More

5.3: Stream Gradient and the Cycle of Stream Erosion - Geosciences


Overview

Stream gradient refers to the slope of the stream’s channel, or rise over run. You have dealt with gradient before in Topographic Maps. It can be calculated using the following equation:

[Gradient =frac{(change ;in; elevation)}{distance}]

Let’s calculate the gradient from A to B in Figure 5.4 below. The elevation of the stream at A is 980’, and the elevation of the stream at B is 920’. Use the scale bar to calculate the distance from A to B. [Gradient = frac{(980’ – 920’)}{2 ;miles}, or; frac{30 ;feet}{mile}].

Stream gradients tend to be higher in a stream’s headwaters (where it originates) and lower at their mouth, where they discharge into another body of water (such as the ocean). Discharge measures streamflow at a given time and location and specifically is a measure of the volume of water passing a particular point in a given period of time. It is found by multiplying the area (width multiplied by depth) of the stream channel by the velocity of the water and is often in units of cubic feet (or meters) per second. Discharge increases downstream in most rivers, as tributaries join the main channel and add water.

Sediment load (the amount of sediment carried by the stream) also changes from headwaters to mouth. At the headwaters, tributaries quickly carry their load downstream, combining with loads from other tributaries. The main river then eventually deposits that sediment load when it reaches base level. Sometimes in this process of carrying material downstream, the sediment load is large enough that the water is not capable of supporting it, so deposition occurs. If a stream becomes overloaded with sediment, braided streams may develop, with a network of intersecting channels that resembles braided hair. Sand and gravel bars are typical in braided streams, which are common in arid and semiarid regions with high erosion rates. Less commonly seen are straight streams, in which channels remain nearly straight, naturally due to a linear zone of weakness in the underlying rock. Straight channels can also be man-made, in an effort at flood control.

Streams may also be meandering, with broadly looping meanders that resemble “S”-shaped curves. The fastest water traveling in a meandering stream travels from outside bend to outside bend. This greater velocity and turbulence lead to more erosion on the outside bend, forming a featured called a cut bank. Erosion on this bank is offset by deposition on the opposite bank of the stream, where slower moving water allows sediment to settle out. These deposits are called point bars.

As meanders become more complicated, or sinuous, they may cut off a meander, discarding the meander to become a crescent-shaped oxbow lake. Check out Figure 5.6 to see the formation of an oxbow lake.

Even though streams are not living, they do go through characteristic changes over time as they change the landscape. The ultimate goal of a stream is to reach the base level (the low elevation at which the stream can no longer erode its channel often a lake or other stream; ultimate base level is the ocean). While trying to reach this goal, the stream will experience the cycle of stream erosion, which consists of these stages:

  • Youthful (early) stage – these streams are downcutting their channels (vertically eroding); literally they are picking up sediment from the bottom of their channels in an effort to decrease their elevation. The land surface will be above sea level, and these streams form deep V-shaped channels.
  • Mature (middle) stage – these streams experience both vertical (downcutting) and lateral (meandering) erosion. The land surface is sloped, and streams begin to form floodplains (the flat land around streams that are subject to flooding).
  • Old age (late) stage – these streams focus on lateral erosion and have very complicated meanders and oxbow lakes. The land surface is near base level.

An interruption may occur in this cycle. If a stream suddenly begins to downcut again, if sea level dropped (so base level dropped) or if the area around it was uplifted (think building mountains), then the stream would become rejuvenated. If the rejuvenated stream was in the old age stage, it will begin to form a deep V-shaped channel within that complicated meandering pattern that it has. This creates a neat geologic feature called an entrenched meander (Figure 5.8).


5.3: Stream Gradient and the Cycle of Stream Erosion - Geosciences

Fresh water in streams, ponds, and lakes is an extremely important part of the water cycle if only because of its importance to living creatures. Along with wetlands, these fresh water regions contain a tremendous variety of organisms.

Streams are bodies of water that have a current they are in constant motion. Geologists recognize many categories of streams depending on their size, depth, speed, and location. Creeks, brooks, tributaries, bayous, and rivers might all be lumped together as streams. In streams, water always flows downhill, but the form that downhill movement takes varies with rock type, topography, and many other factors. Stream erosion and deposition are extremely important creators and destroyers of landforms and are described in the Erosion and Deposition chapter.

Rivers and streams complete the hydrologic cycle by returning precipitation that falls on land to the oceans (Figure 10.1). Ultimately, gravity is the driving force, as water moves from mountainous regions to sea level. Some of this water moves over the surface and some moves through the ground as groundwater . As this water flows it does the work of both erosion and deposition. You will learn about the erosional effects and the deposits that form as a result of this moving water.

Figure 10.1: As rivers and streams move towards the ocean, they carry weathered materials.


