GEOGRAPHY As LEVEL(FORM FIVE) NOTES THE DYNAMIC-EARTH AND CONSEQUENCE (3)

Why does a large river like the Congo not have a delta?

The Congo River has a large load but a high velocity near its mouth, which enables most of its load to be carried far out to sea, thereby preventing the formation of a delta.

The River Niger also has a large load, but its velocity near its mouth is low. Much of its load is deposited at its mouth where an extensive delta has formed.

Value of Rivers / Economic Importance of Rivers

1. Water supply for domestic use, industrial uses, and irrigation purposes.
2. Rivers are used for local transport (navigation); they provide inland ports on their courses, e.g., St Louis on the Mississippi River in the USA.
3. Provide sites for hydro-electric power generation. Harnessing of hydro-electric power is common worldwide, e.g., Mtera Dam in Tanzania, Seven Forks Dams on the Tana River in Kenya.
4. Rivers are sources of building materials. Sand for building is scooped from river beds and valleys, like in Machakos, Kenya.
5. Rivers also form sources of various alluvial minerals like gold and diamonds, e.g., alluvial diamonds along the Orange River in South Africa and Namibia.
6. Rivers deposit alluvial soils along their valleys during floods and at their deltas. These alluvial soils are fertile and suitable for agriculture, e.g., along the Nile Valley and its delta in Egypt.
7. Rivers have features that provide tourist attractions, such as waterfalls and gorges, e.g., Victoria Falls.
8. Rivers provide rich fishing grounds, e.g., Nile River, Tana River, River Nguruka.
9. Rivers form natural boundaries between communities, districts, provinces, and countries, e.g., Kagera River between Tanzania, Uganda, and Rwanda.

Drainage Patterns

Drainage refers to the removal of water from the surface.

A drainage pattern is the actual arrangement or layout of its tributaries over the surface.

Factors Which Influence Drainage Patterns

1. Slope: This determines the direction and speed of flow; the steeper the slope, the higher the speed, and vice versa.
2. The function of structure: Uniformity of rocks, whether they have joints or are uniform rock, e.g., granite will differ from limestone which has joints. Rocks with joints cause the drainage pattern to follow lines of weakness, but in uniform rocks, the slope determines the drainage system.
3. Nature of rock: Hard and soft rock. Drainage patterns develop more easily on soft rock because water penetrates it easily, unlike hard rock where drainage patterns are difficult to develop. In alternating layers of soft sand and hard rock, drainage patterns develop on the soft rocks only.

Types of Drainage Patterns

1. Dendritic: This pattern resembles the trunk and branches of a tree without leaves. Tributaries join one another at a low angle (less than 90°) from many directions. It develops where there is no structural control, such as in uniform rock; slope is the main factor influencing the drainage pattern. Example: Granite.
2. Trellis: Develops in regions made up of alternate belts of hard and soft rock. The shape is rectilinear or almost rectangular. Tributaries join one another and eventually join the main river at right angles (90°). Major control is rock structure and nature of rock with joints or alternating layers. This drainage pattern gives rise to various types of rivers (streams):
   • Subsequent River: Any tributary which joins the consequent stream at a right angle.
   • Consequent River: The main river which flows down slope.
   • Consequent Stream: Any stream which flows in the opposite direction to the consequent stream and joins the subsequent stream, almost at a right angle.
   • Minor Consequent River: A stream which flows parallel to the consequent stream and joins the subsequent stream.
3. Radial: Shape is like spokes of a wheel radiating from the center, which can be a conical hill (volcano). Develops on volcanoes. Major control is slope. Examples: Granite, volcanic rocks, basalts.
4. Centripetal: Streams from various directions converge to the center. Common in inter-mountain basins or basins between highlands. Major control is slope.
5. Annular: A pattern with streams often joining at sharp angles but arranged in a series of curves about a dissected dome, basin, or crater area. Major control is the nature of the rock.
6. Accordant and Discordant:
   • Accordant: A normal drainage system where the river flows in accordance with the rock structure and slope, following lines of weakness.
   • Discordant: Drainage systems opposed to the dominant structure (rock structure, slope, and landforming processes).
7. Superimposed: Some rivers have developed drainage patterns unrelated to the structure of the region in which they occur. The drainage pattern is discordant to the land surface structure. It has forced itself into a place without any relationship with geological structure.

