Geomorphology (Grade 12 NSC Matric Geography): Revision Notes
Geomorphology
When studying geomorphology on topographic maps and orthophotos, you need to understand how landforms develop and how to identify different features. Geomorphology questions in NSC exams focus on four main areas that help you interpret the physical landscape.
Determining river flow direction
Understanding which way rivers flow is fundamental to reading any topographic map. Rivers always follow the basic principle that water moves from higher elevations to lower elevations due to gravity.
Fundamental Principle: Rivers ALWAYS flow from high to low elevations. This is the most basic rule for determining flow direction on any topographic map.
To determine river flow direction, you should examine the height values marked on contour lines throughout the map. Find where the river begins (its source) and where it ends (its mouth). The river will flow downhill from the point with the highest elevation to the point with the lowest elevation.
Useful Technique: Study the tributaries that feed into the main river. These smaller streams and rivers join the main channel, and they flow in the same direction as the main river. By observing how tributaries connect to the main river, you can confirm the overall flow direction of the entire river system.
Identifying drainage patterns
Drainage patterns reveal important information about the underlying landscape and geological structure. These patterns show how water networks have developed across the terrain over long periods.
The main drainage patterns you need to recognise include dendritic, radial, and trellis patterns. Each pattern provides clues about the geological conditions beneath the surface. When analysing drainage patterns, look at how the river channels branch and connect across the mapped area.
Pattern Characteristics:
- A dendritic pattern resembles the branching structure of a tree, with streams flowing in many different directions
- A radial pattern shows streams flowing outward from a central high point, like water flowing down the sides of a dome
- A trellis pattern displays a rectangular grid-like arrangement where streams follow parallel valleys with shorter tributaries joining at right angles
Determining underlying rock structure
The type of rock beneath the surface strongly influences how drainage patterns develop. By studying these patterns carefully, you can make educated conclusions about the geological foundation of an area.
Different rock types create characteristic drainage patterns. When you observe a dendritic drainage pattern, this often indicates that the underlying rocks are horizontal sedimentary rocks, igneous rocks, or metamorphic rocks. These rock types generally offer uniform resistance to erosion, allowing streams to flow in various directions without being controlled by structural weaknesses.
Key Relationship: The relationship between drainage patterns and geology helps geographers understand landscape evolution. Areas with different rock types will show distinct drainage characteristics, making it possible to map geological boundaries even when the rocks themselves are not visible on the surface.
Identifying river course stages
Rivers change their characteristics as they flow from their source to their mouth. Recognising these changes helps you determine whether you're looking at the upper, middle, or lower course of a river system.
Upper Course Characteristics: The upper course of a river occurs in mountainous or hilly areas where the river has significant energy. Here you'll find steep valley sides that create a distinctive V-shaped valley profile. The river channel tends to be narrow with a steep gradient, allowing the water to flow rapidly and erode vertically into the landscape.
In contrast, the lower course develops in flatter terrain where the river has less energy for vertical erosion. The valley becomes much wider, creating an extensive floodplain alongside the river channel. In these areas, you'll commonly observe meandering river patterns, marshes, vleis (seasonal wetlands), and oxbow lakes formed by the river's sideways movement across the landscape.
Lower Course Characteristics:
- Wide floodplains with gentle slopes
- Meandering patterns showing lateral erosion
- Presence of marshes and vleis
- Oxbow lakes formed from cut-off meanders
- Extensive depositional features
The transition between upper and lower courses happens gradually, with the middle course showing intermediate characteristics. Understanding these stages helps you interpret the river's erosional and depositional processes at different points along its journey.
Key Points to Remember:
- Rivers always flow from higher elevations to lower elevations - check contour line values to determine direction
- Tributaries flow in the same direction as the main river they join
- Drainage patterns (dendritic, radial, trellis) reveal information about underlying rock structure
- V-shaped valleys with steep sides indicate upper river courses in mountainous areas
- Wide floodplains with meanders, marshes and oxbow lakes indicate lower river courses in flat terrain