14.1 The Hydrological Cycle

Water is constantly on the move. It is evaporated from the oceans, lakes, streams, the surface of the land, and from plants (transpiration) by solar energy (Figure 14.2). It is transported in its gaseous form through the atmosphere by the wind and condenses to form clouds of water droplets or ice crystals. It falls to the Earth’s surface as rain or snow and flows through streams, into lakes, and eventually back to the oceans. Water on the surface and in streams and lakes infiltrates the ground to become groundwater. Groundwater slowly moves through soils, surficial materials, and pores and cracks in the rock. The groundwater flow paths can intersect with the surface and the water can then move back into streams, lakes and oceans.

Figure 14.2: The various components of the water cycle. Black or white text indicates the movement or transfer of water from one reservoir to another. Yellow text indicates the storage of water . Source: Steven Earle (2015) CC BY-SA 3.0 view source after Wikimedia user “Ingwik” (2010) CC-BY-SA 3.0. view source

Water is stored in various reservoirs as it moves across and through the Earth. A reservoir is a space that stores water. It can be a space we can easily visualize (such as a lake) or a space that is more difficult to visualize (such as the atmosphere or the groundwater in a region). The largest reservoir is the ocean, accounting for 97% of the total volume of water on Earth (Figure 14.3). Ocean water is salty, but the remaining 3% of water on Earth is fresh water. Two-thirds of our fresh water is stored in the ground and one-third is stored in ice. The remaining fresh water (about 0.03% of the total) is stored in lakes, streams, vegetation, and the atmosphere.

Figure 14.3: The storage reservoirs for water on Earth. Glacial ice is represented by the white band, groundwater the red band, and surface water the very thin blue band at the top. The 0.001% stored in the atmosphere is not shown. Source: Steven Earle (2015) CC BY 4.0 view source, data from USGS Water Science School (2016) view source

To put these percentages in perspective, we can compare a 1 litre container of water to the entirety of the Earth’s water supply (Figure 14.4). We start by almost filling the container with 970 ml of water and 34 g of salt, to simulate all the sea water on Earth. Then we add one regular-sized (ca 20 ml) ice cube (representing glacial ice) and two teaspoons (ca 10 ml) of groundwater. All of the water that we see around us in lakes and streams and in the atmosphere can be represented by adding three more drops of water from an eyedropper.

Figure 14.4: Representation of the Earth’s water in a 1 litre container. The three drops represent all of the fresh water in lakes, streams, and wetlands, plus all of the water in the atmosphere. Source: Steven Earle (2015) CC BY 4.0 view source

Although the water in the atmosphere is only a small proportion of the total water on Earth, the volume is still very large. At any given time, there is the equivalent of approximately 13,000 km 3 of water in the air in the form of water vapour and water droplets in clouds. Water is evaporated from the oceans, vegetation, and lakes at a rate of 1,580 km 3 per day, and each day nearly the same volume falls back as rain and snow over the oceans and land. Most of the precipitation that falls onto land returns to the ocean in the form of stream flow (117 km 3 /day) and groundwater flow (6 km 3 /day). Most of the rest of this chapter is about this 117 km 3 /day of streamflow.

14.1 Exercise 14.1 How Long Does Water Stay in the Atmosphere?

The residence time of a water molecule in the atmosphere (or any of the other reservoirs) can be estimated by dividing the total amount of water in the reservoir by the rate at which it is removed. For the atmosphere, we know that the reservoir size is 13,000 km 3 , and the rate is 1,580 km 3 /day. If we divide 13,000 km 3 by 1,580 km 3 /day, we get 8.22 days. This means that, on average, a molecule of water stays in the atmosphere for just over eight days. “Average” needs to be emphasized here because some molecules remain in the air for only a few hours, while others may remain in the air for weeks.

The volume of the oceans is 1,338,000,000 km 3 and the rate of removal of water from the oceans is approximately the same as the atmosphere (1,580 km 3 /day). What is the average residence time of a water molecule in the ocean?


13.3 Stream Erosion and Deposition

As we discussed in Chapter 6, flowing water is a very important mechanism for both erosion and deposition. Water flow in a stream is primarily related to the stream’s gradient, but it is also controlled by the geometry of the stream channel. As shown in Figure 13.14, water flow velocity is decreased by friction along the stream bed, so it is slowest at the bottom and edges and fastest near the surface and in the middle. In fact, the velocity just below the surface is typically a little higher than right at the surface because of friction between the water and the air. On a curved section of a stream, flow is fastest on the outside and slowest on the inside.

