. When velocity slows down, larger sediments settle to the channel bottom. When the velocity increases, those larger sediments are entrained and move again. Transported sediments are grouped into bedload, sus­ pended load, and dissolved load as illustrated in the above image. Sediments moved along the channel bot- Figure 11.12: A stream carries dissolved load, suspended load, and tom are the bedload that typically consists of the largest bedload. and densest particles. Bedload is moved by saltation (bouncing) and traction (being pushed or rolled along by the force of the flow). Smaller particles are picked up by flowing water and carried in suspension as suspended load. The particle size that is carried in suspended and bedload depends on the flow velocity of the stream. Dissolved load in a stream is the total of the ions in solution from chemical weather­ ing, including such common ions such as bicarbonate (-HCO3–), calcium (Ca+2), chloride (Cl-1), potassium (K+1), and sodium (Na+1). The amounts of these ions are not affected by flow velocity. One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=is-qcxrKKBI Video 11.4: Bed load sediment transport. If you are using an offline version of this text, access this YouTube video via the QR code. 320 | WATER A floodplain is the flat area of land adjacent to a stream channel inundated with flood water on a regular basis. Stream flooding is a natural process that adds sediment to floodplains. A stream typically reaches its greatest velocity when it is close to flooding, known as the bankfull stage. As soon as the flooding stream overtops its banks and flows onto its floodplain, the velocity decreases. Sediment that was being carried by the swiftly moving water is deposited at the edge of the channel, forming a low ridge or natural levée. In addition, sediments are added to the floodplain during this flooding process contributing to fertile soils. Zone of Sediment Deposition Deposition occurs when bedload and suspended load come to rest on the bottom of the stream channel, lake, or ocean due to decrease in stream gradient and reduction in velocity. While both deposition and erosion occur in the zone of transport such as on point bars and Figure 11.13: Profile of stream channel at bankfull stage, cut banks, ultimate deposition where the stream reaches a lake or flood stage, and deposition of natural levee. ocean. Landforms called deltas form where the stream enters quiet water composed of the finest sediment such as fine sand, silt, and clay. Equilibrium and Base Level All three stream zones are present in the typical longitudi­ nal profile of a stream which plots the elevation of the channel at all points along its course (see figure 11.14). All streams have a long profile. The long profile shows the stream gradient from headwater to mouth. All streams attempt to achieve an energetic balance among erosion, transport, gradient, velocity, discharge, and channel char- acteristics along the stream’s profile. This balance is called equilibrium, a state called grade. Another factor influencing equilibrium is base level, the elevation of the stream‘s mouth representing the lowest level to which a stream can erode. The ultimate base level is, of course, sea-level. A lake or reservoir may also repre- Figure 11.14: Example of a longitudinal profile of a stream; Halfway Creek, Indiana. sent base level for a stream entering it. The Great Basin of western Utah, Nevada, and parts of some surrounding states contains no outlets to the sea and provides internal base levels for streams within it. Base level for a stream enter- ing the ocean changes if sea-level rises or falls. Base level also changes if a natural or human-made dam is added along a stream‘s profile. When base level is lowered, a stream will cut down and deepen its channel. When base level rises, deposition increases as the stream adjusts attempting to establish a new state of equilibrium. A stream that has approxi- mately achieved equilibrium is called a graded stream. 11.5.5 Fluvial Landforms Stream landforms are the land features formed on the surface by either erosion or deposition. The stream-related land­ forms described here are primarily related to channel types.  WATER | 321 Channel Types Stream channels can be straight, braided, meandering, or entrenched. The gradient, sedi­ ment load, discharge, and location of base level all influence channel type. Straight channels are rela- tively straight, located near the headwaters, have steep gradients, low discharge, and narrow V- shaped valleys. Examples of these Figure 11.15: The braided Waimakariri river in are located in mountainous areas. Figure 11.16: Air photo of the meandering river, New Zealand. Río Cauto, Cuba. Braided streams have multiple channels splitting and recombining around numerous mid-channel bars. These are found in floodplains with low gradients in areas with near sources of coarse sediment such as trunk streams draining mountains or in front of glaciers. Meandering streams have a single channel that curves back and forth like a snake within its floodplain where it emerges from its headwaters into the zone of transport. Meandering streams are dynamic creating a wide floodplain