GG101 Running Water II

GEOLOGY/GEOPHYSICS 101 Program 24

Running Water II


Well, hello, again, and welcome. Today's program is about the development of landscape.That's Lesson 20, which covers the last half of Chapter 16 in the textbook.

You know, from the understanding that we gain by studying the dynamics of streams we can apply uniformitarianism to understand the types of changes that streams go through in geologic time.

There's a whole branch of Geology called"Geomorphology," which applies these studies to understand how a particular landscape has developed and evolved through time. We can understand whatprocesses have been active to produce the landscapes that we actually see,and in the process we discover that uniformitarianism although a good model is not entirely correct; in fact, geologic changes do occur in episodes on all time scales although the average of these changes over time may be uniform so that we can still apply uniformitarianism at least on a broad scale.

We can also from studying streams and landscapes understand how man's activities affect and are affected by these various stream processes, and we can try to predict the outcome of our interference with these processes as we make attempts to stabilize our environment.

Landscapes are the most visible results of geologic processes. They actually represent the current state of interaction between tectonic and erosional processes of the various types, and we'll study some of these other erosional processes, such as wind and ice in future programs, so in landscapes we can see and appreciate the results of the various processes that we've studied up to this point in the course.

Before I go on then, let me remind you of the text assignment. We are in Chapter 16, which is the chapter on "Streams and Landscapes," and you might want to review the first part of the chapter before approaching this lesson. That's on pages 349 to 368, and then when you feel like you understand the first part go on and study the second half of the chapter, which is from page 360 to the end. That section begins with "Valley Development," and then when you're done, reread the summary of Lessons 19 and 20, and you'll get a sense of how these two work together, and, of course, when you're done, go back to the learning objectives and make sure you've learned each one.

From studying drainage patterns on maps, we learn that stream patterns and stream systems develop characteristic patterns which can help us in several ways; for example, they may help us to identify the underlying rock structures even if we can't be there in the field to examine the rocks,and it also helps us to understand the way landscapes of different types evolve, so let me review with you briefly these four main types of stream patterns. They're called "dendritic,""radial," "rectangular," and "trellis."

"Dendritic" patterns are the most natural. The word "dendritic" comes from the Latin word for "tree," so these dendritic patterns are branching treelike patterns of the streams and the tributaries. This type of pattern usually develops on soft sediments or on flat lying sedimentary rocks where there's no structure to control the shape.

"Radial"patterns develop on certain types of structures; in fact, anywhere where a structure is higher in the center than it is on the edges. Examples would be structural domes and on volcanic edifices like the Hawaiian Islands; in fact, radial drainage patterns, as we'll see later in today's program, are quite common in theHawaiian Islands.

You may want to go back to the section on structures and review the concept of structural domes, and the concepts of anticlines, and synclines, and some of the other things we studied earlier on.

"Rectangular" patterns develop on rocks with joints. The erosion is usually enhanced along the joints, and you may remember or you may want to review that joints usually occur in patterns which are nearly at right angles to each other. "Trellis" patterns, on the other hand, develop where there are alternating ridges and valleys caused by rocks, and folds, or dipping rock layers. Interestingly enough, the folded rocks usually develop anticlinal valleys and synclinal ridges; that is, the tops of the folds generally turn into valleys while the bottoms generally turn into ridges. The reason for this is quite simple, that compression in the troughs as the folds form make the rocks in the troughs more resistant, and by the same line of reasoning, tension in the crests makes the rocks less resistant along the crests of the anticlines, so we generally find that the main streams in this type of folded rock structure generally flow along the crests of anticlines and valleys, but the tributaries of these streams may cut across the synclinal ridges, giving this trellis shaped pattern. These trellis patterns are common in the Appalachian Mountains, and it's even called the valley-enriched province. You may want to refer back to Figure 1.6 on page 8 and 9 in the text which shows the various geologic provinces. These type of structures are also common in Central California; for example, the Sacramento and San Joachim Rivers flow north and south, respectively, through the Central California valley drained by many small tributaries which drain the coastal ranges on the west and the Sierra Nevada on the East.

