G4-RIVERS

RIVERS

. 6.1 A river flows down the hill

Of all the geological processes, rivers may have the greatest impact on people. We depend upon rivers for drinking , irrigation, electricity travel etc. Fertile floodplains of the rivers have fostered human progress since the dawn of civilization. As the dominant agent of landscape alteration, rivers have shaped much of our physical environment. A river starts as a tiny trickle, or rill, on a slope. Rainfall, snowfall, a spring, or the melting of glacial ice may be its source. As it flows downhill it is joined by other trickles to make a brook. Several brooks add their waters to form a stream, which broadens into a creek. As the water continues its downward journey, it gains in volume and finally becomes a river.

A river does three different kinds of work. They are:

1. Erosion, 2. Transportation and 3. Deposition

1. Erosion: Rivers erode their channels in several different ways: a.by lifting loosely consolidated particles, b.by abrasion, and c.by solution activity. The last of these is by far the least significant. Although some erosion results from the solution of soluble bed rock and channel debris, most of the dissolved material in a river is contributed by the groundwater.

a. By lifting: When the flow of water in the river is turbulent, the water whirls and eddies. When an eddy is suffeciently strong, it can dislodge rock pieces and lift them into the moving water. In this manner the force of running water swiftly erodes lossely consolidated materials on the bed and banks of the river. Stronger the current, more effectively the stream will lift the rock pieces. In some instances water is forced into cracks and bedding planes with sufficient strength and moves pieces of rock from the bed and banks of the channel.

b. By abrasion: Observing a muddy river will reveal that currents of water can lift and carry debries. However, it is not obvious that a river is capable of eroding solid rock. Nevertheless, the solid particles carried by a river are capable of abrading a bedrock channel. The ceaseless bombardment of these rock pieces against the bed and banks of the river channel have created many steep sided gorges cut through solid rock. In addtion, the individual rock pieces are also abraded by the many impacts with the channel and with one another. Thus, the rock pieces by scraping, rubbing and bumping, erodes bedrock channel and the banks. Simultaneously these rock pieces also get reduced in their size and smoothend to form pebbles. This process of erosion is known as abrasion.

Abrading tools such as pebbles and sand particles that are swirling in fast-moving eddies, act as drill to bore holes on the river beds. In course of time, the rotational motion of pebbles and sand particles create rounded depressions several meter across and just as deep. Such large depressions are known as potholes. As the abrading particles wear down to nothing, they are replaced by new ones that continue to drill the river bed.

2. Transportation: The solid material carried by a river is called its load. A river transports its load in three different ways. They are:

a. in solution (dissolved load)

b. in suspension ( suspended load) and

c. by saltation (bed load).

a. In solution: Some of the material a river carries in solution, is acquired when running water dissolves bedrock channel. However, the greatest portion of the dissolved load transported by most rivers is supplied by groundwater. As water percolates through the ground, it first acquires soluable soil compounds. As the water seeps deeper through cracks and pores in the bedrock below, additional mineral matter is dissolved. Eventually much of mineral-rich water finds its way into rivers. The velocity of river flow, which is very important to the transportation of solid particles, has essentially no effect upon a river’s ability to carry its dissolved load. After material is in solution, it goes wherever the stream goes, regardless of velocity.

b. In Suspension: The major portion of the river load is carried in suspension. Usually only fine sand, silt and clay-sized particles can be carried this way, but during floods larger particles are carried in suspension as well. The finest materials such as silt and clay remain suspended in the water for most of their journey and sink very slowly. No. 6.2 Potholes

3.By saltation: A portion of a river’s bedload consists of solid rock fragments. They are too large to transport in suspension. So they are moved along the bottom by saltation, rolling, and sliding. Particles moving by saltation appears to jump or skip along the river bed. This occurs as particles are propelled upward by collisions or sucked upward by the current and then carried down stream a short distance until gravity pulls them back to the bed of the river. Particles that are too large or heavy to move by saltation, either rolled or pushed along the bottom,depending upon their size and shape. Unlike the suspended and dissolved loads, which are constantly in motion, the bed load is in motion only intermittently. The movement of bedload is determined by two factors. They are:

a. the competence and

b. the capcity of the river .