by eroding and extending meander loops side-to-side. The highest velocity water is located on the outside of a meander bend. Ero­ sion of the outside of the curve creates a feature called a cut bank and the meander extends its loop wider by this erosion. The thalweg of the stream is the deepest part of the stream channel. In the straight parts of the channel, the thalweg and highest velocity are in the center of the channel. But at the bend of a meandering stream, the thalweg shifts toward the cut bank. Opposite the cutbank on the inside bend of the channel is the lowest stream velocity and is an area of depo­ sition called a point bar. In areas of tectonic uplift such as on the Colorado Plateau, meandering streams that once flowed on the plateau surface have become entrenched or incised as uplift occurred and the stream cut its meander­ ing channel down into bedrock. Over the past several million years, the Colorado River and its tributaries have incised into the flat lying rocks of the Figure 11.17: Point bar and cut bank on the Cirque de la Madeleine in France. plateau by hundreds, even thousands of feet creating deep canyons including the Grand Canyon in Arizona. 322 | WATER Figure 11.18: An entrenched meander on the Colorado River in the eastern entrance to the Grand Canyon. Figure 11.19: Panoramic view of incised meanders of the San Juan River at Gooseneck State Park, Utah. Figure 11.20: The Rincon is an abandoned meaner loop on the entrenched Colorado River in Lake Powell.  WATER | 323 Many fluvial landforms occur on a floodplain associated with a meandering stream. Meander activity and regular flooding contribute to widening the floodplain by eroding adjacent uplands. The stream channels are confined by natural levees that have been built up over many years of regular flooding. Natural levees can isolate and direct flow from tributary channels on the floodplain from immedi- ately reaching the main channel. These isolated streams are called yazoo streams and flow parallel to the main trunk stream until there is an opening in the levee to allow for a belated confluence. Figure 11.21: Landsat image of Zambezi Flood Plain, Namibia. One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=persGpc6-Dw Video 11.5: How is a levee formed? If you are using an offline version of this text, access this YouTube video via the QR code. To limit flooding, humans build artificial levees on flood plains. Sediment that breaches the levees during flood stage is called crevasse splays and delivers silt and clay onto the floodplain. These deposits are rich in nutrients and often make good farm land. When floodwaters crest over human-made levees, the levees quickly erode with potentially catastrophic impacts. Because of the good soils, farmers regularly return after floods and rebuild year after year. Through erosion on the outsides of the meanders and deposition on the insides, the channels of meandering streams move back and forth across their floodplain over time. On very broad floodplains with very low gradients, the meander bends can become so extreme that they cut across themselves at a narrow neck (see figure 11.22) called a cutoff. The former channel becomes isolated and forms an oxbow lake seen on the right of the figure. Eventually the oxbow lake fills in with sediment and becomes a wet- land and eventually a meander scar. Stream meanders can migrate and form oxbow lakes in a relatively short amount of time. Where stream channels form geographic and political boundaries, this shifting of channels can cause conflicts. Figure 11.22: Meander nearing cutoff on the Nowitna River in Alaska. 324 | WATER One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=8a3r-cG8Wic Video 11.6: Why do rivers curve? If you are using an offline version of this text, access this YouTube video via the QR code. Alluvial fans are a depositional landform created where streams emerge from mountain canyons into a valley. The channel that had been confined by the canyon walls is no longer confined, slows down and spreads out, dropping its bedload of all sizes, forming a delta in the air of the valley. As distributary channels fill with sedi­ ment, the stream is diverted laterally, and the alluvial fan devel- ops into a cone shaped landform with distributaries radiating from the canyon mouth. Alluvial fans are common in the dry climates of the West where ephemeral streams emerge from canyons in the ranges of the Basin and Range. Figure 11.23: Alluvial fan in Iraq seen by NASA satellite. A stream emerges from the canyon and creates this cone-shaped deposit. Complete this interactive activity to check your understanding. If you are using an offline version of this text, access this interactive activity via the QR code. An interactive H5P element has been excluded from this version of the text. You can view it online here: https://pressbooks.lib.vt.edu/introearthscience/?p=765#h5p-78  WATER | 325 A delta is formed when a stream reaches a quieter body of water such as a lake or the ocean and the bedload and suspended load is deposited. If wave erosion from the water body is greater than deposition from the river, a delta will not form. The largest and most famous delta in