Let's turn our attention now to the actual downcutting of the erosion by the stream. You see, if downcutting was the only process that was taking place, then all streams would form narrow, thin, steep-walled canyons. Now, these types of geologic features do exist, and they're called "slot canyons" when they exit, but they're not common. The reason is because mass wasting and sheetwash remove material from the valley walls, so they widen this steep, narrow canyon into a "v" shaped valley, a narrow "v" shaped valley. These features may persist in very resistant rock, features like slot canyons, or where downcutting is very rapid, but we know that downcutting of this type cannot continue indefinitely because a stream cannot erode below the level of the stream at its mouth. Think about that. In order for the stream to erode below the level of its mouth, it would have to flow uphill to get to its mouth, and it can't do that, so base level we define as the "lowest level" to which a stream can erode. It represents the lower limit of downcutting. The base level may be "local"; it may be "regional"; or it may be an "ultimate" base level. "Local" base levels are things like lakes, layers of resistant rock. A "regional" base level may be a desert floor; but the "ultimate" base level for all downcutting by streams is sea level. I think we can understand here that if a stream is flowing, and its base level is changed, the change in base level can then affect the equilibrium of the stream; for example, if a stream has reached a period of equilibrium, lowering its base level can give the stream renewed energy and renewed erosional capacity, so the stream may begin to erode. If we raise the base level, it may cause the stream to have less energy and then cause deposition. A good example of this is building a dam in the middle of a stream. The dam creates a lake which represents a local base level, and we find when this happens that sedimentation occurs upstream of the dam and the lake; whereas renewed erosion takes place below the dam, and this can very often cause problems downstream if there are structures, houses, cities, and so forth built along the stream below the dam. Okay, the stream not only cuts downward when it erodes, but it also cuts sideways. We refer to this as "lateral" erosion. "Lateral" means "sideways." The stream actually widens its valley by both lateral cutting and also mass wasting. Mass wasting, of course, removes material from the valley walls and puts it where the water can remove it. As the valley widens, it also increases the size of the river's flood plane, and you may recall from our last program that water velocity is greatest on the outside of a bend, so when the stream goes around the corner the water may undercut the banks, which eventually collapse under their own weight, and this in itself is a form of mass wasting. The stream, then, may easily removes these debris left behind after the collapse of the stream bed, and, of course, the stream may also erode the banks during flood stage when it overflows those banks. Not only does the stream cut or erode downward and sideways, but it also tries to lengthen its stream course; that is, the stream undergoes what we call "headward" erosion, and by increasing its length, the stream can also decrease its gradient. It can decrease its gradient and increase its length by several other methods, too, by building a delta, for example, or by meandering, but also by headward erosion, so we can think of headward erosion as a slow lengthening or upward growth of the valley by gullying, mass wasting, sheetwash, or waterfalls. Another interesting phenomenon of headward erosion is something called stream "piracy."

Stream "piracy" is a term we use to describe what's happened when one stream erodes faster than another in the same area, and if this happens, the waters of one stream may be diverted into the waters of another stream that's growing faster, so that a major stream may actually capture the water and, therefore, the drainage area of the smaller stream. These may take place by headward erosion or by lateral erosion; in any case, it's erosion of some form which cuts across a drainage divide diverting the water. This, by the way, also makes the pirate stream, the one who's done the capturing, even a stronger stream because it now has the discharge of its own drainage area plus the drainage area that it's pirated, so these terms are covered fairly well in the text, and I'm trying to give you just a brief overview of these various things, and you're going to have to go back to the text to read more details on these various processes. I want to get to the idea of "equilibrium" in streams. Like most processes that occur in nature, processes happen in a way that try to come to equilibrium, a sense of balance, so in geology we refer to a stream that's in equilibrium as a "graded" stream. "Graded" in this sense simply means that there's an equilibrium between the sediment that the stream has available and the amount of the sediment the stream can carry, so if we look at a stream early in its development, a youthful stream say, for example, in a mountain range, we find that its longitudinal profile is generally irregular during the early stages. You find many rapids and waterfalls, for example, which represent these irregularities. At this point the energy of the stream is mainly used for downcutting the stream; in other words, it's mostly erosion, but eventually it smooths out these irregularities to develop a reasonably smooth profile, so we can say that a graded stream exhibits a very delicate balance between the sediment transport and capability and the load of the sediment. It's an equilibrium between erosion and deposition at various places along the stream core. You might say that it's a cutting a filling process that the stream undergoes to balance the available energy with the load of sediment that the stream has to carry, so we also remember that the gradient or the slope of the stream increases the load or it influences the load. The steeper gradient causes an increase in velocity, so that if a stream gradient is increased, it may increase both competence and velocity, but we also know that the load might influence gradient; for example, a reduction in load if the sediment upstream, for example, is diverted somehow, this may cause the stream to erode further downstream, and with the same line of reasoning, an increased load, sediment being dumped in, for example, from a high rain in the mountain area, may actually cause deposition of material somewhere further downstream, so it's an interesting process, an interesting concept, the idea of the graded stream and equilibrium, but, of course, real streams are never completely in equilibrium. There are seasonal changes in water and sediment supply, and these seasonal changes may actually have different results in different streams or in one stream at different times.

Well, what we can say is when the sediment load, or discharge, or channel shape changes, that a compensation of some kind will occur in the stream. These may represent changes in base level due to tectonic activity or stream dynamics either upstream or downstream over a particular area, and it may also be influenced by seasonal or climatic changes somewhere in the drainage area of the stream. Remember that the stream is a system, so a change in one part of the drainage area is going to be reflected somehow and usually in unpredictable ways all the way down the stream and other regions of the drainage area. Okay, a decreased load may cause erosion of sediment in the stream bed, which causes downcutting, lateral erosion or increasing roughness of the stream bed, and we usually can't say exactly what's going to happen. The changes that are caused by any particular change of these various factors are pretty much unpredictable. What we do know is that interference in the stream dynamics may have unexpected and harmful results, and, again, since they're unpredictable, we don't know exactly if building a dam here is going to cause erosion at Point A, or Point B, or somewhere else along the stream, so construction of dams, channelization of streams, the paving of cities, changing the amount of water that's dumped into streams with sewers and so forth can have tremendous effects, and we're only now beginning to understand the variety of these effects, so I think this gives a fairly good background to understand the material presented in the video.

