Geomorphology Topics - Drainage Systems, Fluvial Processes, and River Management

Drainage Systems in South Africa

A drainage system refers to a river system, including numerous streams (tributaries) that flow to form it. A river is a body of water flowing downslope in a defined channel from a source to its mouth.

• Drainage Basin/Catchment Area: The land area drained by a river and its tributaries.
• Watershed: High land separating drainage basins.
• Tributaries: Smaller rivers feeding into a main river.
• Confluence: Where two rivers meet.
• River Mouth: Where a river flows into another body of water.
• Interfluve: Land separating tributaries.

Rivers are fed by surface run-off and groundwater (baseflow).

Types of Rivers

1. Permanent Rivers: Flow year-round, associated with high rainfall areas (e.g., Tugela River).
2. Periodic/Seasonal Rivers: Flow only in the rainy season (e.g., Limpopo River).
3. Episodic Rivers: Flow briefly after heavy rainfall, common in arid areas.
4. Exotic Rivers: Originate in high rainfall areas but flow through dry areas (e.g., the Orange River).

Drainage Patterns

Underlying rock structure and type influence drainage patterns:

1. Dendritic: Resembles tree branches, occurs in uniform rocks.
2. Rectangular: 90-degree bends, forms in jointed igneous rocks or faulted sedimentary rocks.
3. Trellis: Parallel main streams with tributaries at right angles, found in rocks with varying resistance to erosion.
4. Parallel: Tributaries stretch parallel to the slope, occurs in steeply folded bedrock.
5. Radial: Streams flow outwards (centrifugal) or inwards (centripetal) from a central point.
   • Centrifugal: River flows away from a central point. Forms in areas with domes or volcanoes.
   • Centripetal: River flows toward a central point. Tributaries flow inwards from a higher elevation.
6. Deranged: Disrupted patterns with numerous lakes/swamps, forms in geologically young areas.

Drainage Density

Drainage density is calculated as Drainage Density = {Total Stream Length
/ Drainage Area}. Higher drainage density indicates faster water drainage and is influenced by:

• Impermeable surfaces
• Thin vegetation
• Steep gradients

Stream Order

Stream order measures a river's size relative to others in the system. Streams begin as 1st order; when two of the same order join, the order increases.

River Discharge and Flow

• Discharge is the water volume flowing in a river, measured in cumecs (m^3/s). It depends on river size, flow speed, gradient, surface roughness, and flow type.
• Laminar Flow: Smooth, layered flow, usually in lower courses.
• Turbulent Flow: Complex, uneven flow with high erosion rates.

Urbanization increases surface runoff and discharge, leading to more turbulent flow.

Hydrographs

Hydrographs record river discharge over time. Storm hydrographs show discharge changes caused by rainfall.

Key components include:

• Rising Limb: Increasing discharge.
• Peak Flow: Maximum discharge.
• Recession Limb: Falling discharge.
• Basin Lag Time: Time between peak rainfall and peak flow.
• Base Flow: Normal river discharge.
• Overland Flow: Surface run off.
• Through Flow: Water volume reaching the river.

Factors influencing storm hydrographs: Area, shape, slope, rock type, soil, land use, drainage density, precipitation, temperature, and tidal conditions.

Fluvial Processes

• Longitudinal Profile: A river's view from source to mouth.
• Transverse Profile: A river's view from bank to bank.
• Vertical Erosion: Downward erosion deepening the river.
• Lateral Erosion: Sideways erosion widening the river.

Rivers erode, transport, and deposit materials, forming various landforms. The base level is the lowest level to which a river can flow.

River Course

The river is divided into 3 categories: Upper, Middle and Lower.

Fluvial Landforms are formed by erosion, transport, and deposition:

• Erosion: Waterfalls, Rapids
• Deposition: Braided Streams, Natural Levees, Delta's
• Both: Meanders and Oxbow Lakes, Floodplains

Graded vs. Ungraded Rivers

A graded river is in equilibrium (deposition = erosion) with a gently concave profile. An ungraded river is not in equilibrium and has water falls and lakes. Stream piracy occurs when one river captures another through headward erosion.

River Capture Features

• Captor Stream: The river that captures another river.
• Captured Stream: The river that is captured.
• Misfit Stream: A stream too small for its valley.
• Elbow of Capture: The point where piracy occurred.
• Wind Gap: A dry river valley.

Drainage Patterns

• Superimposed: A river develops a pattern on a surface that has eroded over time. Now the river flows over the rocks that have been uncovered. (New river over old landforms)
• Antecedent: After the river formed, folding or faulting occurred but the river maintained its pattern (Old river over new landforms)