a. The Competence of the river: Competence is a measure of the maximum size of particles the river is capable of transporting. The river’s velocity determines its competence; the stronger the flow, the larger the particles it can carry in suspension or as bed load. It is a general rule that the competence of a river increases as the square of its velocity. Thus, if the velocity of a stream doubles, the impact force of the water increases four times; If the velocity triples, the force increases nine times, and so forth.

b. The capacity of the river: The maximum load of solid particles that a river can transport is termed its capacity. The capacity of a river is related to its discharge. The greater the amount of water flowing in a river, the greater the river’s capacity for hauling sediments. Hence, the large boulders that are often visible during a low-water stage and seem immovable, can be transported during floodstage because of stream’s increased velocity. With rising velocity the water becomes more turbulent, and larger and larger particles are moved by the river. In course of just a few days, or perhaps just a few hours, a stream in floodstage can erode and transport more sediments than it does during months of normal flow.

3. Deposition:

Whenever a river’s velocity subsides, its competence is reduced. As the velocity of a river diminishes, particles of sediment are deposited according to size. As the river flow drops below the criticle settling velocity of a certain particle size, sediment in that category begins to settle. Thus river transport provides a mechanism by which solid particles of various sizes are seperated. This process, called sorting, explains why particles of similar size are deposited together. The well sorted material typically deposited by a river is called alluvium. Many different depositional features are composed of alluvium. Alluvium deposits are found in three places. They are:

a.Foothill deposits

b. River bed deposits and

c.Plain deposits

a. Foothill deposits: When a river leaves a narrow valley in the mountainous terrain and comes out suddenly on to a broad, flat plain such as a plateau, or a valley floor , the velocity of the river is reduced. So, the river deposits its load in response to the abrupt drop in gradient, combined with the change from a narrow channel of a mountainous stream to the unconfined flow on the slopes of the plain. The sudden drop in velocity causes the river to dump its load of sediments quickly in a distinctive cone-or-fan-shaped accumulation at the foot hills. This accumulation is called foothill deposit or alluvial fans. The surface of the alluvial fan slopes outward in a broad arc from an apex at the mouth of the steep valley. Usually, coarse material is dropped near the apex of the fan, while finer material is carried toward the base of the deposit. One can see such alluvial fans in the Kambam valley of Madurai district.

2. River-bed deposits: In a river, volume of water flowing into is highly variable. During rainy season, the volume of water in a river is very high. In contrast, during dry season the volume of water in the same river is very low or completly dry.. When the volume of water in a river reduces, its velocity and the capcity to transport the coarser materials also reduces. The river deposits some of its materials on its bed. Such deposits are known as ‘river bed deposits’. River bed deposits are often composed of sand and gravel, the coarser components of a river’s load and are commonly referred to as bars. Such features, however, are only temporary, for, the material will be picked up again by the running water and be transported farther down stream.

3. Plain deposits: Before entering into plain, the river deposits all the larger particles and other smaller particles on the river bed at the foot hill . Only very fine particles are transported into the plain region. As the slope of the plain is very gentle, the velocity of the river is very much reduced. The forward motion of the river is very much arrested at the place where it enters the sea. As a result, finer particles are deposited near the mouth of the river. Such deposits are called as ‘plain deposits’. One form of plain deposits is known as Delta.Fig.No. 6.4 River bed deposits

Did you know?

Many large rivers have deltas that extend over thousands of squre kilometers. The delta of the Mississippi river is one such feature. It resulted from the accumulation of huge quantities of sediment derived from the vast region drained by the river and its tributaries. During the past 5000-6000 years, the mississipi river has built a series of seven subdeltas. The numbers in the figure indicate the order in which the subdeltas were deposited. The present bird foot delta (number7) represents the activity of the past 500 years.

