Well, hello, again, and welcome. I'm glad you could join us again today for Program 23 on "Running Water."

This, in fact, is the first of two programs on the dynamics and the effects of streams. This one is Lesson 19 on the dynamics of streams. Next time is Lesson 20 on the effects of streams and stream erosion.

You know, rivers are very important for many of man's activities; in fact, we rely upon rivers for commerce, for transportation, for agriculture,for recreation, and even for water supplies, and for electrical energy, and rivers have soothing qualities.

Many poems have been written about streams. They've been the subject of art, and of course, rivers also seem to concentrate and become centers of population, but unfortunately, as we'll see in these two programs, man's activities are not always in harmony with the stream.

The stream tends to do things like flood, and erode, and deposit sediment in places where we don't always think is convenient, so rivers are important geologically, not just in erosion, and transportation, and deposition of sediments, but also in shaping the landscape both by erosion and by deposition. You see, one of the things that we need to understand about steams is that streams and rivers represent dynamic changing systems, always changing in fact. In fact, a given stream is always striving for equilibrium state, which it never quite reaches. There's an equilibrium between input and output of water, for example, through the stream. There's also, the stream is looking for a balance of energy, and also, of course, balancing that energy between the amount of sediment that's available, and the stream's ability to carry that sediment along. A stream system includes not only the water in the stream, but also includes the landscape that the stream flows through as well. It includes the sediments that are in transit in the stream. It includes mountains, and flood plains, and deltas. We often forget this when trying to live with rivers because we concentrate our attention simply on the amount of water in the river at any given time, so in today's program we'll try to understand the dynamics of stream flow, so understanding these processes even at a basic level should increase your appreciation for the complexities and the aesthetics of rivers and streams, and in the next lesson, we'll try to understand the way in which streams and their processes modify the landscape.

Well, as it turns out, there's much more to the dynamics of streams than we can cover even in these two lessons. The studies of streams, in fact, encompass several different subfields of geology. The dynamics themselves can involve sophisticated data gathering as well as mathematical analysis of the amount of water in sediment, so today we'll try to summarize the main features of stream folds and dynamics for you. Let me remind you of the text assignment. This lesson is in Chapter 16 in the text; it's actually the first half of Chapter 16, the title "Steams and Landscapes," so be sure to read the introduction and the summary and examine the photograph facing page 349. In this photograph note the little waterfalls and the turbulence, and the waterworn boulders, and also a little mass wasting on the far side of the stream, so you should read the chapter at least as far as "Valley Development" on page 349 to 368 and pay close attention to all the photographs and diagrams; they illustrate these various features, describe quite well, and there's also quite a few graphs, and lined diagrams, and so forth and look for these features as you drive and fly around Hawaii and elsewhere. The remainder of this chapter, the second half, will be covered in the next program, which will be Lesson 20 and don't forget to follow the study plan and the study guide, and when you've done that, be sure to go back to the learning objectives and make sure you've learned each one. You might want to take a look at those objectives before you actually start reading the chapter, and, again, I won't review the objectives with you this time. Well, there are several things that we need to define or to get a sense of before we watch the video, and I'd like to run through some of those with you.

First of all, what's the difference between a river and a stream? In fact, we use many words to describe streams, and usually these words somehow imply a size. We use words like "rill" or "rivulet," or "creek," or "brook," or "stream," or even "river," but geologically speaking we don't really make a distinction. We call all of these things "streams," and all of them share features in common that we can use to understand streams of various sizes, so when we use the word "stream," we're encompassing all of these possibilities, so a "stream" basically is a narrow body of water flowing in a channel slowly downhill under the influence of gravity. The stream includes all the features that are actively modified by a given stream and its tributaries, and, of course, a "tributary" is also a stream, but it's important to note here that not all flowing water flows in streams all the time; it sometimes flows down slope as sheet wash before it collects into rills and rivulets, and the sheet wash is capable of tremendous erosion, but eventually small streams of all sizes coalesce to form larger streams and eventually to create a tree-like network that we call a "drainage system." The stream includes stream deposits like bars, and deltas, and flood plains. These just happen to be sediments that the stream is not using at a particular moment, and we want to note that we use words to describe the various parts of the stream like the "head" of the stream, for example, is at the upper end, near the source of the stream, and usually in the mountains. The "mouth" of the stream is at the lower end where the water empties into something else. That "something else" might be and ocean, a lake, a valley, or another stream, so I also want to recall the concept of the "hydrologic cycle."

