Drainage
Senior 1 · Geography
Drainage
Lesson 3 of 4
20 min
Look down on a landscape from an aeroplane and the rivers form shapes: a tree, a ladder, the spokes of a wheel. These shapes are not an accident. They are drawn by the slope of the land and by the rocks underneath, so a geographer can read the land from the pattern of its rivers.
Definition
Drainage pattern: the shape made on a map by a river and its tributaries, seen from above. It depends mainly on the slope of the land and on the kind and arrangement of the rocks. Example: the Nyabarongo and its many tributaries form a tree-like (dendritic) pattern.
Figure 1. The five main drainage patterns, seen from above. Drawn by Ganzaa.
| Pattern | What it looks like | Where it forms | Example |
|---|---|---|---|
| Dendritic | Tributaries join the main river at acute angles, like the branches of a tree | On rocks of the same hardness everywhere, so the water can cut valleys in any direction | The Nyabarongo–Akagera basin of central and eastern Rwanda |
| Trellis | Long parallel streams in valleys, with short tributaries joining at right angles, like a garden trellis | Where long bands of hard rock (ridges) and soft rock (valleys) alternate, as in folded rocks | The Appalachian Mountains of the eastern USA |
| Radial | Streams flow outwards in all directions from a high centre, like the spokes of a wheel | On a volcanic cone or a dome-shaped hill | The Virunga volcanoes; Mount Kilimanjaro; Mount Elgon |
| Centripetal | Streams flow inwards from all sides to a low centre | In a basin or crater, often with a lake or swamp in the middle | Lake Chad's basin; on a small scale, streams from the hills around the Rugezi marsh |
| Parallel | Streams run side by side in the same direction | On a long, steep, even slope | The short streams falling west from the Congo–Nile ridge into Lake Kivu |
Definition
Dendritic pattern: a tree-like drainage pattern in which tributaries join the main river at acute angles. It forms where the rocks are of similar hardness throughout. Example: the Nyabarongo basin.
Definition
Radial pattern: a pattern in which streams flow outwards in all directions from a high central point, such as a volcanic cone. Example: streams and gullies running off the slopes of Karisimbi and Muhabura.
Worked example 1: reading patterns on the Rwanda map
Use the map of Rwanda's rivers in lesson 1 (Figure 1 there). Name the pattern (a) east of the Congo–Nile divide; (b) on the slope between the divide and Lake Kivu.
(a) Look at how the Nyabarongo's tributaries join it: the Mbirurume, Mwogo, Mukungwa, Nyabugogo and Akanyaru come in from many directions and meet the main river at sharp angles, with smaller streams branching off them. This is a dendritic pattern, formed on the old, deeply weathered rocks of the central plateau.
(b) The Sebeya, Koko and other short streams all run in the same direction, straight down the steep slope into the lake, side by side. This is a parallel pattern.
Worked example 2: drainage density
Geographers measure how closely an area is cut by streams with the drainage density. A small catchment of 48 km² contains streams with a total length of 132 km. Find its drainage density.
Interpretation. A high density means many streams: rain runs off quickly over rock or clay that lets little water in. A low density means rain soaks into the ground. On the cracked, porous lava of the Virungas much rain sinks in and comes out later in springs, so many of the radial channels carry water only after heavy rain.
Important
Case study: the radial drainage of the Virungas
The Virunga volcanoes stand along Rwanda's border with Uganda and the DR Congo: Karisimbi (4,507 m), Bisoke, Sabyinyo, Gahinga and Muhabura. Each is a cone, so rain falling on it runs outwards in every direction, down gullies that spread from the summit like the spokes of a wheel. Water running off the northern and western slopes is lost to Uganda and the DR Congo; water running south and east feeds the Mukungwa and the twin lakes Burera and Ruhondo, and so reaches the Nile. After very heavy rain these steep, radial channels fill in minutes and carry mud and boulders onto the farms and roads below, so the pattern matters to the families of Musanze and Nyabihu. Unit 11 returns to these floods.
Warning
Radial or centripetal? Both look like a wheel. Check the arrows: water flowing out from a high centre (a volcano) is radial; water flowing in to a low centre (a lake in a basin) is centripetal. "Centripetal" means "seeking the centre".
Formation of Drainage Patterns (CSEC Geography), by Geography Journey (6 min). Watch for how the rock underneath decides the pattern, then sketch each pattern as it is described.
Engineers want to build a road around the foot of a volcano. Using what you know about radial drainage, where along the road would they need the most bridges and culverts (pipes under the road)? What might happen if they built too few?
Tip
Picture the name. Dendron is Greek for "tree"; a trellis is a garden frame of squares; a radius runs out from the centre of a circle; centripetal seeks the centre; parallel lines never meet.
Practice
Remember
Sources and credits
The lesson text, pattern diagrams, table, worked examples and practice are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