Figure 13.14 The relative velocity of stream flow depending on whether the stream channel is straight or curved (left), and with respect to the water depth (right). [SE]

Other factors that affect stream-water velocity are the size of sediments on the stream bed — because large particles tend to slow the flow more than small ones — and the discharge, or volume of water passing a point in a unit of time (e.g., m 3 /second). During a flood, the water level always rises, so there is more cross-sectional area for the water to flow in however, as long as a river remains confined to its channel, the velocity of the water flow also increases.

Figure 13.15 shows the nature of sediment transportation in a stream. Large particles rest on the bottom — bedload — and may only be moved during rapid flows under flood conditions. They can be moved by saltation (bouncing) and by traction (being pushed along by the force of the flow).

Smaller particles may rest on the bottom some of the time, where they can be moved by saltation and traction, but they can also be held in suspension in the flowing water, especially at higher velocities. As you know from intuition and from experience, streams that flow fast tend to be turbulent (flow paths are chaotic and the water surface appears rough) and the water may be muddy, while those that flow more slowly tend to have laminar flow (straight-line flow and a smooth water surface) and clear water. Turbulent flow is more effective than laminar flow at keeping sediments in suspension.

Stream water also has a dissolved load, which represents (on average) about 15% of the mass of material transported, and includes ions such as calcium (Ca +2 ) and chloride (Cl-) in solution. The solubility of these ions is not affected by flow velocity.

Figure 13.15 Modes of transportation of sediments and dissolved ions (represented by red dots with + and – signs) in a stream. [SE]

The faster the water is flowing, the larger the particles that can be kept in suspension and transported within the flowing water. However, as Swedish geographer Filip Hjulström discovered in the 1940s, the relationship between grain size and the likelihood of a grain being eroded, transported, or deposited is not as simple as one might imagine (Figure 13.16). Consider, for example, a 1 mm grain of sand. If it is resting on the bottom, it will remain there until the velocity is high enough to erode it, around 20 cm/s. But once it is in suspension, that same 1 mm particle will remain in suspension as long as the velocity doesn’t drop below 10 cm/s. For a 10 mm gravel grain, the velocity is 105 cm/s to be eroded from the bed but only 80 cm/s to remain in suspension.

Figure 13.16 The Hjulström-Sundborg diagram showing the relationships between particle size and the tendency to be eroded, transported, or deposited at different current velocities

On the other hand, a 0.01 mm silt particle only needs a velocity of 0.1 cm/s to remain in suspension, but requires 60 cm/s to be eroded. In other words, a tiny silt grain requires a greater velocity to be eroded than a grain of sand that is 100 times larger! For clay-sized particles, the discrepancy is even greater. In a stream, the most easily eroded particles are small sand grains between 0.2 mm and 0.5 mm. Anything smaller or larger requires a higher water velocity to be eroded and entrained in the flow. The main reason for this is that small particles, and especially the tiny grains of clay, have a strong tendency to stick together, and so are difficult to erode from the stream bed.

It is important to be aware that a stream can both erode and deposit sediments at the same time. At 100 cm/s, for example, silt, sand, and medium gravel will be eroded from the stream bed and transported in suspension, coarse gravel will be held in suspension, pebbles will be both transported and deposited, and cobbles and boulders will remain stationary on the stream bed.

Exercise 13.3 Understanding the Hjulström-Sundborg Diagram

Refer to the Hjulström-Sundborg diagram (Figure 13.16) to answer these questions.

1. A fine sand grain (0.1 mm) is resting on the bottom of a stream bed.

(a) What stream velocity will it take to get that sand grain into suspension?

(b) Once the particle is in suspension, the velocity starts to drop. At what velocity will it finally come back to rest on the stream bed?

2. A stream is flowing at 10 cm/s (which means it takes 10 s to go 1 m, and that’s pretty slow).

(a) What size of particles can be eroded at 10 cm/s?

(b) What is the largest particle that, once already in suspension, will remain in suspension at 10 cm/s?

A stream typically reaches its greatest velocity when it is close to flooding over its banks. This is known as the bank-full stage, as shown in Figure 13.17. As soon as the flooding stream overtops its banks and occupies the wide area of its flood plain, the water has a much larger area to flow through and the velocity drops significantly. At this point, sediment that was being carried by the high-velocity water is deposited near the edge of the channel, forming a natural bank or levée.

Figure 13.17 The development of natural levées during flooding of a stream. The sediments of the levée become increasingly fine away from the stream channel, and even finer sediments — clay, silt, and fine sand — are deposited across most of the flood plain. [SE]


13.4 Stream Types

Stream channels can be straight or curved, deep and slow, or rapid and choked with coarse sediments. The cycle of erosion has some influence on the nature of a stream, but there are several other factors that are important.

Youthful streams that are actively down-cutting their channels tend to be relatively straight and are typically ungraded (meaning that rapids and falls are common). As shown in Figures 13.1 and 13.18, youthful streams commonly have a step-pool morphology, meaning that the stream consists of a series of pools connected by rapids and waterfalls. They also have steep gradients and steep and narrow V-shaped valleys — in some cases steep enough to be called canyons.