All water on Earth is in involved in this continuous cycle that we call the "hydrologic cycle." You might want to refer back to Figure 1.5 on page 6 early in the textbook. The "hydrologic cycle" basically is a model that we use to understand the movement of water between various reservoirs on Earth's surface. These reservoirs we all know. The ocean is the most abundant reservoir; in fact, about 98 percent of all the water stored on Earth is in the ocean at any given time. The remaining 2 percent is mostly in glaciers; these are glaciers that form icecaps and glaciers that form mountain glaciers; and again, we'll study glaciers in a future lesson. The remainder, much less than 1 percent, is in streams, in lakes, in the groundwater system, and small amounts in soil moisture, and organisms like ourselves, and, by the way, just because these reservoirs are small doesn't mean that they're not important, so the idea of the hydrologic cycle is that water is transferred between these reservoirs by various processes. These various processes include evaporation from the surface; precipitation, rainfall, and snow, and so forth; runoff, which is the stream component; infiltration, which is water that soaks into the ground; and transpiration, which is water which is passed through the biological system in one form or another. Aristotle, the great Greek philosopher, thought that water in streams issued somehow from the center of the Earth through springs, but that doesn't really make sense, and even Aristotle wondered if that's the case, then why don't the oceans eventually fill up and overflow? Measurements of precipitation versus runoff in a given drainage area prove that water doesn't, in fact, come from underground springs. The amount of rainfall in a given area is generally four to six times the amount of runoff for a given drainage area; that means that much more water is falling than is actually running off in the streams. The remainder of this water, that which doesn't flow out of the streams, infiltrates, percolates through the soil and into the bedrock to enter the groundwater system, and I want to note at this point although we'll study this in some detail later on that there is an active transfer between these two systems, between the stream system and the groundwater system. Even desert streams get their water either from rainfall, or streams, or springs in mountain areas. The springs often tap the groundwater system. It's also important to note here that the hydrologic cycle interacts with the tectonic cycles of uplift; in fact, the hydrologic cycle is mostly erosive, and it's largely destructive of tectonically created features. Streams also build structures, but they're only temporary deposit, and we may recall that erosion would have long ago leveled all the land above sea level if it was not for this continued uplift of tectonic processes.

Well, let's look now at profiles of streams. If we want to get a sense of what a stream looks like, we can draw a cross section or a profile, and there are two ways we can do this. One is a longitudal profile where we look at the stream along its length from it's headwaters to its mouth; the other is a cross section where we look at a section across the channel of the stream, so let's look at the longitudinal profile first. When we look at the longitudinal profile of streams of all sizes, we find certain patterns; for example, the gradient is usually highest in the head waters and becomes lower near the mouth. The "gradient" is the steepness of the stream bed, and here in the United States we usually measure this in feet per mile, so this is the drop of water over a certain horizontal distance. We note that the longitudinal profile or the longitudinal gradient usually decreases along the length from the source to the mouth; in other words, the stream has a steeper bed in the upper regions than it does in the lower regions, and we find that the profiles of all streams are somewhat similar. Now, the details of a particular stream may show bumps; in other words, the profile may not be completely smooth, and, in fact, the sizes, and shapes, and gradients, are different, but in all streams we see this general trend for them to be steeper in the headwaters than they are near the mouth. A cross section of the stream we draw perpendicular to the channel at some point, and if we did this for any stream, we'd find that there are many variations in the shape of the channel along the length and also over time. The valley, for example, may be deep, may be narrow, may be "v" shaped, or may be "u" shaped. Generally, the stream is more "v" shaped near the headwaters and more "u" shaped near the mouth. And the stream channel itself may lie in bedrock, or it may be in a flood plain of its own making, and generally near the source of the stream it tends to be in bedrock,and near the mouth it tends to be in the flood plain. Okay, we also talk about a stream in terms of its drainage basin, and basically the drainage basin of the stream is the total area drained by a stream and all its tributaries; in other words, water in a given drainage area that runs off will all eventually make it back into a particular stream that drains that area; in fact, streams can be classified based upon the number of branches or the number of tributaries, and it turns out as you might suspect that there are only a few large streams which empty only into the ocean. Most streams discharge into larger streams. A drainage area is separated from another drainage area by a divide, and, for example, in the United States the Continental Divide separates streams which discharge into the Pacific Ocean from those which discharge into the Atlantic Ocean or the Gulf of Mexico, so within a given drainage area, we find that there are smaller divides which separate the drainage areas of smaller streams from one another. We also note that flooding occurs in a stream when there's more water than can be contained within the stream channel, and this is a natural part of every stream system, and we'll come back and look at the flooding a little bit later in this program or next, so let's look at the speed of the water flowing in the stream.