Stages in the Formation of a Superimposed Drainage Pattern

1. Original folded surface.
2. Region is reduced to a plain due to erosion.
3. Subsidence results in the region being buried by new rocks, but subsequent uplift sees the formation of a drainage pattern. The main river drains at right angles to the axis of the original structure.
4. Tributaries to the main river develop wide valleys in the weaker rocks. As the main river erodes vertically, it cuts across ridges of strong rock and forms gorges. The stronger rock forms ridges because the weak rocks are worn away, not because the region has been uplifted.

Antecedent Drainage: A river pattern disturbed by earth movement (uplift or folding). A river capable of maintaining its course after uplift and erosion is called antecedent. Examples: Ganges River, Snake River, and Colorado River.

River Capture (Piracy)

It is the process where one river diverts the headwaters of a neighboring river into its own course/valley (upper course).

Conditions Necessary for River Capture to Occur

1. The capturing stream should flow at a lower level than its victim stream (capture stream).
2. The capturing stream must be stronger and flowing at a steeper slope than its victim.
3. The capturing stream must flow over easily eroded, weaker rocks.
4. River rejuvenation.

Processes

1. Two streams which are adjacent.
2. There is headward erosion which makes stream S extend headward to C₂.
3. After years of erosion, C₂ is diverted to C₁ where all its water enters C₁.

Evidence / Features Related to River Capture

1. Elbow of Capture: A point where there is a sharp turn of the river; a sharp change in the river course direction at the point of capture.
2. Wind Gap (Dry Valley): A gap between the elbow of capture and the beheaded stream which is dry. The beheaded stream will not dry because it receives water from other sources.
3. Misfit Stream (Beheaded River): A stream carrying less water than the valley depth; it becomes too small for its valley. A river flowing in a valley wider than the size of the river.
4. Rejuvenation: Features of river rejuvenation can be seen in the capturing stream.

Examples: Great Berg River capture in South Africa, Volta River capture in Ghana, Niger River capture in Nigeria.

River Regime

River regime is the seasonal variation of the volume of water in its channel.

Factors Influencing River Regime

Climate

• Precipitation causes variation in rainfall and snowmelt.
  • High rainfall and snowmelt increase river volume.
  • River regime follows rainfall regime.
  • During winter, volume decreases; in early summer, volume increases.
• High temperature increases melting and volume but also increases evaporation, reducing volume.
• Low temperature causes freezing, decreasing volume and causing fluctuations.

Nature of the Rock

• Porous and permeable rocks allow water percolation, reducing river volume.
• Impermeable rocks do not allow water to sink, increasing volume.

Slope

The steeper the slope, the less percolation and evaporation, so volume is maintained. Gentle slopes increase water loss due to longer water retention and more evaporation.

Vegetation

Variation depends on the surface the river flows through. Dry or bare land has less water during dry seasons due to increased evaporation compared to rivers flowing through forests where vegetation retains water.

• During rainy seasons, bare land has more volume because there is nothing to retain water, while forests retain water.

Number of Tributaries Joining the River

The larger the number of tributaries, the larger the volume, and vice versa.

Human Activities

Activities along the river or basin, e.g., irrigation schemes during dry seasons, tend to reduce water volume.

• Agriculture clears forests, reducing water volume.
• Clearing land increases evaporation, reducing water volume.

Types of River Regime

There are three types of river regime:

1. Simple River Regime: Seasonal variation with one high water period and one low water period. Occurs where there is one dry and one wet season, common in tropical regions. Examples: Ruvu, Wami, Ruvuma Rivers.
2. Double River Regime: Two distinct high water periods, possibly due to snowmelt or double rainfall maxima. Occurs in equatorial regions with two rainfall peaks, e.g., Congo and Amazon Rivers.
3. Complex River Regime: Evidenced by the longest rivers with wide basins and numerous tributaries of different regimes. They cut across different climates. Example: Mississippi River.

Importance of Studying River Regime

All development schemes planned on rivers and their valleys should have proper knowledge of river regimes.