Figure 13.18 The Cascade Falls area of the Kettle River, near Christina Lake, B.C. This stream has a step-pool morphology and a deep bedrock channel. [SE]

In mountainous terrain, such as that in western Alberta and B.C., steep youthful streams typically flow into wide and relatively low-gradient U-shaped glaciated valleys. The youthful streams have high sediment loads, and when they flow into the lower-gradient glacial valleys where the velocity isn’t high enough to carry all of the sediment, braided patterns develop, characterized by a series of narrow channels separated by gravel bars (Figure 13.19).

Figure 13.19 The braided channel of the Kicking Horse River at Field, B.C. [SE]

Braided streams can develop anywhere there is more sediment than a stream is able to transport. One such environment is in volcanic regions, where explosive eruptions produce large amounts of unconsolidated material that gets washed into streams. The Coldwater River next to Mt. St. Helens in Washington State is a good example of this (Figure 13.20).

Figure 13.20 The braided Coldwater River, Mt. St. Helens, Washington. [SE]

A stream that occupies a wide, flat flood plain with a low gradient typically carries only sand-sized and finer sediments and develops a sinuous flow pattern. As you saw in Figure 13.14, when a stream flows around a corner, the water on the outside has farther to go and tends to flow faster. This leads to erosion of the banks on the outside of the curve, deposition on the inside, and formation of a point bar (Figure 13.21). Over time, the sinuosity of the stream becomes increasingly exaggerated, and the channel migrates around within its flood plain, forming a meandering pattern.

Figure 13.21 The meandering channel of the Bonnell Creek, Nanoose, B.C. The stream is flowing toward the viewer. The sand and gravel point bar must have formed when the creek was higher and the flow faster than it was when the photo was taken. [SE]

A well-developed meandering river is shown in Figure 13.22. The meander in the middle of the photo has reached the point where the thin neck of land between two parts of the channel is about to be eroded through. When this happens, another oxbow lake will form like the others in the photo.

Figure 13.22 The meandering channel of the Nowitna River, Alaska. Numerous oxbow lakes are present and another meander cutoff will soon take place. [Oliver Kumis, http://commons.wikimedia.org/wiki/File:Nowitna_river.jpg]

Exercise 13.4 Determining Stream Gradients

Gradient is the key factor controlling stream velocity, and of course, velocity controls sediment erosion and deposition. This map shows the elevations of Priest Creek in the Kelowna area. The length of the creek between 1,600 m and 1,300 m elevation is 2.4 km, so the gradient is 300/2.4 = 125 m/km.

1. Use the scale bar to estimate the distance between 1,300 m and 600 m and then calculate that gradient.

2. Estimate the gradient between 600 and 400 m.

3. Estimate the gradient between 400 m on Priest Creek and the point where Mission Creek enters Okanagan Lake.

At the point where a stream enters a still body of water — a lake or the ocean — sediment is deposited and a delta forms. The Fraser River has created a large delta, which extends out into the Strait of Georgia (Figure 13.23). Much of the Fraser delta is very young in geological terms. Shortly after the end of the last glaciation (10,000 years ago), the delta did not extend past New Westminster. Since that time, all of the land that makes up Richmond, Delta, and parts of New Westminster and south Surrey has formed from sediment from the Fraser River. (You can see this in more detail at Geoscape Vancouver http://www.cgenarchive.org/vancouver-fraserdelta.html.)

Figure 13.23 The delta of the Fraser River and the plume of sediment that extends across the Strait of Georgia. The land outlined in red has formed over the past 10,000 years. [September 2011, SE after NASA: http://earthobservatory.nasa.gov/IOTD/view.php?id=77368]


Davisian Model of Geographical Cycle of Erosion | Geography

In this article we will discuss about the Davisian model of geographical cycle of erosion.

William Morris Davis, an American geomorphologist, was the first geomorphologist to present a general theory of landform development.

In fact, his theory is the outcome of a set of theories and models presented by him from time to time e.g.:

(i) ‘complete cycle of river life’, propounded in his essay on. ‘The Rivers and Valleys of Pennsylvania’ in 1889,

(ii) ‘geographical cycle’ in 1899,

He postulated the cyclic concept of progressive development of erosional stream valleys under the concept of ‘complete cycle of river-life’, while through ‘geographical cycle’ he described the sequential devel­opment of landforms through time.

The general theory of landform development of Davis is not the ‘geographical cycle’ as many of the geomorphologists believe.

His theory may be expressed as follows:

‘There are sequential changes in landforms through time (passing through youth, mature and old stages) and these sequential changes are directed to­wards a well-defined end product-development of peneplain.’