Basically there are two types of ways in which water can flow in the stream. It can be "laminar " flow, or it can be "turbulent" flow. At slow speeds the water usually flows in what we call "laminar " flow. The word "laminar " simply means "layers." In laminar flow, the flow is very organized and is a relatively steady motion of the water without much mixing of the water, but as the water flows faster, this nice organized pattern breaks up into turbulent flow where we find many eddies, much swirling and mixing of the water with a disorganized pattern of swirls. The water that flows in a fast moving stream typically ranges between maybe 3 and 25 miles per hour. The reason we care about stream velocity is because it's important in the balance between erosion, transportation, and deposition. In general, the higher the stream velocity, the greater the tendency for it to erode, so what is it that controls the speed of the water in a stream? Well, it makes sense that it's controlled by the gradient. It's also controlled by the shape of the stream channel, and also by the amount of water in the stream, which we call the "discharge," so obviously the steeper the gradient, the higher velocity, and as it turns out, the gradient of a stream that exists at a particular time actually represents a balance between erosion and deposition for that stream. The shape and roughness of the channel also affect the speed. Friction with the bottom of the stream bed robs energy from the stream and slows the water, so we generally, for example, find a wider channel and softer rock, and geologists or hydrologists usually talk about something called the "wetted perimeter," which is the total length of the stream bed that's in contact with the water, and then we see that the greater the wetted perimeter, the more friction, so the less velocity that the water would have, all other things being equal, so the discharge, as I mentioned before, represents the total amount of water flowing past a given point, and obviously the more water there is in a stream channel, the faster it will flow in order to get rid of the water. We also note that the velocity of a stream is fastest in the center of the channel because of less frictional contact with the bottom, and it's usually deepest in the fastest part. The water also flows faster on the outside of a bend. Centrifugal force concentrates the water on the outside of the bend, and also the water must flow a greater distance on the outside of the bend in order to make it around the curve. Okay, all rivers carry sediment of some kind. This material is carried in three different ways: That's "dissolve" load, which represents the products of chemical weathering; the "bed" load, which represents material which is bounced and rolled along the stream bottom; and "suspended" load, which represents material which is carried along in suspension by the material in the stream. I might note here that the dissolved load of a stream averages about a hundred parts per million of dissolved material world around. That seems like not very much, and it isn't in fact, if you compare with about 35,000 parts per million for seawater, so the water in streams is not pure water, but it's nowhere near as salty as water in the ocean, but even at these small concentrations, it's estimated that there are about 2.5 times 10th to the 15th grams of dissolved material added to the oceans each year, and not only that, but there are about 9 times 10 to the 16th grams of detrital sediment added to the oceans each year. I might note that this dissolved material is also responsible for the chemical precipitates in sedimentary rocks. Okay, the suspend load, by contrast, is that material which is small enough to be lifted and carried by the turbulence of the water. These are usually silt and clay sized particles that are suspended throughout the water, and it gives water a muddy appearance. The bed load is kind of interesting, too, because you see the larger material; that's material that's too big to be kept in suspension, may roll, or slide, or bounce along the stream bed. Rolling and sliding is called "traction," and these are usually gravel or boulder size particles. The bouncing part is called "saltation." It comes from the Latin word that means "to bounce." Medium sized grains like sand might be lifted by turbulence of the water, but since the turbulence is irregular and not constant, eventually the material settles back to the bottom, and in the process has been moved somewhat downstream. Material can move quite a distance this way in a series of small jumps. Okay, we also want to distinguish now between two words that are used in relation to the amount of sediment that can be carried. These two words are "capacity" and "competence." Stream "capacity" refers to the total load or total amount of material the stream can carry; that is material of all different sizes. The "capacity" obviously increases as the discharge increases. The capacity is related to the total amount of water. "Competence," on the other hand, refers to the largest size of particle that a stream can move, and the competence also increases with velocity. This has to do with the fact that different materials of different sizes settle through water at a different rate, so the higher the velocity of the water, the more likely it is that it can lift larger particles. You might want to note Box 16.2 on page 359 for a graph which shows this relationship, and from a graph of this type, you can see that a sand sized particles of about 3 tenths of a millimeter, for example, would be deposited if a current speed is less than about 2 centimeters per second, but it would be eroded if the current speed was more than 20 centimeters per second.This is an important diagram because it explains the relationship between erosion, transportation, and deposition of a stream particle. Note, by the way, that higher current speeds are required to erode material than to transport it; In other words, it takes more energy to lift something off of the stream bed than it does to carry it once the material is in suspension. A large part of this is because of the cohesion of small particles like clay particles.

Okay, so we note here that the velocity of a stream may vary longitudinally, that is, along the course of the stream, and also over time. This is responsible then for the sorting of sediments; in other words, a given stream velocity can and will move smaller particles, leaving larger particles behind, so we might also note here that the force exerted by the water changes with the square of the current speed, so doubling the velocity results in four times the force being exerted, and that means that the particle size that can be carried by doubling the velocity also increases by four times. From this we see that very fast flowing streams can move boulders the size of cars, and, in fact, in mountain streams we often find huge boulders littering the stream.

