Drainage Patterns (Grade 12 NSC Matric Geography): Revision Notes

Drainage Patterns

What are drainage patterns?

When rivers and streams flow across the landscape, they create distinctive arrangements called drainage patterns. These patterns develop as water naturally follows the easiest path to erode through different types of rock and terrain. Understanding these patterns helps geographers analyse the underlying geology and predict where water will flow during floods or droughts.

Individual streams carve out channels as they flow over the surface, cutting into rock where it offers the least resistance to erosion. The way these channels connect and branch creates recognisable patterns that tell us a great deal about the area's geological structure.

Types of drainage patterns

There are three main drainage patterns you need to know for your NSC Geography exam: dendritic, trellis, and radial patterns. Each develops under specific geological conditions.

Dendritic pattern

The dendritic drainage pattern gets its name from the Greek word for tree, because it resembles the branching structure of a tree with its trunk, main branches, and smaller twigs.

Key characteristics:

• Tributaries join the main river at sharp angles (acute angles)
• Branching becomes finer and more detailed as you move away from the main channel
• Resembles the root system of a tree when viewed from above

Where it forms: Dendritic patterns develop in areas where the underlying rock is homogeneous, meaning it has uniform resistance to erosion throughout. This could be:

• Flat-lying sedimentary rocks of similar hardness
• Massive igneous rocks like granite
• Metamorphic rocks with consistent structure

Because the rock offers equal resistance in all directions, water can flow and erode equally in any direction, creating the natural tree-like branching pattern.

Trellis pattern

The trellis drainage pattern creates a distinctive rectangular network that looks similar to a garden trellis or lattice fence, with parallel main streams connected by shorter tributaries.

Key characteristics:

• Main streams run parallel to each other
• Short tributaries join the main streams at right angles ($90°$)
• Creates a rectangular or grid-like appearance

Where it forms: Trellis patterns develop in two main geological situations:

1. Alternating rock resistance: Areas with bands of hard and soft rock running parallel to each other. Rivers flow along the soft rock bands (which erode easily) while tributaries cut through the harder rock at right angles.
2. Folded sedimentary rock: In mountainous areas where sedimentary layers have been folded, rivers follow the soft rock in the valley floors between ridges of harder rock.

Radial pattern

The radial drainage pattern spreads outward from a central point like the spokes of a bicycle wheel or rays from the sun.

Key characteristics:

• All streams flow outward from one central elevated point
• Rivers radiate in multiple directions from the highest point
• Forms a star-like or wheel-spoke pattern when viewed from above

Where it forms: Radial patterns develop around isolated elevated landforms such as:

• Volcanic cones: Rivers flow down all slopes of a volcano
• Domes: Uplifted areas where rock has been pushed upward
• Isolated mountains: Any single peak rising above surrounding terrain

Water naturally flows downhill and away from the highest central point, creating this radiating pattern.

Identifying drainage patterns in exams

When analysing drainage patterns in your NSC Geography exam, you may be asked to:

• Identify the pattern type from a map or diagram
• Describe what the pattern looks like
• Explain the geological conditions that created the pattern

Exam tips:

• Look for the overall shape and arrangement of streams first
• Pay attention to the angles where tributaries join main rivers
• Consider what the underlying rock structure might be
• Use proper geographical terminology in your answers

The diagram above shows three different drainage patterns. Practice identifying whether each represents dendritic (A), trellis (B), or radial (C) characteristics.