Examples: RUBADA – Rufiji Basin Development Authority, TVA – Tennessee Valley Authority, Kagera River Development Authority.

1. Navigation: Navigation and vehicles should be planned according to water depth. Different vehicles are used during high and low water levels.
2. Construction of Hydro-Electric Power Stations: Machines must be placed considering varying water levels to ensure operation during low water.
3. Construction of Bridges: Engineers must plan for varying water levels and energy to build strong bridges that withstand floods.
4. Flood Control: Knowing flood timing and water levels helps in flood control, e.g., dam construction by TVA in the USA.
5. Agricultural Activities on Flood Plains: Irrigation depends on water levels, e.g., Nile River irrigation during dry seasons.
6. Settlement: Settlements should be established above flood-prone areas to avoid flooding.

Concept of Graded Profile

• Considers the river’s long profile from source to mouth.
• Refers to a river profile in dynamic equilibrium, balancing erosion and deposition rates.
• A graded river has a gentle slope with decreasing gradient towards its mouth.
• It is concave and smooth due to higher erosion in the middle course and less erosion at the source and lower course.

Criticisms of the Concept of the Graded River Profile

Several obstacles distort the equilibrium rivers attempt to attain:

• Variation in rock nature along the riverbed and banks (hard and soft rocks).
• Climatic variation affecting rainfall and regions.
• Presence of water bodies like lakes in the river course, which become sediment deposition centers.
• River rejuvenation, sea level changes, and river capture.
• Continuous erosion along the river channel.
• Vegetation occupying parts of the river channel.

River Rejuvenation

• Juvenile: Young or youth stage.
• Juvenation: The process of rejuvenation or renewal.
• Repeat: To do again.

River rejuvenation is the renewal of erosive activity of a river valley after it has reached its old stage. Instead of deposition, it starts eroding again.

Causes of River Rejuvenation

A.

1. Eustatic Change: Caused by a fall in sea level (negative movement of base level). The river profile adjusts to the new sea level. Examples include sea level changes during glacial periods.
2. Diastrophic Change (Isostatic): Fall of sea level floor relative to land or rise of land relative to sea floor causes renewed erosion from upstream.

B. Static Rejuvenation

1. Discharge: Increase in river discharge due to increased volume caused by increased precipitation or river capture.

Features/Landforms Resulting from River Rejuvenation

1. Knick Point: A sharp break in river valley slope where old base level joins new base level after rejuvenation.
2. Waterfall / Rapid: Formed at knick points with sharp breaks, e.g., Bathurst Falls on the Congo River.
3. River Terraces (Paired): Steps or benches on either side of the river valley formed by renewed erosion and downcutting.
4. Incised Meanders: Curved bends of river valleys cut deeply into the land surface, winding between steep valley walls.
   • Ingrown Meanders: Asymmetrical valleys with one side undercut due to resistant rocks on the other side.
   • Entrenched Meanders: Steep-sided symmetrical meanders formed by vertical erosion on uniform rock.
5. Valley within a Valley: Deep, steep-sided valley within a former valley caused by rapid rejuvenation.

Wind

Wind is air in motion, moving from regions of high pressure to low pressure.

Action of Wind

Wind action is more effective in arid regions, especially deserts, due to:

1. Abundance of loose, unconsolidated dry sand and gravel.
2. Absence of vegetation cover.
3. Strong tropical storms causing air molecule expansion and increased wind.

Aridity

Aridity refers to land deficient in moisture, leading to scarce vegetation. Areas receiving less than 250 mm of rainfall are considered arid.

A desert is an arid area with sparse vegetation and very low, unreliable rainfall.

Types of Desert

• Sand Desert (erg): Surface covered mostly by sand, e.g., Sahara Desert.
• Stony Desert (reg or serir): Surface covered by angular boulders, gravel, and pebbles, e.g., Algeria, Libya, Egypt.
• Rock Desert (hamada): Surface made up of bare rocks with scarce vegetation.