Okay, we might also note that both capacity and competence may increase by a significant factor during flooding; in fact, a single flood may move as much material as 50 or even a hundred years of normal stream flow. Well, with this background, I think we have enough that we can relate to the video, so let's watch the video. Major funding for "Earth Revealed" was provided by the Annenberg C.P.B. Project. This ancient meandering river provided sustenance for one of the earliest civilizations on Earth. The culture which thrived along its fertile valleys forever changed Western Civilization. Egypt, said Robidus, is the gift of the Nile. Puzzled by the source of all this water, many early philosophers theorized that the waters of the Nile, as well as all other rivers, originated from a system of boundless underground fountains. Then, in the Seventeenth Century, French scientist, Pierre Pereau, conducted a simple experiment that would yield a startling discovery. Pereau reasoned that rivers transport snow and rain from the land to the ocean. To test this hypothesis, he compared rainfall with the flow or discharge of a river. He first measured the amount of water flowing annually in the River Seine in France. Then, he calculated rainfall for the upstream drainage basin surrounding the river. Pereau found, to his surprise, that rainfall was six times as large as the flow of the river, so early speculation about rivers actually addressed the wrong question. The problem was not where does river water come from, but where does all the excess rainfall go? Only about one quarter of Earth's annual precipitation flows in rivers. The rest seeps into ground, becomes groundwater, or stored as glacial ice, or soil moisture, or is returned to the atmosphere by evaporation, and growing plants. Rivers are among the most common land forms on Earth. Although they appear to vary a great deal in their behavior and characteristics, careful study has shown that all rivers actually have a great deal in common. The impact of rivers on the landscape is often spectacular. They can gouge out deep canyons, create gentle valleys with verdant meadows, or build enormous deltas. In creating these diverse landscapes, all rivers function in the same manner. They erode, transport, and deposit sediment. These processes enable rivers to continuously reshape the surrounding land. One of the most important factors influencing the geologic impact of a river is the velocity of its water. A swiftly flowing river erodes and transports more sediment than a slow river. Velocity generally increases with the slope of the river, but channel shape also plays a role. If a channel has a nearly perfect semicircular cross section, the frictional resistance is minimal, so the water loses very little energy flowing over the channel. If it's a wide, flat channel, it's a fairly shallow river, there's a greater surface area along the banks and the bottom, and that tends to slow the stream down, too. The texture of a stream bed also influences stream velocity. Roughness is a function of the materials over which it flows, so if it's flowing over gravel and boulders, there'll be more resistance to the flow, and that will slow the river down. If it's flowing over muds and clays like along the lower part of the Mississippi River, there will be less resistance, and it will flow a little faster. The velocity of a river also tends to increase if the amount of water in the river channel increases.

The quantity of water moving through a river is called its "discharge." The discharge of a river is how much water it's actually carrying. We usually measure this as a volume per unit time. In the United States we commonly say cubic feet per second. Most of the rest of the world would talk about the number of cubic meters per second that are moving down the channel. Discharge increases from the head of the stream to the mouth of the stream as the drainage basin increases in size. There's simply a larger area to contribute discharge, contribute flow to the streams. The primary way that a river functions geologically is to transport not just water, but sediment down slope and toward the ocean. The faster a river flows, the more efficient this process becomes, so geologists are acutely interested in flow velocity. When the flow velocity of a stream is relatively high, the energy of the moving water is converted into processes that lift chunks of bedrock or sedimentary particles from the bottom and carry them downstream. This is known as erosion, and there are three different erosional processes that operate in rivers. The first of these is "hydraulic action." The turbulence of a rapidly flowing stream applies vertical forces that can lift sedimentary grains off of the bottom. The flowing current also pushes against these particles and carries them downstream. If you've ever waded across a river and felt the sandy bottom moving beneath your feet, you've experienced hydraulic action first hand. The faster the river flows, the greater the turbulence, and the swirling flow of a very rapid stream can even wrench chunks of fractured bedrock off of the channel bottom. The rapid current of a sediment laden river can also generate a sandblasting effect, which can scour its way down through sediment or even solid rock. In this process called "abrasion" the energy of the moving water is converted into collisions between sedimentary grains and the bedrock at the channel bottom. Abrasion not only smoothes river cobbles into rounded shapes, it can also wear away the bedrock many times faster than hydraulic action alone. Running water can also to some degree dissolve any type of rock or mineral. This process of erosion called "dissolution" is controlled in part by the mineral composition of the bedrock. For example, a riverbed made of limestone will dissolve more rapidly than one made of granite. The rate of erosion by dissolution is also controlled by temperature, the acidity of the water, and, of course, by flow velocity. Erosion in its various forms is only one of the ways that rivers interact with the sediment and bedrock of the Earth's crust. Once this material is picked up and put into motion, it becomes part of the river's flow and is transported downstream by one of several processes of sediment transport. The shape, size, and composition of sediment influence how the sediment will be carried along in the stream and where it will be deposited. Larger particles stay near the bed of the stream and are transported by rolling, or bouncing, or skidding along the bottom. This is called "bed load." When material is moved as bed load in a stream, exactly how it moves is largely a function of size. Larger particles, things which are gravel, cobble, or boulder size stay in contact with the bed virtually all of the time except in extreme discharge events where the velocities are very high. These particles move by rolling, or by being pushed, or by sliding along the bottom. This is the traction load of the stream, the part that is continuously in contact with the bed. Smaller particles in transport as bed load, sand grains, for example, stay close to the bed of the stream but aren't in contact continuously. These particles actually move along in a series of jumps, hopping up into the flow, being pulled forward by the discharge, hitting the bottom, sometimes bouncing up themselves again, or injecting another particle from the bed of the stream, which jumps up into the flow. This style of bed load transport is called "saltation." Although considerable amounts of sediment are transported as bed load, most of the stream's sediment is typically carried in suspension and in solution. Suspended load includes material like silts or clays. It is light enough to be swept along in the current without touching bottom. Dissolved load is invisible. It is the ever-present soluble material which results from chemical weathering of the rocks along the channel. Because precipitation varies seasonally, as well as from year to year, the discharge and velocity of a stream also fluctuates. As the river slows down, the turbulence of the moving water begins to subside, and the amount of energy available to erode and transport sediment, decreases abruptly. Much of the sediment can no longer remain in motion and is deposited instead.