The video does have a fairly long segment here about a particular problem on the Mississippi River, dealing with the Mississippi River's tendency to change its location and, of course, changing the location of a major river like the Mississippi often has very long term results and very damaging results on commerce and other activities of man, so with this background and with this information, let's watch the video. A close look at the Earth's intricate system of running water is a close look at the evolution of Earth's landscape. As they continue to shape the land around us, rivers and streams leave behind evidence of their enduring power. Unlike earthquakes and volcanoes which can cause sudden change, running water works slowly, almost imperceptibly, in shaping Earth's landscape. We usually think of the Grand Canyon in terms of its rocks and the fascinating story that they contain, one that spans almost half of Earth's history, but there's more to the Grand Canyon than rocks. The Canyon itself is a geologically active feature, a changing and evolving land form that's a monument to the power of running water. The Colorado River carved this enormous valley over the last nine million years; in fact, the River carries about half a million tons of sediment past any point of the Canyon every day. No wonder this great River has been described as "too thin to walk on, but too thick to drink." Rivers like the Colorado are powerful geologic agents that disrupt and reshape the surface of continents. The energy of running water in a river channel is transformed into processes that erode rock and sediment at the bottom of the channel and carry it downstream. As the river deepens its channel, the sides of the valley steepen and grow unable. Eventually, mass wasting processes are triggered causing these slopes to fail. This delivers even more sediment to the river and widens the river valley even further.

In order to understand the influence of running water on the Earth's surface, we're going to look at a variety of rivers and also at different land forms at various stages of their development. In doing so, we'll explore the connections between the geologic process of running water and the evolution of the surface of the Earth. The connection between a river and its deep wide valley is not an obvious one. At one time valleys were thought to have formed independently of the rivers which flow through them. Today geologists are well aware that valleys usually form by the downcutting of running water combined with the mass wasting of slopes. As a river cuts its channel deeper, it carries away sediment fed to it from surrounding hillsides. Now there are limits to how deep that a stream can erode its valley, and those limits come up several kinds, which we generally refer to as "base level." The ultimate base level, a grand base level is sea level. Streams don't degrade their valleys below the level of the sea, so we don't find great canyons arcing down to the ocean filled by water which is flowing back in from the sea. The stream as it approaches the sea level loses velocity and, therefore, loses ability to erode. Modern concepts of landscape evolution began with an American geomorphologist, William Morris Davis. He believed that rivers and streams gradually wear down rugged mountain slopes to form planes. He classified landscapes by their maturity and used the terms "youthful," "mature," and "old aged" to categorize their stages of development. Davis's work done mostly in the Appalachian Mountains conceived of landscapes as going through a distinct series of stages that began with an uplift of the area supplying streams with potential energy which they could then use to carve their valleys. During the earliest stages after uplift, streams were predominantly cutting downward incising their valleys and creating a steep sided landscape. Davis referred to this as "youth." As this process continued, the streams gradually spent some of their energy carving from side to side, wandering back and forth across the valleys. The valleys then ceased to be "v" shaped and become somewhat more flat floored, and eventually all of the original upland surface is consumed by erosion from tributaries, so that all of the landscape is now in hillslopes, and this is what Davis considered to be the stage of "maturity," and finally as the stream works down, it reaches near base level or the limit to which it can erode downward, and then does very little vertical erosion, expending almost all of its energy eroding from side to side, and as it does so, these valley bottoms become more and more broad. The tributary streams erode their slopes eventually creating flood planes of their own, and finally, virtually all of the material that was uplifted is destroyed and brought to an equal level, which Davis referred to as a "peneplane," as the ultimate stage in the cycle of erosion, which he thought ended in "old age." In order to make his model easier for geologists to apply, Davis described uplift and erosion as events occurring separately in time, but he knew, in fact, these processes occur simultaneously.