Wind Erosion

Wind erosion in deserts involves three processes:

1. Abrasion: Mechanical erosion caused by particles carried by wind hitting and grinding rock surfaces.
2. Deflation: Blowing away of unconsolidated materials like dust and fine particles.
3. Attrition: Wearing down of wind-borne materials as they collide and rub against each other and rock surfaces.

Wind Transportation

Movement of materials by wind depends on wind strength (usually over 20 km/hr), turbulence, and duration. It involves:

• Traction: Dragging or rolling of large particles like pebbles.
• Saltation: Smaller particles bouncing along the surface.
• Suspension: Very fine particles like silt or dust carried in the air.

Erosional Features

1. Rock Pedestals: Tower-like structures of alternating soft and hard rock formed by wind abrasion. Examples: Saudi Arabia, Tibet, Central Sahara.
2. Zeugen: Ridges of hard and soft rock layers formed by weathering and wind abrasion.
3. Yardangs: Elongated ridges of resistant rock separated by soft layers, aligned with prevailing winds. Examples: Atacama Desert, Algeria, East of the Nile.
4. Blowouts (Deflation Hollows, Pans): Shallow depressions formed by wind deflation, sometimes filled with water to form oases. Examples: Kalahari, Botswana, South Africa.
5. Inselbergs: Residual hills of resistant rock left standing after erosion. Smooth ones are called bornhardts. Characterized by many joints.
6. Desert Pavements: Flat areas of bare, polished rock formed by wind abrasion.
7. Ventifacts (Drakantars): Heavier rock blocks left after wind has removed finer materials.

Features Due to Wind (Aeolian) Deposition

• Features formed include dunes (barchans and seifs), loess, and ripples.

Sand Dunes

Hills of sand deposited by wind in deserts, influenced by vegetation cover, sand particle size, material amount, and wind velocity.

1. Barchans (Bar Khans): Crescent-shaped dunes with horns pointing downwind.
2. Seif Dunes: Long, narrow ridges parallel to wind direction, occurring in small sand areas.

Loess

Accumulation of wind-carried sand deposited beyond desert limits, forming fertile soil. Example: China.

Ripples

Smallest wave structures, sometimes less than a centimeter high, commonly found between dunes.

Why Are Deserts Found on the Western Parts of Continents?

1. Cold ocean currents off western coasts cause winds to lose moisture over the sea, arriving dry on land.
2. Prevailing winds and high mountain ranges on the west cause rain shadows on eastern sides.
3. Subtropical high-pressure zones discourage cloud formation and precipitation.

Glaciations

Major ice activity periods occur roughly every 200–250 million years. The most recent significant glaciation occurred during the Pleistocene period of the Quaternary era.

In the last two million years, temperature fluctuations of about 5°C have led to cold phases (glaciers) and warm phases (interglacials).

Causes of Ice Age / Theories

• Variation in solar radiation reaching Earth.
• Volcanic dust injection reflecting and absorbing solar radiation.
• Changes in atmospheric carbon dioxide affecting greenhouse effects.
• Planetary movements to colder latitudes or increased altitude at constructive margins.
• Changes in ocean currents or jet streams.

Snow Accumulation and Ice Formation

• Colder climates increase snowfall and reduce melting time.
• Permanent snowline forms where snow lies year-round.
• Snow compresses into firn or neve, a dense form of snow.
• Firn turns into solid ice over decades, eventually flowing downhill as glaciers.

Glacier and Ice Masses

Glaciers are classified by size and shape, identifiable by field observation.