The sediment is usually deposited in the river channel itself in a series of piles called "bars." Most river bars are ridges made of sand and gravel that are covered with small migrating ripples; in fact, the bars themselves are actually large ripples that migrate downstream during sporadic cycles of erosion and deposition. Bars are especially common in graded streams which form where sediment choked rivers flow across broad easily eroded slopes. Bars also commonly occur in meandering rivers. A meandering river typically wanders across wide valleys and lowlands in a series of "s" shaped curves. Erosion and deposition occur continuously side by side along the banks of meandering rivers. Low velocity on the inside of a meandered curve results in the deposition of point bars. On the outside of a curve where velocity is high and erosion normally takes place, cut banks form. Because of this erosion and deposition, both the sizes and positions of meanders continuously change. We're not altogether clear why meandering occurs, why it's such a common phenomenon. It probably has to do with the equalization of energy distribution as the flow moves down valley. By meandering the amount of work done by a stream in some unit of discharge is more or less constant, and that seems to be a principle of nature to try to equalize the amount of work and minimize the amount of work at the same time it's being done. The Mississippi is a prime example of a meandering river. As the crow flies, the distance between New Orleans and Memphis is about 550 kilometers. By boat, it's over 1,000. Meandering rivers are associated with one of the world's most significant geological hazards: flooding. Floods are absolutely a naturally part of the river's cycle, and, in fact, flooding, that is to say, overbank discharge, is common enough that it shouldn't surprise anyone. Geologists have looked at this pretty carefully over the last few years. We have records which indicate that most streams overtop their banks about every two and a half years; that's nothing unusual at all. Anyone who is surprised by flooding are the ones who are not paying attention. Shifting meanders and repeated flooding along rivers produce broad flatlands called "flood plains." When a river goes into flood, the water level in the river channel rises until water spills over the riverbank, rounding the adjacent landscape and giving the floodplain its name. Human population centers have historically been closely linked to the flood plains of major rivers like the Tigress and Euphrates in Ancient Mesopotamia, the Yangtze and Huang Ho in China, and the Nile in Egypt. Flood plains are good places to grow crops because as each flood inundates the plain, it carries with it a muddy sediment rich in organic matter and nutrients. The sediment is deposited in flat layers atop the flood plain and is naturally irrigated by the floodwaters, but life on the flood plain is a double edged sword. The agriculture benefits of these periodic floods are offset by damage to homes and cities, and in some cases, to the people who inhabit them. The edges of flood plains are marked by levees, ridges of sediment left atop riverbanks by floods. Once formed, levees serve as natural barriers confining rivers during periods of ordinary flow. They may even protect low lying areas from flooding if the level of a river isn't too high. For this reason, artificial levees designed to contain a river during flood stages are often built, but artificial levees can themselves create problems. By confining the river to a narrow channel, levees may also confine sediment, raising the riverbed higher and higher, and levees can provide a false sense of security. If a river overtops its levees to flood the surrounding land, the levees can actually prolong flooding by preventing water from draining back into the river. Most people don't appreciate the fact flood plain is very much a part of the stream itself, not something separate from it. The flood plain is the place where rivers store discharge during periods of high flows and also places where rivers store sediment during periods of low flows. When we move on to the flood plain, we're moving on to the river, and it really isn't very much different from being in the channel itself. It's just that the river doesn't use it all of the time. One way to reduce floods is by constructing dams through which a river's discharge can be regulated, but while dams solve some problems, they can create others. All manmade structures in river valleys have an effect upon the stream, the most profound effect being caused by dams. A dam essential creates an artificial base level. Sea level for all practical examples that causes the stream to deposit all of the load that it's been moving. The quiet water of the lake doesn't allow that sediment to continue to move, so it's dumped at the upper end of the reservoir. The water which comes through the spillways of the dam is now without the sediment that has been transported and will go about eroding new sediment to replace that which has been lost. A river replaces this sediment by eroding the river channel downstream from the dam. Sometimes this erosion can be severe.