Landscapes form by a continuous interplay of tectonic activity and erosion. Other crucial elements also influence the shape of the land, including rock type, rock structure, and climate. Solid rock, for example, can hold up a much steeper cliff than sand or clay, and when structures such as folds or faults appear at the Earth's surface, shapes adjust accordingly. The slope of the Grand Canyon is a result of many of these factors. The rocks are layered sedimentary strata with contrasting resistance to weathering. This naturally results in slopes with differing steepness and rates of erosion. In addition, the arid climate and relative lack of vegetation contribute to the sharp angular features of the Canyon walls. Ultimately, rock type and rock structure also affect river drainage patterns. By studying different stream patterns, geologists can infer a great deal about the nature of the underlying rock. If we have homogenous rocks or flat lying sedimentary rocks, streams typically form what's called a "dendritic" pattern, and it looks very much like the branching of a tree. Where the rocks are not homogenous or where there are definite structures in rocks, other patterns occur. For example, if we have intersecting fractures or faults, streams commonly follow those intersecting patterns. They make sharp bends at acute or even right angles forming a different kind of stream pattern. Where we have alternating layers of strong and weak rocks, the streams will usually branch out as a trellis pattern reflecting the weaker valley rocks and the stronger hill forming rocks. The drainage patterns of streams expand and grow more intricate as the land erodes away. Even after streams and rivers wear a landscape flat, it's possible for erosion to become active again if the landscape is uplifted, or if the regional base level drops. Geologists call this renewal of stream erosion "rejuvenation," and it produces certain characteristic land forms, such as stream terraces. Stream terraces are found frequently in areas of very wide valleys where one of several things have happened. One of the most common ways in which terraces form is because of rejuvenation where there has been uplifting of the area or a downdropping of the base level, so that a stream which was formerly meandering with big wide sweeping turns with a wide flood plane ends up cutting into its own flood plane, leaving the flood plane elevated on either side of the river as a terrace, so that it is no longer an active flood plane but actually an elevated surface above the river. But terraces can also indicate other types of regional change. For example, if the climate grows drier, a river will shrink in size. Rather than carrying away sediments from surrounding slopes, the shrinking stream will eat into its own sedimentary deposits created in whetter times. Gradually terraces form. Another land form that can be produced as a result of rejuvenation and uplift is an "incised meander." The key factor distinguishing incised meanders from normal meanders is that they are cut well below the level of a river's former flood plane. Incised meanders result from downcutting along the thalweg or deepest part of a river's channel. The downcutting is so rapid, the river maintains a meandering pattern while deepening its valley. River valleys form a significant part of Earth's landscape, but they are not the only land form created by running water.

All streams and rivers come to an end. Most ultimately flow into the ocean or another large body of water such as a lake. Due to the sudden loss in velocity at the mouth of a river, most of its sediment is deposited forming a delta. Deltas, of course, form at the mouth of the river where they enter a large lake or the ocean. The gradient or the slope of the river is very gentle. When it hits the water there is no gradient; consequently, the sediment begins to settle out immediately. In so doing, it dams its channel, and the river tends to branch into a series of distributaries. From time to time the certain branches load up with more sediment than others, so the main flow of the river may shift from one locality to another over a long period of time. Distributaries play a vital role in building and enlarging a delta intermittently supplying new sediment to all parts of the delta's shore. The Mississippi River has built one of the largest deltas in the world. Nearly 40,000 square kilometers of land have been added to the State of Louisiana due to the astonishing power of the Mississippi River and its enormous amount of sediment. One million tons of silt, sand, and clay are added to the Mississippi delta each day giving the river its nickname, the "Big Muddy." The Mississippi could not have created this much land if it had stayed in one channel. The southern part of the River has changed course many times over an area some 300 kilometers wide. The key to these changes is the River's natural tendency to follow the path of least resistance, which is almost always the shortest route to the sea. The Mississippi follows a single channel until gradually its channel fills with sediment. At that point, the River easily overtops its banks during periods of high discharge. When that happens, it is free to find a more direct route to the Gulf until, of course, the lengthy cycle begins again. This cyclical shifting of the Mississippi has resulted in an ongoing battle to control the forces of nature. Along most of its lower course, levees have been built to confine the River to its present channel. Cities and ports have grown along the Mississippi, and it has gradually become one of the world's most important economic waterways. If the Mississippi were allowed to change course from its modern channel, 3 major ports built along its shores would be left dry. Elsewhere farms and towns in the path of the new riverbed would be washed away, so the U. S. Army Corps of Engineers has engaged a team of scientists and engineers to hold the River to its present channel. How long the Corps can keep the River where it is really just a matter of money. One of the things about engineering is that you can do almost anything given the money, and we can basically keep the River where it is.