Landforms Produced by Glacial Erosion

1. Niche Glaciers: Small glaciers occupying hollows and gullies on north-facing slopes.
2. Corrie or Cirque: Armchair-shaped hollows in mountains, feeding valley glaciers. Known as corries in Scotland.
3. Arêtes: Narrow, steep-sided ridges formed by adjacent cirques eroding toward each other. Example: Alps, Switzerland.
4. Pyramidal Peak: Sharp mountain peak formed by three or more cirques. Example: Alps.
5. Truncated Spurs: Spurs with ends cut off by river erosion, forming alluvial fans.
6. Crag and Tail: Resistant rock mass with a tapering tail of softer rock behind. Example: Edinburgh, Scotland.
7. Roche Mountonnée: Rock outcrop smoothed on upstream side and steep on downstream side due to glacial abrasion and plucking. Examples: Mobuku Valley (Ruwenzori), Yosemite National Park, Mt. Kilimanjaro.
8. U-Shaped Valley / Glacial Trough: Wide, steep-sided valley formed by glacier erosion, replacing V-shaped river valleys.
9. Hanging Valley: Tributary valley ending abruptly above main valley floor, often with waterfalls.
10. Rock Basin: Irregular depression in glacial valley floor formed by glacier erosion, often forming lakes.
11. Ribbon Lake (Finger Lake or Trough Lake): Elongated lake occupying a glacial trough, e.g., Lake Michelson near Mt. Kenya.
12. Fiord: Long, narrow, deep sea inlet with steep sides formed by glaciers, e.g., Norway.
13. Ice Eroded Plain: Extensive, level lowland area of bare rock smoothed by ice sheets.

Glacier Transportation and Deposition

Glacier movement forms depositional features such as:

1. Moraines: Unsorted ridges of glacial debris.
   • Ground Moraines: Deposited at glacier valley bottom.
   • Medial Moraines: Formed where two glaciers meet.
   • Lateral Moraines: Along glacier valley sides.
   • Terminal Moraines: At glacier end, marking maximum advance.
   • Recessional Moraines: Formed during glacier retreat.
2. Boulder Clay Plain (Till Plain): Extensive lowland of clay and boulders deposited by ice sheets.
3. Erratics: Large boulders differing from local rock, transported by glaciers.
4. Drumlins: Elongated oval hills of clay and boulders aligned in groups.
5. Eskers: Long, steep-sided ridges of sand and gravel deposited by subglacial streams.
6. Kames: Irregular mounds of stratified sand and gravel deposited by melting ice.
7. Outwash Plain: Gently sloping land of gravel and sand at ice sheet edges.

Value / Importance of Glaciated Landscapes to Humans

1. Some glaciated landscapes are fertile and good for agriculture, e.g., dairy belt in the USA, wheat cultivation in Canadian prairies.
2. Hanging valleys are suitable for hydroelectric power generation, e.g., Norway, Sweden, Switzerland.
3. Glaciated landscapes provide attractive scenery for tourism, e.g., Mt. Kenya, Kilimanjaro.
4. Glaciated valleys provide good grazing land in summer, e.g., Alps in Europe.
5. Fiords form natural harbors and fishing grounds, e.g., Oslo Fjord, Norway.
6. Glaciated landscapes contain lakes used for navigation, e.g., Great Lakes in North America.
7. Melting glaciers provide rivers for domestic and industrial use.

Disadvantages of Glaciated Landscapes

1. Boulder clay plains can be poor for agriculture, e.g., central Ireland.
2. Outwash plains often have infertile sands leading to wasteland.

Wave

Wave Action and the Features It Produces

Waves are horizontal movements of ocean water that cause erosion, transportation, and deposition along coasts. Waves differ from tides, which are vertical movements.

Movements Associated with Waves

1. Swash: Movement of water towards the beach.
2. Backwash: Movement of water back to the ocean after waves break.
3. Wave Break: Splitting of waves as they release energy on the coast.
4. Wave Erosion: Occurs along shores, influenced by rock nature and wave strength.

Wave Erosion Processes

1. Corrosive Action: Erosion by rock fragments carried by waves banging and cutting cliff bases.
2. Hydraulic Action: Water thrown against cliffs compresses air in cracks, breaking rocks.
3. Attrition: Breaking down of materials carried by waves as they collide.
4. Chemical Solvent Action: Occurs on limestone coasts where carbon dioxide in water dissolves calcium carbonate into soluble calcium hydrogen carbonate.

Cliff: Raised land facing the sea, usually vertical.

Formation starts with a notch formed by wave action at high tide level, which expands and causes overhanging blocks to collapse, forming cliffs. Waves then attack the cliff base, forming overhanging cliffs.

Wave Cut Platform: Flat shore part formed by wave erosion as cliffs retreat inland, e.g., west coast of Norway.

Wave Deposition

Materials eroded by waves are transported and deposited along coasts. Deposited materials come from the coast itself or rivers.

Wave Deposited Features