The basic goal of Davisian model of geographi­cal cycle and general theory of landform development was to provide basis for a systematic description and genetic classification of landforms. The reference sys­tem of Davisian general theory of landform develop­ment is ‘that landforms change in an orderly manner as processes operate through time such that under uni­form external environmental conditions an orderly sequence of landform develops’.

Various models were developed on the basis of this reference system e.g., normal cycle of erosion, arid cycle of erosion, glacial cycle of erosion, marine cycle of erosion etc. Thus, ‘geographical cycle’ is one of the several possible models based on Davis’ reference system of landform development.

Davis postulated his concept of ‘geographical cycle’ popularly known as ‘cycle of erosion’ in 1899 to present a genetic classification and systematic descrip­tion of landforms.

His ‘geographical cycle’ has been defined in the following manner:

‘Geographical cycle is a period of time during which an uplifted landmass undergoes its transforma­tion by the process of land-sculpture ending into low featureless plain or peneplain (Davis called peneplane).”

According to Davis three factors viz. structure, process and time play important roles in the origin and development of landforms of a particular place.

These three factors are called as ‘Trio of Davis’ and his concept is expressed as follows:

“Landscape is a function of structure, process and time” (also called as stages by Davis’ followers).

Structure means lithological (rock types) and structural characteristics (folding, faulting, joints etc.) of rocks. Time was not only used in temporal context by Davis but it was also used as a process itself leading to an inevitable progression of change of landforms. Process means the agents of denudation including both, weathering and erosion (running water in the case of geographical cycle).

The basic premises of Davisian model of ‘geo­graphical cycle’ included the following as- sumptions made by Davis:

(1) Landforms are the evolved products of the interactions of endogenetic (diastrophic) forces originating from within the earth and the external orexogenetic forces originating from the atmosphere (denudational processes, agents of weathering and erosion-rivers, wind, groundwater, sea waves, glaciers and periglacial processes).

(2) The evolution of landforms takes place in an orerly manner in such a way that a systematic sequence of landforms is developed through time in response to an environmental change.

(3) Streams erode their valleys rapidly down­ward until the graded condition is achieved.

(4) There is a short-period rapid rate of upliftment in land mass. It may be pointed out that Davis also described slower rates of upliftment if so desired.

(5) Erosion does not start until the upliftment is complete. In other words, upliftment and ero­sion do not go hand in hand. This assumption of Davis became the focal point of severe attacks by the critics of the cyclic concept.

Davis has described his model of geographical cycle through a graph below (fig. 16.1):

The cycle of erosion begins with the upliftment of landmass. There is a rapid rate of short-period upliftment of landmass of homogeneous structure. This phase of upliftment is not included in the cyclic time as this phase is, in fact, the preparatory stage of the cycle of erosion.

Fig. 16.1 represents the model of geographical cycle wherein UC (upper curve) and LC (lower curve) denote the hill-tops or crests of water divides (absolute reliefs from mean sea level) and valley floors (lowest reliefs from mean sea level) respectively.

The horizontal line denotes time whereas vertical axis depicts altitude from sea level. AC repre­sents maximum absolute relief whereas BC denotes initial average relief. Initial relief is defined as differ­ence between upper curve (summits of water divides) and lower curve (valley floors) of a landmass. In other words, relief is defined as the difference between the highest and the lowest points of a landmass. ADG line denotes base level which represents sea level. No river can erode its valley beyond base level (below sea level).

Thus, base level represents the limit of maximum vertical erosion (val­ley deepening) by the rivers. The upliftment of the landmass stops after point C (fig. 16.1) as the phase of upliftment is complete.

Now erosion starts and the whole cycle passes through the following three stages:

Erosion starts after the com­pletion of the upliftment of the landmass. The top- surfaces or the summits of the water divides are not affected by erosion because the rivers are small and widely spaced. Small rivers and short tributaries are engaged in head-ward erosion due to which they extend their lengths.

The process is called stream lengthening (increase in the lengths of the rivers). Because of steep slope and steep channel gradient rivers actively deepen their valleys through vertical erosion aided by pothole drilling and thus there is gradual increase in the depth of river valleys. This process is called valley deepening. The valleys become deep and narrow characterized by steep valley side slopes of convex plan.

The youthful stage is characterized by rapid rate of vertical erosion and valley deepening because:

(i) The channel gradient is very steep,

(ii) Steep channel gradient increases the velocity and kinetic energy of the river flow,

(iii) Increased channel gradient and flow velocity increases the transporting capacity of the rivers,

(iv) Increased transporting capacity of the rivers allow them to carry big boulders of high calibre (more angular boulders) which help in valley incision (valley deepening through vertical erosion) through pothole drilling.