Course of a river and its associated features: Nearly all rivers have an upper, middle, and lower course. Each level has its own characteristics.

The Upper Course: The upper course begins at higher elevations. Here the river is smaller and usually has a rapid, tumbling flow that cuts a narrow channel through rocky hills or mountains. Over thousands or millions of years the river erodes, or wears away, soil and rocks and carves a canyon or a deep, V-shaped valley. The type of formation it creates depends on the force of the river and the type of material it erodes.

V-shaped valley: A deep, steepwalled, V-shaped valley cut by a river through resistant rock is often called a canyon, from the Spanish word canon, meaning “tube.” Such valleys often occur in the upper courses of rivers, where the stream has a strong, swift current that digs its valley relatively rapidly. Smaller valleys of similar appearance are called gorges . The largest and most famous canyons have been cut through arid or semiarid lands by swift streams fed by rain or melting snow of moister regions. The walls remain steep and angular because they are not worn and softened by frequent rainfall and surface drainage.

The middle course: When the river descends to lower elevations, it runs more slowly over the gently sloping land lley

of its middle course.Its current no longer has the force to carry stones or gravel. This material drops to the riverbed, where it forms bars of sand or gravel or builds islands. These formations are continually changing shape as the river deposits or erodes material. The formations affect the river in turn by altering its course. River currents swing toward one bank or the other, gradually undercutting the banks and widening a V-shaped valley into a U shape.

As the river flows downstream it reaches the still gentler slope of its lower course. It drops more of its load than it did upstream and begins to build up its bed instead of tearing it down. The valley has been eroded into a wide plain. The river swings in great S-shaped curves, forming loops called meanders

Did you know?

The Grand Canyon, one of the most majestic natural wonders in the world, is nature’s handiwork. Nature’s greatest example of sculpture, the Grand Canyon in northern Arizona is the most spectacular canyon in the world. It is a 446-kilometer long gorge cut through high plateaus by the Colorado River. It is noted for its fantastic shapes and colors. Within the walls of the canyon stand imposing peaks, canyons, and ravines. In general, the color of the canyon is red, but each stratum or group of strata has a distinctive colour such as gray, delicate green or pink, and, in its depths, brown, gray, or violet. No other place on Earth compares with the kilometerdeep Grand Canyon for its record of geological events. Some of the canyon’s rocks date back about 4 billion years. The river’s speed and such “cutting tools” as sand, gravel, and mud account for its incredible cutting capacity.

Oxbow lakes: Meandering channels form where streams are flowing over a relatively flat landscape with a broad floodplain. When a river floods, it may cut across the narrow part of the loop, making a new, shorter channel. The loop is left as a lake known as an oxbow lake.

The lower course: At times heavy rain or melting snow rushes from the upper or middle course of a river into the shallow channel of the lower course. The river floods the surrounding country, leaving a thin layer of sediment. If the flooding is seasonal, in time the layers of sediment accumulate and build a broad, fertile floodplain.

Flood Plain: As its name implies, a floodplain is that part of a valley that is inundated during a flood. Most rivers are bordered by floodplains. Although some are impressive features that are many kilometers across, others are very modest, having widths of just a few meters.

Levees : Rivers that occupy valleys with broad, flat floors sometime create a landform called a natural levee that parallels the stream channel. Natural levees are built by successive floods over a period of many years. Levees are ridges found along the sides of the stream channel composed of sand or gravel. Levees are approximately one half to four times the channel width in diameter. The area behind the levee is characteristically poorly drained for the obvious reason that water cannot flow up the levee and into the river. Marshes called back swamps often result.Upon retreat of the flood waters, stream velocity is reduced causing the deposition of alluvium. Repeated flood cycles over time can result in the deposition of many successive layers of alluvial material. The well-sorted materials typically deposited by the stream is called alluvium, a general term applicable to any river-deposited sediments. Some of these features may be found within river channels, some occur on the valley floor adjacent to the channel, and some exist at the mouth of river.