The basis of river dynamics is a state of balance between erosion, transportation, and deposition. This is what every body of running water naturally seeks from its headwaters to its mouth. Water literally has the power to move mountains in its quest for equilibrium. The stream will always try to exist in a state of equilibrium between the load it's carrying and the discharge that it has. If the load decreases, the stream has excess energy which will usually be used to erode the bed and banks. If the load increases, the stream will not be able to handle it all, and so some of it will be deposited. One place where human activity has come into conflict with a great river seeking to maintain its equilibrium is the Mississippi. Stretching almost 4,000 kilometers, the Mississippi drains approximately 42 percent of the United States. It is a sediment laden river, shifting an estimated 516 million tons per year from its headwaters in Minnesota all the way to the Gulf of Mexico. Along the great length of this River, the process of deposition sometimes causes serious problems. If bars build up in important areas of navigation, they can disrupt shipping and regional commerce. In the industrial corridor between New Orleans and Baton Rouge lies one of North America's most important navigational routes. In order to keep the River open to the many ocean- going vessels which use it year round,the United States Army Corps of Engineers must continually grapple with the forces of nature. One frequent trouble spot lies just south of Baton Rouge in a stretch of the River called "Red Eye Crossing." Here the River tends to deposit sediment, threatening to close the channel to deep water ships. A detailed study of Red Eye Crossing is currently underway at the Army Corps Waterways Experiment Station or "WES" in Vicksburg, Mississippi. Tom Bickrefke is chief of the River engineering branch and heads the Red Eye investigation. The problem that we're studying on Red Eye Crossing is the Crossing itself, when you go from a low water situation to a high water situation, tends to fill with sediment where you go from high water to low water. There's just not enough energy in the water to clear that Crossing out and maintain the channel deep enough for ship type navigation in that part of the River. Basically the Red Eye Crossing area's been kept open in the past using dredging. When the Channel fills during going from a low water situation to a high water situation, the Corps of Engineers went out and dredged the Channel to make sure it was deep enough. The Army Corps of Engineers would like to minimize the amount of dredging necessary to keep the Channel clear. At the core of their study is a scale model of the River. Graded crushed coal is used to represent the bed material. By studying how the coal moves as water is discharged through the model, the Core's engineers hope to better understand the Red Eye Crossing problem and come up with solutions. Each experiment with the model has a known fixed amount of water discharge. Water fills the channel and is allowed to run for a measured length of time. Almost immediately the bed load begins to move. Confetti thrown on the water surface during the experiment clearly indicates the water's flow pattern through the Crossing. The white beads are another indicator of how the model is performing. The beads build up where expected, at the place where in nature itself, the point bar exists. This indicates the model simulates nature accurately. Periodically, the model is drained and its sediment is carefully mapped. This detailed mapping gives engineers a better understanding of sedimentation processes in the River. Using data from the moveable bed model and from the field, WES engineers have created a computer model which calculates the movement of sediment through Red Eye Crossing. It mimics the sediment's behavior through time as water discharge and velocities change. The light blue area is deep water; the dark blue, shallow water, which corresponds to sediment buildup. As the model goes from low discharge levels to high discharge and back to low again, the dark blue area grows in size indicating that sediment is moving in causing the deep water channel to narrow. Potential solutions to the problem at Red Eye Crossing are tested on the computer. The construction of walls or dikes within the channel is factored into the program. According to the computer, dikes help to eliminate the point bars. Eventually, dikes will be built and tested on the physical model to test their effects on sediment transport. The way dikes function as far as opening the channel and making it wide enough and deep enough is they actually take the channel that has a relatively wide width from top bank to top bank, and it contracts it normally on one side and makes it a little bit narrow. What that does is Mother Nature and the River itself says "I need to have so much area available to me," so when you pinch the size, the only thing that can happen is the bed scour. The thing is that you want to make sure that the bed scour is enough, that the channel is wide enough and deep enough year round, be it high water or low water. The other thing is you don't want to pinch it down too much that all of a sudden the velocities start getting high going through that dike field, and then it becomes a problem of navigation also. Dikes seem to be the most effective way to reduce the need for dredging and keep the channel open, but before they are installed at Red Eye Crossing the engineers want to determine how the dikes will affect the people who actually use the channel, the ship and towboat pilots. The pilots of the Mississippi River have been part of the region's lore for many years. They know the River better than anyone else possibly could. Guiding a ship or a boat down the Mississippi means far more than simply memorizing a route from Point A to Point B for the mighty Mississippi is a dynamic system, always shifting and churning. As Mark Twain so knowingly wrote in "Life on the Mississippi, " Two things seem pretty apparent to me." One was that in order to be a pilot a man had to learn more than any one man ought to be allowed to know, and the other was that he must learn it all over again in a different way every 24 hours. The WES Facility includes a ship tow simulator, which functions much like a flight simulator. Here the navigating instrumentation can be configured or either a tow boat or a ship. The screen is an accurate representation of the view from the pilot house. Many pilots are brought to the Waterways Experiment Station during the study. Each spends a week repeatedly steering up and down the computer simulated course of Red Eye Crossing. The proposed dikes are factored into the simulation. Every run down the Crossing is different. The computer changes many parameters, such as River discharge, the number and placement of passing ships, and channel depth. As the pilot wends his way down Red Eye, the computer records the exact course of each run, the time from start to finish, and whether or not there were any collisions or other safety problems along the way. Although dikes seem to be a promising solution, the Army Core Study indicates that they might create some navigational problems, so the investigation continues, and the mighty River remains unshackled as it flows through Red Eye Crossing. The power of running water extends far beyond the Mississippi. Indeed, it is the dominant force shaping the Earth's landscape. The combined discharge of all of the rivers on Earth is only one ten-thousandth of one percent of all the water on this planet, but few geologic processes have exerted a greater influence on human history and civilization. Many of the world's great cities were first established as riverside settlement, and throughout their history, these cities have depended on the river for food, a water supply, and an avenue of transport and trade, but like all natural systems, rivers undergo relatively rare but extreme events. River flooding is a threat to nearly every nation on Earth. In the United States, floods exact the greatest toll of any geologic hazard causing billions of dollars in property damage and killing about a hundred people every year, and this loss is modest when compared to the destruction in countries with primitive flood control systems or the devastation in pre-industrial society which were visited by floods without warning. Like most natural systems, rivers change and evolve through time in response to a variety of geologic factors that are themselves changing, factors such as regional climate, hill slope, tectonic activity, vegetation, and the bedrock composition of the Earth's crust, so the behavior of rivers is controlled by physical laws and geologic processes that can be observed and understood. Rivers do much more than rain water from the land and carry sediment to the sea.