The lower curve (LC valley floor) falls rapidly because of valley deepening but the upper curve (UC summits of water divides or interestream areas) remain almost parallel to the horizontal axis (AD, in fig. 16.1) because the summits or upper parts of the landmass are not affected by erosion. Thus, relative relief continues to increase till the end of youthful stage when ultimate maximum relief (EF, in fig. 16.1) is attained.

In nutshell, the youthful stage is characterized by the following char­acteristic features:

(i) Absolute height remains constant (CF is par­allel to the horizontal axis) because of insig­nificant lateral erosion.

(ii) Upper curve (UC) representing summits of water divides is not affected by erosion.

(iii) Lower curve (LC) falls rapidly because of rapid rate of vally-deepening through vertical erosion.

(iv) Relief (relative) continues to increase.

(v) Valleys are of V shape characterized by con­vex valley side slopes.

(vi) Overall valley form is gorge or canyon.

(vii) Long profiles of the rivers are characterized by rapids and water falls which gradually diminish with march of time and these prac­tically disappear by the end of late youth. The main river is graded.

The early mature stage is heralded by marked lateral erosion and well integrated drainage network. The graded conditions spread over larger area and most of the tributaries are graded to base level of erosion. Vertical erosion or valley deep­ening is remarkably reduced. The summits of water divides are also eroded and hence there is marked fall in upper curve (UC) i.e., there is marked lowering of absolute relief.

Thus, absolute relief and relative relief, both decrease. The lateral erosion leads to valley widening which transforms the V-shaped valleys of youthful stage into wide valleys with uniform or recti­linear valleys sides. The marked reduction in valley deepening (vertical erosion or valley incision) is be­cause of substantial decrease in channel gradient, flow velocity and transporting capacity of the rivers.

(3) Old Stage:

Old stage is characterized by almost total absence of valley incision but lateral erosion and valley widening is still active process. Water divides are more rapidly eroded. In fact, water divides are reduced in dimension by both, down-wasting and back-wasting. Thus, upper curve falls more rapidly, meaning thereby there is rapid rate of decrease in absolute height. Relative or available relief also de­creases sharply because of active lateral erosion but no vertical erosion. Near absence of valley deepening is due to extremely low channel gradient and remarkably reduced kinetic energy and maximum entropy.

The valleys become almost flat with concave valley side slopes. The entire landscape is dominated by graded valley-sides and divide crests, broad, open and gently sloping valleys having extensive flood plains, well developed meanders, residual convexo-concave monadnocks and extensive undulating plain of ex­tremely low relief. Thus, the entire landscape is trans­formed into peneplain. As revealed by fig. 16.1 the duration of old stage is many times as long as youth and maturity combined together.

Evaluation of the Davisian Model of Geographical Cycle:

Davisian model of geographical cycle received world-wide recognition and the geomorphologists read­ily applied his model in their geomorphological inves­tigations. The academic intoxication of Davis’ model of cycle of erosion continued from its inception in 1899 to 1950 when the model had to face serious challenges though his model was being criticised from the very beginning of its postulation.

S. Judson (1975) while commenting on Davis’ geographical cycle remarked, “his grasp of time, space and change his command of detail and his ability to order his information and frame his arguments remind us again that we are in the presence of a giant.” C.G. Higgins (1975) admitted that “Davis system came to dominate both teaching and research in the descriptive and genetic-historical aspects of geomorphology. Its continued validity is attested in part by continuing objections to it by recent critics such as R.C. Flemal (1971) and C.R. Twidale (1975) that such an obviously flawed doctrine could have enjoyed such prolonged popularity among large segment of the geomorphic community suggests that there must be compelling reasons for its appeal”.

Positive Aspects of Davis’ Model:

(1) Davis’ model of geographical cycle was highly simple and applicable.

(2) He presented his model in a very lucid, com­pelling and disarming style using very simple but expressive language. Commenting on the language of Davis used in his model Bryan remarked, “Davis rhetorical style is just ad­mired and several generations of readers be­came, slightly bemused by long though mild intoxication of the limpid prose of Davis remarkable essay.”

(3) Davis based his model on detailed and careful field observations.

(4) Davis’ model came as a general theory of landform development after a long gap after Hutton’s cyclic nature of the earth history.

(5) This model synthesized the current geologi­cal thoughts. In other words, Davis incorpo­rated the concept of ‘base level’ and genetic classification of river valleys, the concept of ‘graded streams’ of G.K. Gilbert and French engineers’ concept of ‘profile of equilibrium’ in his model.

(6) His model is capable of both predictions and historical interpretation of landform evolu­tion.

Negative Aspects of Davis Model:

(1) Davis concept of upliftment is not accept­able. He has described rapid rate of upliftment of short duration but as evidenced by plate tectonics upliftment is exceedingly a show and long continued process.