So far we have learnt about how a river works and the different features found along the course of a river. Now Let us take river Cauvery that run accross Tamilnadu as an example and try to understand the different course of a river. River Cauvery is the only major river in Tamilnadu. This is an interstate river with many tributaries lying in Karnataka and Tamilnadu States. It has a catchment area of 80,290 Sq.kms., of which around 44,000 Sq.kms., is in Taminadu.

Upper course of river cauvery: River Cauvery originates from Coorg in Karnataka and it flows through the deep valleys of the mountain. The river cauvery is so narrow at its origin that a sheep could jump across it easily. The important tributaries that join the river Cauvery are the Hemavathi, the Lakshmanatheertha, and the Kabini. After crossing the mountainous region, cauvery river flows through Deccan plateau. The river enters Tamilnadu state at Hogenekal falls, in Dharmapuri district.

Middle course of river Cauvery: The tributaries such as Bhavani,Amaravathi, and Noyyal join the river cauvery while flowing through the Deccan plateau. As the volume in the cauvery increases, its bank becomes wider. Hence, The cauvery that flows in Tamilnadu is known as Aganda (wide) Cauvery.AUVERY BASIN

Lower course of river cauvery: The Aganda cauvery enters the the plain, and branches off into northern and southern branch near Thiruchirapalli district. The Coleroon, the northern branch is really the main distributary of the river and carries the bulk of floodwater to the sea. It flows in an eastern direction and before entering into sea forms a delta, little south of Portonova town. The southern branch, retains the name cauvery and branches off into Vennaru, Vettaru and Vadavaru. Eventually, before entering into sea at Kaveripattinam about 12 kms., north of Tranquebar forms a delta.

Points to remember:

1. The rotational motion of the sand and pebbles act as a drill to bore the holes. These smooth depressions are known as potholes.

2. A deep, steep-walled, V-shaped valley cut by a river through resistant rock is often called a canyon. Smaller valleys of similar appearance are called gorges .

3. The valley has been eroded into a wide plain . In such a the river swings in great S-shaped curves, forming loops called meanders.

4. When a river floods, it may cut across the narrow part of the loop, making a new, shorter channel. The loop is left as a lake known as an oxbow lake.

5. The materials that are deposited on the river bed is called river bed deposits. The materials that are deposited at the foot hills is known as foot hill deposits. The finest particles that are deposited on the plains, near the mouth of the river are called as plain deposits.

6. Alluvial fans develop when streams carrying a heavy load reduce their velocity as they emerge from mountainous terrain to a nearlyhorizontal plain.

7. The forward motion of the river is checked upon when a river enters an ocean. So, the river deposits its load of sediments near the mouth of the river. This is known as Delta.

Geologically, groundwater is also an important erosional agent. Besides groundwater is an important source for domestic use , irrigation and industrial use. In addtion, groundwater contamination due to human activities is a real and growing threat in many places.

Let us investigate all these aspects in the follwing lesson.

EXERCISES

I. Choose the correct answer:

1. Potholes are the product of

a) Corrosion b) solution c) suspension

2. The river that swings in great S-shaped curves, forming loops are called

a) meanders b) oxbow lakes c) levees

3. A deep, steep-walled, V-shaped valley cut by a river through resistant rock is often called a

a) Depression b) lakes c) canyon.

II. Fill in the blanks

1. When rivers erode their channel by lifting lossely consolidated particles by -------------- is called mechanical erosion.

2. The major portion of the river load is carried in ---------------.

3. A deep, steep-walled, V-shaped valley cut by a river through resistant rock is often called a ----------------.

4. The particles composing the ---------- is moved along the bottom by rolling, sliding and saltation.

5. The finest particles that are deposited on the plains, near the mouth of the river are called --------------- deposits.

IV. Answer briefly

1. How do rivers form?

2. Describe briefly about potholes?

3. Write a note on V shaped valley.

4. What are oxbow lakes.

5. Differentiate chemical and mechanical erosion?