the United States is the Mississippi River delta formed where the Mississippi River flows into the Gulf of Mexico. The Mississippi River drainage basin is the largest in North America, draining 41% of the contiguous United States. Because of the large drainage area, the river carries a large amount of sediment. The Mississippi River is a major shipping route and human engineering has ensured that the channel has been artificially straightened and remains fixed within the floodplain. The river is now 229 km shorter than it was before humans began engineering it. Because of these restraints, the delta is now focused on one trunk channel and has created a “bird’s foot” pattern. The two NASA images below of the delta show how the shoreline has retreated and land was inundated Figure 11.24: Location of the Mississippi River drainage basin with water while deposition of sediment was focused at end of the and Mississippi River delta. distributaries. These images have changed over a 25 year period from 1976 to 2001. These are stark changes illustrating sea-level rise and land subsidence from the compaction of peat due to the lack of sediment resupply. Complete this interactive activity to check your understanding. If you are using an offline version of this text, access this interactive activity via the QR code. An interactive H5P element has been excluded from this version of the text. You can view it online here: https://pressbooks.lib.vt.edu/introearthscience/?p=765#h5p-79 The formation of the Mississippi River delta started about 7500 years ago when postglacial sea level stopped rising. In the past 7000 years, prior to anthropogenic modifications, the Mississippi River delta formed several sequential lobes. The river abandoned each lobe for a more preferred route to the Gulf of Mexico. These delta lobes were reworked by the ocean waves of the Gulf of Mexico. After each lobe was abandoned by the river, isostatic depression and compaction of the sediments caused basin subsidence and the land to sink. 326 | WATER A clear example of how deltas form came from an earth­ quake. During the 1959 Madison Canyon 7.5 magnitude earthquake in Montana, a large landslide dammed the Madison River forming Quake Lake still there today. A small tributary stream that once flowed into the Madison River, now flows into Quake Lake forming a delta composed of coarse sediment actively eroded from the mountainous upthrown block to the north. Deltas can be further categorized as wave-dominated or tide-dominated. Wave-dominated deltas occur where the tides are small and wave energy dominates. An example is the Nile River delta in the Mediterranean Sea that has the classic shape of the Greek character (Δ) from which the landform is named. A tide-dominated delta forms when ocean tides are powerful and influence the shape of the Figure 11.25: Delta in Quake Lake Montana. Deposition of this delta delta. For example, Ganges-Brahmaputra Delta in the Bay began in 1959, when the Madison river was dammed by the landslide caused by the 7.5 magnitude earthquake. of Bengal (near India and Bangladesh) is the world’s largest delta and mangrove swamp called the Sundarban. At the Sundarban Delta in Bangladesh, tidal forces create linear intru­ sions of seawater into the delta. This delta also holds the world’s largest mangrove swamp. Figure 11.26: Sundarban Delta in Bangladesh, a tide-dominated delta of the Ganges River.  WATER | 327 Figure 11.27: Nile Delta showing its classic “delta” shape. Lake Bonneville was a large, pluvial lake that occupied the western half of Utah and parts of eastern Nevada from about 30,000 to 12,000 years ago. The lake filled to a maximum elevation as great as approximately 5100 feet above mean sea level, filling the basins, leaving the mountains exposed, many as islands. The presence of the lake allowed for deposition of both fine grained lake mud and silt and coarse gravels from the mountains. Variations in lake level were controlled by regional climate and a catastrophic failure of Lake Bonneville’s main outlet, Red Rock Pass. During extended peri­ ods of time in which the lake level remained stable, wave-cut ter­ races were produced that can be seen today on the flanks of many mountains in the region. Significant deltas formed at the mouths of major canyons in Salt Lake, Cache, and other Utah valleys. The Great Salt Lake is the remnant of Lake Bonneville and cities have built up on these delta deposits. Figure 11.28: Map of Lake Bonneville, showing the outline of the Bonneville shoreline, the highest level of the lake. 328 | WATER Figure 11.29: Deltaic deposits of Lake Bonneville near Logan, Utah; wave cut terraces can be seen on the mountain slope. Stream terraces are remnants of older floodplains located above the existing floodplain and river. Like entrenched meanders, stream terraces form when uplift occurs or base level drops and streams erode downward, their meanders widening a new flood plain. Stream terraces can also form from extreme flood events associated with retreating glaciers. A classic example of multiple stream terraces are along the Snake River in Grand Teton National Park in Wyoming. Figure 11.30: Terraces along the Snake River, Wyoming.