(2) Davis’ concept of relationship between upliftment and erosion is erroneous. Accord­ing to him no erosion can start unless upliftment is complete. Can erosion wait for the comple­tion of upliftment? It is a natural process that as the land rises, erosion begins. Davis has answered this question.

He admitted that he deliberately excluded erosion from the phase of upliftment because of two reasons:

(i) To make the model simple, and

(ii) Erosion is insignificant during the phase of upliftment.

(3) The Davisian model requires a long period of crustal stability for the completion of cycle of erosion but such eventless long period is tectonically not possible as is evidenced by plate tectonics according to which plates are always in motion and the crust is very often affected by tectonic events. Davis has also offered explanation to this objection. Accord­ing to him if crustal stability for desired pe­riod is not possible, the cycle of erosion is interrupted and fresh cycle of erosion may start.

(4) Walther Penck objected to over emphasis of time in Davis’ model. In fact, Davisian model envisages ‘time-dependent series’ of landform development whereas Penck pleaded for time- independent series’ of landforms. According to Penck landforms do not experience pro­gressive and sequential changes through time. He, thus, pleaded for deletion of ‘time’ (stage) from Davis’ ‘trio’ of ‘structure, process and time’. According to Penck “geomorphic forms are expressions of the phase and rate of upliftment in relation to the rate of degrada­tion”.

(5) A.N. Strahler, J.T. Hack and R.J. Chorley and several others have rejected the Davisian con­cept of ‘historical evolution’ of landforms. They have forwarded the dynamic equilib­rium theory for the explanation of landform development.

It may be pointed out that non- cyclic concept of ‘dynamic equilibrium’ as valid substitute of Davis’ cyclic concept of landform development and other so called ‘open system’ and non-cyclic models of landform development could not arouse any enthusiasm among the modern geomorphologists.

It may be concluded in the words of Charles Higgins (1975) that “If the desire for a cyclic, time- dependent model stems from an unacknowledged fun­damental postulate that the history of the earth is itself cyclic, then no non-cyclic theory of landscape develop­ment can win with general acceptance until this postu­late is unearthed, examined, and possibly rejected.”


10.6 KARST TOPOGRAPHY

The sedimentary rock limestone is composed of the mineral calcite, which is water soluble, meaning it will dissolve in water that is weakly acidic. In humid areas where limestone is found, water dissolves the rock, forming large cavities and depressions which vary in size and shape. As more dissolution occurs, the caves become unstable and collapse, creating sinkholes. These broad, crater-like depressions are typical of karst topography (Figure 10.12), named after the Karst region in Slovenia. Karst topography is characterized by sinkholes, sink lakes (sinkholes filled with water), caves, and disappearing streams (surface streams that disappear into a sinkhole).

Figure 10.12 | Appearance of a sinkhole on a topographic map.
Source: Randa Harris (2015) CC BY-SA 3.0 view source

Living in karst topography poses challenges for building infrastructure sinkholes can appear rather rapidly and cause great damage to any structures above them. In Canada, there are examples of karst across the country aside from on the Canadian Shield. For example, there is karst topography in Wood Buffalo National Park and near Norman Wells, NWT (Figure 10.13) and in southern Ontario.

Figure 10.13 | Sinkhole near Norman Wells, NWT.
Source: Dennis McBeth (1994) CC BY-SA 4.0.


6.3 Summary

The topics covered in this chapter can be summarized as follows:

6.3.1 What Is a Rock?

A rock is a solid mass of geological materials. Geological materials include individual mineral crystals, inorganic non-mineral solids like glass, pieces broken from other rocks, and even fossils.

6.3.2 The Rock Cycle

There are three main types of rock. Igneous rocks form when melted rock cools and solidifies. Sedimentary rock forms from fragments of other rocks, or when crystals precipitate from solution. Metamorphic rocks form when existing rocks are altered by heat, pressure, and/or chemical reactions. The rock cycle summarizes the processes that contribute to transformation of rock from one type to another. The rock cycle is driven by Earth’s internal heat, and by processes happening at the surface that are driven by solar energy.


Sediment Transport Depends on Stream Velocity and Turbulence

If you drop a piece of gravel into a glass of water, it will sink quickly to the bottom. If you drop a grain of sand into the same glass, it will sink more slowly. A grain of silt will take longer yet to get to the bottom, and a particle of fine clay will take a long time settle out. The rate of settling is determined by the balance between gravity and friction, as shown in Figure 14.16.