The evolution of a river exerts a powerful influence on the surface of the Earth; in fact, much of the continental landscape, especially those areas where people live, was formed by the power of running water. Well, again, this video shows us, I think, many things that we can go out and see in the real world, but we wouldn't know what to look for, I think, without the guidance that we get from other people's study. Well, you know, one of the things that we know today that people didn't used to know is simply that streams for the most part cut the valleys in which they flow. It was originally thought a long time ago that streams simply followed the path of least resistance flowing in valleys that were created by other processes; in fact, it was Leonardo da Vinci who first surmised that streams actually cut their own valleys. Well, now we generally accept that streams cut their own valleys although we also understand that the underlying rock structures may determine the actual course and the location of the valley. We also know that streams cut their own valleys by erosion which may either downward or sideways, but mostly downward. We also know that mass wasting plays an important role in the valley widening, but moving material into the stream bed where it can be eroded and transported away by water.

Streams erode downward by three principal methods. These are "abrasion," "solution," and "hydraulic action." "Abrasion" is just what it sounds like; it's the scouring of the stream bed by moving particles both by impact and by virtue, and it turns out that it's probably the most effective method of downcutting; in fact, the more sediment a stream carries, the more effective it is at abrading. Potholes are a special aspect of abrasion; these are caused by particles of sediment which are trapped in a depression in the stream bed, and the swirling action of the rocks may actually enlarge and deepen the potholes. Many potholes still contain those pebbles which are responsible for their creation. "Solution" is a slow but effective process similar to chemical weathering except it takes place in the stream bed itself, but it's important to note here that solution may also release sediment particles into the stream as solution dissolves the cement, for example, between holding the grains of sedimentary rock together. The last of these methods is by irtue of its flow. The water may flow under a rock, lift it; it may fracture rocks that are already existing. It's not as effective as the other two methods, but it's still relatively important.