Now, we may end up having a whole mess of control structures up and down the River because the River is going to try to change its course. It's going to try to find the shortest distance to the Gulf of Mexico, and it might be not this flood, but it might be the next flood where a levee might break, or else a structure might be flying through something like that where the River is going to try to change its course again, but the Corps realized that it could not really let this happen. The economies of Baton Rouge and New Orleans depend on the River for its fresh water, for its commerce, its transportation. Industries all up and down the River use the fresh water in their processing. In its continuing search for the shortest route to the sea, the Mississippi has found a comrade. At one time a mere trickle compared to the Mississippi, the Atchafalaya is now a mighty predator; the Mississippi, a willing prey. The fight to control the Mississippi has escalated from a battle into a war. Approximately 150 miles north of New Orleans, these two rivers have come perilously close together linked by an abandoned loop of the Mississippi called "Old River." The Atchafalaya offers the Mississippi a route to the Gulf that is 175 miles shorter than its present course. The Corps of Engineers realized that there was a potential problem with Atchafalaya capturing the Mississippi back in the 1950s. A gentlemen by the name of Fiske, who was a geologist, did a report for the Corps of Engineers, the Mississippi River Commission, in which he studied old delta systems of the Mississippi and old diversions of the Mississippi, and compared to those to what was happening on the Atchafalaya and the Mississippi, and he theorized that the Atchafalaya and the Mississippi were in an intermediate stage of capture, and that if something was not done by about the 1970s, about 1975, we would reach a critical stage of capture in which the Mississippi would no longer be able to carry any more flow because of this build up of sediment, and the flows would go down the Atchafalaya, and he said that this would happen when about 40 percent of the flow was going down the Atchafalaya from the Mississippi. In 1954, as the result of the Army Corps Report, the United States Congress authorized the Old River Control Project. Essentially, this funded the construction of a series of control structures and channels all situated in the Old River Area. Under the plan, the flow of water and sediment between the Mississippi and the Atchafalaya was to remain at its then current rate, a 70/30 split. Thirty percent of the combined discharge of the Mississippi and Red Rivers was to flow into the Atchafalaya while the remaining 70 percent would be kept within the Mississippi itself. First, the Old River Channel was dammed; this meant that the only natural connection between the Atchafalaya and the Mississippi was closed. Since the Corps did not want to disrupt boat traffic between the two Rivers, a navigational lock was built on Old River at a cost of $15 million. To enforce the mandated 70/30 flow rate, construction began on two control structures at a combined cost of $15 million. These structures were completed and operational by 1963. The low sill structure is 566 feet wide and has 11 gates which allow the Corps to control the flow rate. The low sill sits on a manmade outflow channel connecting the Mississippi to the Atchafalaya. On the flood plane next to the low sill structure sits an emergency facility, the overbank structure built to assist the low sill during major floods. Well over a half mile long, it has 73 gates. For a time this elaborate and costly system managed to keep the Mississippi in place, but in 1973, ten years after the system went on line, the Corps efforts were tested to their limits. Well, 1973 we had a flood which happened to be the second greatest flood that man has observed since he kept records on the Mississippi, the greatest being the 1927 flood which is what resulted in the Mississippi River and tributaries system and all the structures that go with it that you see today. We actually had to open up several structures between Old River and New Orleans to alleviate flood waters between here and New Orleans so that New Orleans wouldn't go underwater. What happened was that the River decided that it basically wanted to continue going down the Atchafalaya. The thalwag of the Mississippi just moved almost right into the entrance to the low sill structure and basically took out a wing wall, the forces were that powerful. It also undermined the foundation of the low sill structure. The water actually in addition to try to go around it actually did go underneath the structure, and it was only because the structure's pile founded, and that the piles were very, very deep that the structure remained standing, and we basically just had to open the gates and let the River go because they were just so afraid to lose the structure. Although emergency repair work to strengthen the low sill structure began immediately, it was obvious that more control was needed. At an additional cost of almost $300 million, the Army Corps proceeded with the construction of another control structure and accompanying channel. We learned that we would basically not be able to control the flows unless we did construct another structure. The low sill was just too damaged. The Corps was authorized in the late 70s to build the auxiliary structure. The auxiliary structure which was completed in 1986 basically is what its name says. It's an auxiliary. It serves to complement the low sill structure, allows us to get a better control of the flows so that in the event that we have a 1973 flood, which was just a tremendous amount of water, that we would not get ourselves in a situation that we did where we almost lost the River. The 1973 flood demonstrated how suddenly the River's condition could change. The Corps realized it had to more closely monitor the 70/30 ratio of flow between the Mississippi and the Atchafalaya. Until 1973 the Corps only looked at the average annual flow. As a result of the flood, the flow rate is currently monitored each and every day. It is the New Orleans District Office of the Army Corps of Engineers, which oversees the day-to- day operation of the control structures. The flow data from the structures is reviewed here, and river conditions are closely monitored. Based on these data, major decisions are made about which gates are to be opened or closed. A small device is lowered into the water to measure velocity. These measurements are made along various parts of the River to calculate discharge. Using this information, flow predictions are made, and the gates are raised or lowered accordingly, but the amount of water flow is not the only factor that needs to be considered in keeping the Mississippi in place. Although the Atchafalaya takes water from the Mississippi, it leaves most of the sediment behind. In response, the Atchafalaya scours its own channel, acquiring enough new sediment to restore its equilibrium. The effect of the scouring also deepens the Atchafalaya bed providing an even steeper route for the Mississippi. The effect on the Mississippi of the bed load remaining behind is a buildup of sediment in its channel, so sediment flow, especially bed load as well as water flow, must be kept in check. With this in mind, the auxiliary structure was strategically placed. One of the goals of placing the auxiliary structure where it is to try to increase the amount of sediments being diverted from the Mississippi to the Atchafalaya.

