Quick Summary
- Drainage patterns show how rivers and streams flow across the land
- Five main types: dendritic, trellis, radial, rectangular, and deranged
- Pattern depends on rock type, slope, and geological structure
- Understanding patterns helps predict water flow and erosion
What Are Drainage Patterns?
Drainage patterns are the designs formed by rivers, streams, and tributaries as they flow across the land. Think of how the roads in Lagos form different patterns – some branch out like tree roots, others run parallel like railway lines. Rivers do the same thing, but their patterns depend on the type of rock beneath them and the slope of the land.
When smaller rivers (tributaries) join together to form a larger river (the main river), they create a river system. The total area that all these rivers drain is called a river basin or watershed. For example, the River Niger and all its tributaries form the Niger River Basin, which covers large parts of West Africa.
Types of Drainage Patterns
1. Dendritic Pattern
This is the most common drainage pattern in Nigeria. The word “dendritic” comes from the Greek word for tree. Rivers in this pattern look like the branches of a tree or roots spreading out from a trunk. Streams flow in many directions and join larger rivers at sharp angles.
This pattern develops on uniform rock types like sedimentary rocks where the land slopes gently in one direction. The soil erodes easily and evenly. You can see dendritic patterns in many parts of southern Nigeria where the land is relatively flat.
2. Trellis Pattern
In a trellis pattern, rivers flow in two main directions – one set runs parallel to each other, while shorter streams join them at right angles. It looks like a garden trellis or fence. This pattern forms in areas with folded mountains or where hard and soft rocks alternate in parallel bands.
The main rivers flow along valleys made of soft rock, while short tributaries flow down the sides of hard rock ridges. This pattern is common in areas with ridge and valley landscapes.
3. Radial Pattern
Radial means spreading out from a center point, like the spokes of a bicycle wheel. In this pattern, rivers flow outward in all directions from a high central point, usually a mountain or volcano. Mount Cameroon in Cameroon and the Jos Plateau in Nigeria show radial drainage patterns.
Water flows downhill from the peak in every direction, creating streams that spread out like rays from the sun. This pattern is easy to spot on topographic maps because all the rivers point away from one central high point.
4. Rectangular Pattern
This pattern has streams that make sharp right-angle turns. Rivers flow in straight lines, then suddenly turn 90 degrees to flow in another direction. The pattern looks like a grid or the streets of a well-planned city like Abuja.
Rectangular patterns develop where the bedrock has fractures, faults, or joints running in two directions at right angles. Water follows these lines of weakness in the rock. This pattern is less common but can be found in areas with heavily fractured bedrock.
5. Deranged Pattern
This pattern has no clear organization. Rivers and lakes appear scattered randomly across the landscape with no obvious direction. Streams twist and turn in irregular paths, and there are many lakes and swamps.
Deranged patterns develop in areas that were recently covered by glaciers (ice sheets) or where the landscape is very new. When ice melts, it leaves an uneven surface with many depressions that fill with water. This pattern is rare in Nigeria but common in countries like Canada and Finland.
Factors That Control Drainage Patterns
Rock Type: Hard rocks like granite resist erosion, so water flows around them. Soft rocks like shale erode easily, allowing water to cut through them. Areas with mixed rock types develop more complex patterns.
Geological Structure: Faults, folds, and joints in rocks create lines of weakness. Water follows these weak zones, creating patterns that match the rock structure underneath.
Slope: Steep slopes produce fast-flowing streams that cut straight down. Gentle slopes allow water to spread out and meander. The angle of the slope affects how tributaries join the main river.
Climate: Heavy rainfall creates more streams and denser drainage networks. Dry areas have fewer streams that only flow during the rainy season. The amount of vegetation also affects how water flows across the land.
Human Activities: Building dams, digging canals, and draining swamps can change natural drainage patterns. In Lagos, for example, drainage channels have been built to control flooding during the rainy season.
| Pattern Type | Appearance | Rock/Terrain Type | Example Location |
|---|---|---|---|
| Dendritic | Tree-like branches | Uniform sedimentary rock, flat terrain | Niger Delta, Southern Nigeria |
| Trellis | Parallel streams with right-angle tributaries | Folded mountains, alternating hard/soft rock | Ridge and valley regions |
| Radial | Streams flowing from central peak | Volcanic cones, domes, mountains | Jos Plateau, Mount Cameroon |
| Rectangular | Right-angle bends, grid-like | Fractured/jointed bedrock | Areas with faulted terrain |
| Deranged | Random, disorganized with lakes | Recently glaciated or new terrain | Rare in Nigeria (common in Canada) |
Common Exam Mistakes
- Confusing pattern types: Students often mix up dendritic and trellis patterns. Remember: dendritic branches randomly like a tree, trellis has parallel main streams with perpendicular tributaries.
- Not explaining WHY a pattern forms: Don’t just name the pattern – explain what rock type or structure causes it. WAEC examiners expect you to link patterns to geological factors.
- Ignoring Nigerian examples: Use local examples like the Niger Delta (dendritic) or Jos Plateau (radial) instead of only foreign examples.
- Spelling errors: Practice spelling “dendritic,” “trellis,” and “deranged” correctly. These are technical terms that must be spelled properly.
- Vague descriptions: Don’t just say “streams flow differently.” Describe the specific direction and angle of flow for each pattern type.
Practice Questions
Multiple Choice Questions
1. Which drainage pattern resembles the branches of a tree?
a) Trellis
b) Radial
c) Dendritic ✓
d) Rectangular
2. A radial drainage pattern typically develops on:
a) Flat plains
b) Folded mountains
c) Volcanic cones and domes ✓
d) Glaciated areas
3. The area drained by a river and all its tributaries is called:
a) Tributary zone
b) River basin ✓
c) Dendritic area
d) Stream pattern
4. Which pattern forms where rocks have fractures running at right angles?
a) Dendritic
b) Trellis
c) Rectangular ✓
d) Deranged
Essay Questions
1. Explain four factors that influence drainage patterns in an area. (8 marks)
Tip: Discuss rock type, geological structure, slope, and climate. Give specific examples of how each factor affects stream direction and pattern formation.
2. Describe the dendritic and trellis drainage patterns, stating one example location for each. (10 marks)
Tip: For each pattern, describe its appearance, explain the geological conditions that create it, and provide a specific location. Use diagrams if allowed.
3. State three differences between radial and rectangular drainage patterns. (6 marks)
Tip: Compare their appearance, the terrain they form on, and how tributaries join the main river. Be specific and clear in your distinctions.
Memory Aids
DTRRD – The five main drainage patterns:
Dendritic (tree-like)
Trellis (parallel with right angles)
Radial (spoke-like from center)
Rectangular (grid pattern)
Deranged (random, chaotic)
Pattern Memory Trick:
“Did The River Run Down?” – Lists the patterns in order from most common to least common in Nigeria.
Dendritic vs Trellis: Remember “Dendritic = Disorganized” (branches go everywhere) and “Trellis = Trains” (parallel tracks with perpendicular connections).
Related Topics
- River Systems and Basins
- Types of Rivers
- Erosion and Weathering
- Landforms Created by Rivers
- Water Resources in Nigeria