Grade 12 Geomorphology Notes

Introduction

• Author: Bryan Shingange
• Focus: Key concepts in geomorphology, including drainage basins, river types, and fluvial processes.

Key Concepts

Glossary

• River: A large, natural stream of flowing water. Can be defined as water flowing downslope in a defined channel from source to mouth.
• River System: Includes the main river and its tributaries.
• Drainage Basin: Area drained by a river and its tributaries.
• Catchment Area: The drainage area supplying water to a river.
• Infiltration: Movement of water into the ground.
• Surface Run-off: Flow of water on the surface due to excess infiltration.
• Ground Water: Water found underground.
• River Mouth: Where the river ends, either in a sea or lake.
• Tributary: A stream feeding into a larger river.
• Watershed: High ground separating two drainage basins.
• Interfluve: A high area separating two river valleys.
• Source/River Source: Where the river begins.
• Confluence: Point where two rivers join.
• Water Table: Upper level of underground saturated rock.
• Drainage Pattern: Arrangement of rivers within a drainage basin.
• Discharge/River Discharge: Amount of water flowing past a point in a river channel over time.

Drainage Basins

• Drainage basins function as open systems with inputs, stores, flows, and outputs.
• Key features: catchment area, river system, tributaries, watershed, river mouth, source, confluence, interfluve, water table, ground water, surface run-off, infiltration.

Important Watersheds in South Africa

• Drakensberg Mountains: Separates rivers flowing to the eastern plateau slopes and the Orange River system.
• Witwatersrand: Separates the Limpopo and Vaal River systems.

Types of Rivers

1. Permanent Rivers: Flow all year round, e.g., Tugela River.
2. Periodic Rivers: Flow during rainy seasons, e.g., Vaal River.
3. Episodic Rivers: Flow after heavy rain, e.g., Molopo River.
4. Exotic Rivers: Begin in high rainfall regions, flow into dry areas, e.g., Orange River.

Drainage Patterns

• Dendritic: Tributaries join at acute angles; associated with uniform rock.
• Trellis: Tributaries join at right angles; found in folded sedimentary rock.
• Radial: Streams flow away from a central high point.
• Rectangular: Tributaries have bends, join at right angles; found in jointed hard rock.
• Centripetal: Streams flow towards a low area.
• Deranged: Small, stagnant streams on flat areas.
• Parallel: Streams flow parallel, often channeled by hills.

Fluvial Processes

• Types of flow: Laminar (smooth) and Turbulent (bubbling motion).
• Erosion Processes:
  • Abrasion: Sand and silt particles wear away riverbed.
  • Traction: Boulders and rocks rolled along the riverbed.
  • Saltation: Small rocks bounce along riverbed.
  • Suspension: Silt and particles carried by river.

River Profiles

• Longitudinal Profile: Side view from source to mouth.
• Transverse Profile: Cross-section from bank to bank.

River Stages

1. Upper Course (Young Stage): Steep, fast-flowing.
2. Middle Course: Develops meanders, wider valleys.
3. Lower Course (Old Stage): Wide floodplains, slower flow.

Landforms

• Rapids: Turbulent flow over resistant rock.
• Waterfalls: Drop over horizontal or vertical resistant rock.
• Meanders: Form in lower courses with gentle gradients.
• Alluvial Fans: Silt deposit from steep areas onto plains.
• Floodplains: Flat areas filled with silt.
• Levees: Raised banks from silt deposits.
• Braided Streams: Multiple channels separated by islands.

River Grading

• Graded Rivers: Achieve balance with gradient, volume, and shape.
• Overgraded Rivers: Excess energy, erode channels.
• Undergraded Rivers: Insufficient energy, deposit load.

River Rejuvenation

• Occurs due to changes in riverbed levels (e.g., isostatic uplift).
• Increases erosive power, can create new valleys, terracing.
• Knickpoint: Sharp gradient change along river course.

River Capture

• Occurs when one river diverts another's flow through headward erosion.
• Captor Stream: Erodes backward to capture another.
• Captive Stream: Loses flow to captor.
• Misfit Stream: Cannot fill original valley post-capture.
• Elbow of Capture: Point where capture occurs.
• Wind Gap: Now dry area between elbow and misfit stream.

Conclusion

These notes summarize the fundamental principles of geomorphology and hydrological systems, focusing on river dynamics, landforms, and drainage basin characteristics.