One of the lessons we learned since the operation of the low sill structure was that we were not diverted the same proportion of sediments through the structure, through low sill structure, as we were water. The Mississippi River was continuing to show evidence that it wanted to fill up, and the Atchafalaya was continuing to scour, so we felt that if we increase the amount of sediments being diverted here at Old River that we would actually try to stabilize the Mississippi River and the Atchafalaya River, at least slow the trend down in the Atchafalaya, so we located the auxiliary structure on the inside of a bend where there's actually more sediments, and we angled it such that we would get the sediments moving along the bottom of the River. These sediments would want to go into the inflow channel of the auxiliary structure and actually go through the structure and on down to the Atchafalaya. Life around Old River is generally peaceful now. Even more important, the world below Old River carries on normally. Many here are unaware of their upstream fortress. It would take an extraordinary amount of water to test these structures and the will of the Army Corps of Engineers, but rivers are capable of extraordinary things. Our ability to control nature, and in particular, our ability to control rivers is limited. We may be successful for a short period of time, a year, five years, ten, maybe even 25 years, but there's always a larger flood out there. There's always a bigger windstorm. There's always higher waves than the ones we've encountered before, and when we place ourselves and our lives in the path of these processes, it's only a matter of time until we can expect to see the adverse effects. Since its beginnings, civilization has flocked to the riverside, and there has always been a price to pay as the result, but even with their power to destroy, rivers have given back. They have cradled the life in and around their banks and carved out landscapes that are legacies to their power and might.

Today, as always, running water is one of the most significant sculptors of Earth's terrain. Its effects are virtually everywhere; even in places which appear to be dominated by other geologic forces. Indeed, the land forms running water creates and leaves behind are an enduring testament to its power. The sequence of events that takes place in the evolution of landscapes is not completely understood. This is in part because the processes that shape the land surface operate very slowly on a human time scale, but there's no doubt that running water plays a significant role. Land forms that have been shaped and modified by running water are found in every terrestrial environment on Earth. They're even abundant in deserts where sudden rainstorms and flashfloods can produce more geomorphic change in a few hours than many years of desert winds, but there would be no running water on the Earth's surface without slopes, and landslopes are both created and maintained by tectonic activity. Indeed, the shape of much of the Earth's surface is the result of a constant competition between tectonic forces and the destructive effects of running water, and no where is this dual between tectonism and running water easier to appreciate than here at the Grand Canyon where the Colorado River continues to sustain the evolution of one of the most beautiful and distinctive landscapes on Earth. Well, from the video you get a real sense, I think, of the problems that we have in trying to keep a river from attaining its equilibrium. This problem with the Mississippi and the Attahatchee, how much money did they say they spent on it? Thirty million dollars already? And they still haven't solved the problem. What they've done is provide a temporary solution, and eventually, Old Man river, as they say, will eventually do something that's gonna cost even more to fix it. Well, let's turn our attention now to some of the aspects that I think they didn't spend enough time on in the video. One of these is the idea of regional erosion and slope development. The textbook points out that there are two contrasting ideas here, and like we often find in our trying to understand natural processes, the real result or the real processes are some kind of a combination of the two extremes.