Figure 14.16 How quickly a grain settles to the bottom of a stream depends on its mass (affecting the force of gravity acting on it), and the friction between the grain and the water, which slows the fall of the grain. Source: Steven Earle (2015) CC BY 4.0 view source

One of the key principles of sedimentary geology is that the ability of a moving medium (air or water) to move sedimentary particles and keep them moving is dependent on the velocity of flow. The faster the medium flows, the larger the particles it can move. As you probably know from intuition and from experience, streams that flow rapidly tend to be turbulent (flow paths are chaotic and the water surface appears rough) and the water may be muddy. In contrast, streams that flow more slowly tend to have laminar flow (straight-line flow and a smooth water surface) and clearer water. Turbulent flow is more effective than laminar flow at keeping sediments suspended within the water.

Particles within a stream are transported in different ways depending on their size (Figure 14.17). Large particles which rest on the stream bed are known as the bedload. The bedload m ay only be transported when the flow rate is rapid and under flood conditions. They are transported by saltation (bouncing along, and colliding with other particles) and by traction (being pushed along by the force of the flow).

Smaller particles may rest on the bottom occasionally, where they can be transported by saltation and traction, but they can also be held in suspension in the flowing water (the suspended load ), especially at higher flow velocities.

Stream water also has a dissolved load, which represents (on average) about 15% of the mass of material transported, and includes ions such as calcium (Ca +2 ) and chloride (Cl – ) in solution. The solubility of these ions is not affected by flow velocity.

Figure 14.17 Modes of transportation of sediments and dissolved ions (represented by red dots with + and – signs) in a stream . Source: Steven Earle (2015) CC BY 4.0 view source

If you look at a typical stream, there are always some sediments being deposited, some staying where they are, and some being eroded and transported. This changes over time as the discharge of the river changes in response to changing weather conditions.


What is the Cycle of Erosion?

The concept of cycle of erosion was formulated by William Morris Davis, an American geomorphologist, towards the end of the nineteenth century. It is a concept of an orderly sequence of evolu­tionary stages of fluvial erosion in which relief of the available landmass declines with time to reach a late stage when the landscape becomes a peneplain .

The cycle of erosion, as envisioned by Davis, has its initial stage at a time when the landmass is rapidly elevated by internal earth forces, followed by a very long period of tectonic quies­cence.

Once raised high above sea level as a landmass, streams come into existence and erosion begins to operate on the uplifted mass which is gradually worn down almost to a plain. The landmass may, at some later time, be rejuvenated and the cycle begins again and remnants of the earlier cycle of erosion are preserved at new and higher levels.

In a normal cycle three stages have been recognized as: youth stage, mature stage and old stage. These follow each other in a regular sequence.

Youth Stage

In this stage the river flows along an uneven surface and there is intensive bottom erosion, the gradients are steep and the erosion is rapid. The rapid deepening of the channel leads to the formation of V-shaped valleys.

Thus during the youth stage of a river, the valley form undergoes vigorous development, particularly in depth and head ward growth. Lakes, rapids, waterfalls, steep-sided valleys and gorges are of common occurrence during this stage. Besides, the phenomenon of river-capture or river piracy takes place in this stage. Youthful rivers have an irregular long profile (thalweg) from source to mouth.

River Capture

When one of the two rivers flowing in opposite directions from a single divide, becomes more effective in erosion due to steeper gradient (when the slopes are unequally inclined), the divide gradually recedes towards the side with the gentler slope.

In other words, the river with steeper gradient extends its valley head ward thus causing a shift of the divide against the river with gentle gradient.

Gradually deepening of the valley continues head ward with pronounced dissection of the ridge (divide). Sometimes this head ward migration of one river enables it to reach the river on the other side.

But, as the first river has a steeper gradient than the other one, the course of the second river gets diverted and its water starts draining through the channel of the first river. This process of diversion of a river by the head ward migration of another river is known as River-Capture or River-piracy.

The point where the course of the second river is diverted is known as the Elbow of capture. The captured river is known as Misfit and the deserted part of its channel through which no water flows is termed as the Wind-gap.

Mature Stage

In this stage rivers flow with a graded profile i.e. it attains a profile of equilibrium. The land mass is fully dissected and a well-integrated drainage system is developed. Ridges and valleys develop prominently.

Flood plains develop and river meandering takes place. The topogra­phy consists of features such as: hogbacks, cuestas, mesa, butte, meanders, oxbow lakes, natural bridge, flood plains, alluvial fans etc.

In this stage the gradients are gentle and the velocity is low. Accord­ingly the river lose most of its erosive power and flow in a sluggish manner. In old age a river has maximum meandering. The river at this age does little of erosion and transportation but is mostly engaged in deposition. This stage is characterised by the development of distribu­taries and the river flows almost at the base level of erosion.

The topography consists of features like peneplains, natural lev­ees, deltas etc.

Most of the cycles of erosion do not reach the final stage, as sometime during their operation either climatic or tectonic disturbances take place, and thus results in an incomplete or partial cycle.


Watch the video: Earth Science - Stream Erosion u0026 Deposition (October 2021).