Well, let's look very quickly at features of streams. Many of these can be observed in almost any stream of various sizes; in fact, if you are simply watching water flowing from a drainage pipe along the beach into the ocean, you can see all of these aspects of stream dynamics in a little stream that's only four or five inches across. Sand bars or gravel bars are basically large ripples of sand and/or gravel which are deposited temporarily in the middle or on the banks of a stream channel. Basically this is sediment that's moved during times of flood that's simply too large to be moved with the normal discharge or the normal velocity of a stream, but the material may be moved downstream great distances during times of high flow rate or times of flooding, but the same processes often deposit a new bar at the same location. Most of these sand bars are transient in geological terms but may last either a long or short time in human terms. In fact, river pilots, as shown in the video, may have to relearn the location of some of these sand bars with each new trip up and down the river. A kind of extreme example of sand bars in a stream is the concept of the "braided stream." Streams which have a great variability in discharge may deposit great amounts of material which cannot be moved in times of low flow, especially streams draining mountainous areas where melting snow, for example, creates a seasonal surge in discharge, or streams that drain glaciers where there's a tremendous melting in the summertime. In a braided stream the stream may eventually deposit so much gravel and sand that it loses its main channel and then has to erode and flow through these series of small interconnected channels within the sedimentary deposits, and when this happens it develops a wide, but shallow stream bed. Another feature of streams is a feature called a "meander." You see, water which is flowing slowly in a narrow and deep channel tends to flow in a curved path instead of in straight lines. This is especially true in easily eroded material which, in the case of a stream, may often be in its own flood plain. This is especially true in the lower portions of the stream near its mouth. These sinuous or "s" shaped channels are called "meanders." It turns out that we don't really know exactly why a stream meanders, but all water tends to do this, and there's even a correlation that the size of the meander is directly related to the width of the stream, so the wider the stream, the bigger the meanders. Some people think that this is one way that the stream can decrease its gradient and lengthen its course both at the same time. One feature of meanders is "cut banks" and "point banks." You see, when water goes around a curve in a meander, it moves more slowly on the nside of the curve and faster on the outside of the curve. That means that before it moves slower on the inside it deposits the coarse sediment that it can't carry because of the decreased velocity, but at the same time on the outside of the stream bed, it has erosive power because of that velocity, so the stream tends to deposit on the inside of the curve and erode on the outside of the curve. Eventually this erosion may undercut the bank, and the material above drops into the water, a form of mass wasting, where it can then be transported by the water. This process causes the meanders to migrate downstream, very much like a wiggling snake, and this happens over fairly short geologic time. This process leaves behind what we call "meander scars" and "old point banks," and occasionally other features like "oxbow lakes." This results in the sediments of the flood plain continuously being reworked and moving downstream as the stream progresses through its erosional cycles. When the curve of a meander becomes extreme, the stream may simply cut through one loop into another, leaving the former loop of the channel high and dry. It believes behind when it does this a feature that we know as an "oxbow" or an "oxbow lake," and even when the stream no longer occupies that channel, we can often recognize an old oxbow by the presence of trees and other vegetation with high water needs which concentrate there in the sandy material.

Okay, a "flood plain" of a stream is simply a broad strip of sediment on either side of a stream channel, and these usually represent very rich and fertile agricultural soil. These specifically are also part of the river system and represent sediment which is not being used now and may not have been used for a thousand years and may not be used again for another thousand or, in many cases, even a million years. You see, streams which are flowing slowly generally erode more sideways than they do downward, and so in the course of doing this by meandering and making point bars and so forth, the stream actually reworks the sediment that it had formerly deposited. This usually happens in the later stages of landscape development or in regions of low gradient near the mouth of the stream. The stream may also overflow its stream channel during a flood building up the flood plain, but it also forms a feature called a "natural levee." As soon as the stream overflows its bank, its velocity is dramatically decreased, and so it drops those particles which it is not competent to carry and forms a natural levee. The finer material that does stay in suspension is then spread out laterally where the flooding occurs to form horizontal layers of fine sediment which also cover the flood plain. These sediments may be reworked later on as the stream meanders. A "delta" is a feature built by a stream which flows into standing water. Here, again, the decreased velocity decreases the competence, and the load is deposited. Deltas in many ways resemble braided streams, but the surface is marked by distributaries; that is, streams that don't come back together to form a main channel. The actual shape and size of the delta depends upon the balance between sedimentation and erosion by waves and currents offshore. The delta builds three distinctive types of sedimentary structures, and there's a good illustration in the textbook of bottom set beds, foreset beds, and topset beds, and you might want to take a look at those diagrams.

Okay, flooding is an important and natural aspect of a river system. In the video, the geologist, D. Trent, pointed out that anyone who's surprised by the flood of a river simply isn't paying attention. Rivers and streams always represent an attempt to gain equilibrium. Okay, rainfall and climate are variable, and so is the amount of water applied to a given stream; in fact, some streams are dry for most of the year and only flood and do all of their damage. Floods do a great deal of damage, but also provide soil and nutrients to support agriculture. The great civilizations of Ancient Egypt, for example, depended upon the annual flooding of the Nile for water, soil, and nutrients. I also want to note here that urbanization of various types may actually increase the flood hazard because the channelization of streams, the pavement of drainage areas, and the collection of runoff in sewage systems, and also the building of bridges which restrict the channel width. Complete control of flooding is impossible although there are several methods which were being employed to minimize the hazard. The video discussed several of these. Well, that about does it for this program. I hope we've been able to summarize the dynamics of stream erosion, and transportation, and deposition in this brief amount of time. It helps to visualize a stream as constantly changing, moving sediment, continually depositing it temporarily, eventually eroding it again and depositing it somewhere else downstream. Sediments are transported in a series of movements. Like sand on the beach the features that we see seem to be permanent though really consist of different sediment put in the same place, so next time we'll study landscape evolution, Chapter 16, the last half of the chapter, and, in the mean time, study hard, enjoy the planet, and I'll see you next time.