You might want to look at Figure 16.41 and 16.42 on page 372 in the text. It turns out that either one of these features will eventually produce a nearly flat plane in time, but the two basic ideas, one is called the "gradual lessening of slope." The other is called "parallel slope retreat." In the gradual lessening of slope we find rugged mountains forming which are gradually reduced to rounded hills and a flat plane, and most geologists think that this process is probably more active in wet regions in folded sedimentary rocks, for example. "Parallel slope retreat" means that the slope angles, remains relatively constant as the valleys are widened. Most people think that this is probably more active in dry regions on flat lying sedimentary rocks, and in our program of deserts, we'll have a chance to examine some of these features in dry regions. The actual slope, the steepness of the hillside is controlled by several different factors. One of these obviously is the rock type. Resistant rocks can simply hold steeper slopes; that is, they have much more strength, and so they can develop more vertical slopes. Climate also has to play a role, of course, because climate determines whether chemical weathering or physical weathering predominates, and, of course, rock structure also is going to have an influence, whether the rocks are horizontal, or folded, or faulted, or tilted in some other way. Okay, with this in mind let's turn our attention now to the idea of stages of landscape development. A geomorphologist named William Davis, who had done lots of observations over several different regions, noticed that it seems as if landscapes might go through various stages of development; that is, one type of landscape changes into another. Like any kind of process that we're studying in the Earth Sciences, processes involving geologic time, we never get the opportunity to watch these cycles from beginning to end. What we can do is look at landscapes in various stages or in various aspects of erosion and try to picture how one type of landscape might evolve or develop into another, so this stages of landscape development is a model for understanding landscapes. Landscapes, we understand, represent the result of constructive and destructive forces operating on a particular area. This happens through the idea of stream equilibrium, that a stream undergoes changes depending upon its sediment load and the amount of water it has available. The processes and interaction that take place here are actually quite complex, and in reality each different landscape is unique in its details, but certain kinds of landscapes do have certain features in common, and it seems that landscapes do go through predictable changes as tectonic cycles interact with these erosional processes, so it's important to note here that these various stages represent only relative stages, and that real landscapes may combine features of two of the stages, or in many cases, two or more of the stages, and real landscapes may also be modified partly by parallel slope retreat. We also note here that for a landscape to evolve from, say, youth to maturity, it may require a short time in one area and a longer time in another area, and this depends upon the climate and the rock type in rock structures. In the Hawaiian Islands we find many good examples of this. Streams on the windward sides of the islands are much more advanced in their evolutionary landscape development than on the leeward side although the rocks are the same ages and the same types. Windward Oahu, for example, is in a mature state of landscape development while leeward Oahu is in the youthful, or subyouthful, or late youth stage. On the Big Island of Hawaii, windward Kohala and Mauna Kea are youthful. The leeward side of these mountains is virtually undissected and pretty much remains the original shield shape of the volcano, so let's review these three stages, and I'll try to give you a sense of what features there are associated with each of these stages, and you may want to take a look at the figures in the textbook which show the various stages of development. Okay, in the youthful stage, we find the youthful stage characterized by wide drainage divides; that means basically that the streams are separated by fairly large distances, and the divides are more or less undissected; that means they resemble pretty much the original slopes that might have been there before erosion took place. The valleys themselves are narrow and "v" shaped. Here we find stream erosion coupled with mass wasting on these steep slopes, so that the valley widens slowly at the top. We also find that the streams in the youthful stage have high gradients; that means that there's rapid water flow, many rapids and waterfalls, and that the erosion at this time is mainly downward. The sediments in the streams are mainly large sized particles like boulder and gravel, and the total relief of the area is relatively low. By "relief" here we mean the total elevation difference between the highest and lowest points in a particular region, so even though the entire area may be mountainous and fairly high, the difference between the high and low regions is relatively small.

We also find that the streams themselves are few in number and have short tributaries and not too many tributaries. Okay, as the youthful stage gives way to maturity, like people as they mature, changes take place. For example, the valleys themselves tend to become more "u" shaped. The valley floors being to become more rounded. The rate of downcutting by the stream slows down, and the stream spends more of its time and energy eroding sideways and laterally, and at this time the stream begins to develop a flood plane. A "flood plane," you may recall is the deposition of sediments within the stream valley itself as the stream becomes incapable of carrying sediments of a certain size. The drainage divides at this point are narrow ridges. Rather than being undissected, there are very little undissected divide left, and there may actually be ridges between the various valleys. The relief at this stage is high; that means that the total difference between the highest and lowest points is at maximum. Many of our mountainous areas like the Rocky Mountains, for example, or the Alps where we associate with mountainous areas with craggy peaks and steep ridges are in this mature stage. By the way, the stream deposits in this stage are generally small gravel and sand. Keep in mind here, just to insert this, that regions in mountainous areas, the same stream in a mountainous area may be in a youthful stage; whereas, further downstream it may have already developed into a mature stage. In the old age stage, the stream is getting old and tired. We use the analogy "Old Man River" for a river like the Mississippi. Here we find the stream gradient very low. The stream doesn't have much velocity. The hills are broad and rounded, and there's again low overall relief. The stream, in this case, is very near to base level, and none of the hills are very much above base level. The sediment that the stream carries and deposits on the bed are mostly silt and clay fine grain particles. Streams in this stage generally had a very wide flood plane, a flood plane, in fact, that's many times wider than the actual width of the stream. At this point because the stream has so little energy because it's close to base level, it does little or no downcutting. Here the erosion is mostly lateral and, in fact, the work that the stream does is not so much erosion, but it's a balance between erosion and transportation.

We might say that the old age stream is more likely to be a graded stream in equilibrium, so the work that the stream does in this stage is mostly reworking the flood plane deposits. As the stream meanders, it wiggles back and forth and progresses downstream, like a wiggling snake. We find many meanders in the river. We find cutoffs, oxbows, and oxbow lakes, and we find lots of point bars and other deposits that we associate with meandering streams. The stream also usually in this stage has an extensive drainage system with many tributaries, and in many cases a smaller stream may flow parallel to the mainstream in the same valley. The Mississippi River Valley, for example, is nearly a hundred miles wide. The River itself is a mile or two wide at places, but flowing parallel to the Mississippi are several streams which we saw in the example. The Attahatchee is one of these. The ultimate stage is what old geologists called a "peneplane." The word "peneplane" like the word "peninsula" comes from the Latin word which means "almost," so almost a plane. In the peneplane the relief is almost completely flat, there's virtually no hills, and the entire area has been reduced mainly to base level, but this doesn't represent the entire story because we know there are tectonic processes operating as well; that is, while the stream is in process of going through these various stages, something may happen to change the base level of the stream. When this happens, we call this "rejuvenation." There are several features associated with rejuvenation that are called "incised meanders" and "antecedant" and "superposed" streams, so it's important to understand here that rejuvenation may occur at any stage during the stream cycle. We find youthful streams, mature streams, and old age streams that are rejuvenated; in fact, the peneplane is actually the rarest of these three stages. The key here is that the rejuvenated stream may retain features of the stage that it had before the renewed tectonic activity, especially, for example, of the rate of uplift or base level lowering is about the same as the erosion rate; in other words, if the landscape is being uplifted at about the same rate that the stream downcuts, then we find some of these interesting features. In this case, the stream may actually cut across existing rock structures and leave us with features that are hard to explain. One of these, for example, is what geologists call "incised meanders." Here we see old age stream courses with many meanders that are preserved in steep canyons with more or less vertical slopes. These are common in flat lying sediments or in regions with gently dipping folds. Another example is a "superposed" stream. A "superposed" stream is simply that a drainage pattern, which it developed as a result of a middle stage of stream development, develops on flat layers of sedimentary rock, and then the whole area becomes lowered by erosion through an unconformity onto existing geologic structures below, sort of like etching through a stencil. Okay, and in this case the patterns may also cut across the folds. This is common in the Appalachians where we believes that the Appalachian trellis drainage pattern probably started out as a dendritic pattern and was modified by being superposed onto the existing folded rocks below. In the Appalachian are many examples of water gaps that leave steep sided valleys perpendicular to folds and where the valleys actually cut right through a mountain. You can see some of the pictures of this in the text. We also use the term "antecedant" streams to describe streams that may predate tectonic activity; in other words, the drainage pattern of a stream is engrained during a time at near base level, and then further uplift simply preserves that same drainage pattern even though it returns back to its youthful stage.

Well, I want to add a little section here on Hawaiian streams and valleys because here in Hawaii and other Pacific islands, we have an interesting combination of features which combine to create our landscape which is not unique, but it's different from some of the landscapes that we see on continental areas. First of all, we find that in the Hawaiian Islands radial drainage patterns are common because of the shield volcanoes; that is, the stream patterns tend to radiate outward from the central or summit areas of the volcanoes. We also find that valleys develop rather slowly for two reasons: Number 1, because the lava is fairly rough, and especially aa lava, as we remember, is quite permeable. Not only that, but because the lava is quite permeable, we find that the amount of water which runs off as opposed to infiltration, is relatively large. Infiltration is about half of or about the same amount as runoff, and we find mass wasting playing a very important role as well. We find steep amphitheater headed valleys. The walls are steep because they consist of layers of alternating resistant and nonresistant aa and pahoehoe flows. We also find that stream piracy is quite common by both lateral and headward erosion. We find many hanging valleys, for example, on the sides of the main valleys, and the valleys themselves may have flat floors because sediment was deposited during times of plasticing glaciation when sea level was higher. Well, we have several illustrations that can show the stages of development of the Hawaiian landscape. In the early stages, amphitheater headed valleys formed. Mass wasting of alternating layers of resistant and nonresistant rock are responsible for both the steep sides and the steep head of the valley. It was once thought that these amphitheater headed valleys might have been caused by glaciation, but we since know that it's not. We also find higher rainfall in higher elevations, so the valleys tend to be wider at higher elevations, and there is a large amount of infiltration so that the discharge of the stream is greater in the upper regions. Triangular facets called "planeze"form between valleys, and there are many examples of this here in Honolulu. Wilhelmina Rise, for example. As it develops, a knife-edged ridge forms between the planezeses as the valleys coalesce, and this gradually reduces the height and the size of the planezes. Eventually, the planezes disappears, leaving only the sharp ridge as the valley floor becomes more rounded and less steep, and, finally, continued erosion of the ridge leaves long scalloped cliffs formed by coalescence of successive amphitheater valley heads, much as we see now on the windward side of Oahu and the older islands.

Three Example of Planeze on Oahu's Koolau Mountains in East Honolulu. Click on each photo for a larger image.

Well, I hope this has helped like our other programs to visualize some of these processes and to get an understanding of how these processes work. To truly understand how they work, you must spend lots of time in the field observing, driving around, flying over things to see the variety of processes, but we do know that landscape is the most visible result of geologic processes. We also understand that landscapes influence man's activities and man's culture. The types of agriculture, the types of recreation, the types of structures that we build, and the things we do with our lives all are affected by landscape, and we also know that everybody has different feelings when surrounded by landscapes of different types, whether you're in the desert or high mountains.

Landscapes seem unchanging, but they're modified quickly and significantly in geologic time, so I want to remind you before we close, don't forget about your field trip and your report. The report is due at the end of the class when you return the fourth exam, and also don't forget in studying that you have to use the text, the study guide, and the video all together, so next time we'll study ground water. That's Lesson 21, Chapter 17 and pay attention to the landscape as you travel around both at home and here in Hawaii. So study hard, and I'll see you next time.