Drainage
Senior 1 · Geography
Drainage
Lesson 2 of 4
25 min
A mountain stream in Nyungwe rushes over rocks in a deep, narrow valley. Near Kigali the Nyabarongo winds slowly across a wide, green valley floor. At Lake Rweru it splits into several channels among the reeds. These could be three different rivers, but they are three stages of one river system. As a river travels from its source to its mouth, the land it flows over gets flatter, and the work the water does changes.
Definition
Long profile: the side view of a river's course from its source to its mouth, showing how the height of the river bed falls with distance. It is steep near the source and almost flat near the mouth, a concave curve. Example: Figure 1.
Definition
Cross-section (cross profile): the shape of a river valley seen across it, from one side to the other, at one point. Example: the V-shaped valley at A in Figure 1.
Figure 1. The long profile of a river and valley cross-sections at A (youthful), B (mature) and C (old). Drawn by Ganzaa.
Geographers divide the course of a river into three stages. Each stage has its own gradient, its own main process and its own landforms.
| Stage | Gradient and speed | Main work of the river | Landforms |
|---|---|---|---|
| Youthful (upper course) | Steep; fast, turbulent water in a small channel | Vertical erosion: the river cuts down into its bed | V-shaped valley, interlocking spurs, waterfalls, rapids, gorges, potholes |
| Mature (middle course) | Gentler; more water, a wider and deeper channel | Lateral erosion: the river cuts sideways; some deposition | Meanders, river cliffs, slip-off slopes, a wider valley with a narrow flood plain |
| Old (lower course) | Almost flat; slow, full of fine mud | Deposition: the river drops its load | Wide flood plain, ox-bow lakes, levees, swamps, braided channels, delta |
Definition
Erosion, transport and deposition: the three kinds of work a river does. Erosion is wearing away the bed and banks; transport is carrying the eroded material (the load); deposition is dropping it when the water slows down. Example: the Nyabarongo turns brown in the rains because it is transporting soil eroded from hillsides.
Definition
V-shaped valley: a narrow, steep-sided valley shaped like the letter V, cut by a fast river eroding downwards while rain and gravity wear back the valley sides. Example: the valleys of the streams that fall from the Congo–Nile ridge to Lake Kivu.
Definition
Waterfall: a place where a river drops vertically over a step in its bed, usually where a band of hard rock lies over softer rock. The falling water scours a deep plunge pool, undercuts the hard rock, and the fall slowly retreats upstream, leaving a gorge. Example: Rusumo Falls on the Akagera.
Definition
Meander: a large loop or bend in a river. On the outside of the bend fast water erodes a steep river cliff; on the inside slow water deposits sand and silt to form a gentle slip-off slope. Example: the loops of the Nyabarongo on its valley floor south of Kigali.
Definition
Ox-bow lake: a curved lake formed when a river cuts through the narrow neck of a meander and the old loop is sealed off by deposits. Example: Figure 2, step 4.
Figure 2. From meander to ox-bow lake in four steps (plan view). Drawn by Ganzaa.
Definition
Flood plain and levee: a flood plain is the wide, flat valley floor on either side of a river in its old stage, built of silt dropped in floods. Levees are low natural banks of coarser silt that build up along the river's edges, because the water slows down first as it spills over them. Example: the broad swampy valley of the Nyabarongo and Akanyaru south-east of Kigali.
Definition
Delta: a flat area of deposits at a river's mouth, where the river slows down on entering a lake or sea and splits into several channels called distributaries. Example: the small delta of the Nyabarongo where it enters Lake Rweru.
Worked example 1: how gentle is the old stage?
In its lower 80 km the Akanyaru falls from 1,465 m to 1,400 m above sea level. Find its average gradient.
Step 1 Vertical fall: .
Step 2 Put both distances in metres: .
Answer. About 1 in 1,230: the river falls only 1 m for every 1.2 km. No wonder it spreads into a swamp belt about 7 km wide.
Worked example 2: comparing the stages
Suppose a headwater stream in Nyungwe falls 800 m over 20 km. How many times steeper is it than the lower Akanyaru?
Answer. 1 in 25 against 1 in 1,231: the mountain stream is about 49 times steeper, which is why it cuts down fast while the lower Akanyaru drops its load.
Worked example 3: is this river meandering?
On a map, a river between two bridges measures 18 km along its channel, but the straight line between the bridges is 12 km. Geographers call a river meandering when its sinuosity is 1.5 or more.
Answer. 1.5, so the river is meandering. A perfectly straight river would score 1.
Important
Case study: Rwandan rivers do not follow the textbook exactly
Warning
Don't mix up the two sides of a meander. The river cliff is on the outside of the bend, where the water is fastest and erodes; the slip-off slope is on the inside, where the water is slowest and deposits. Also: a gradient must use the same units top and bottom (65 m ÷ 80 km is not 1 in 1.2).
River Landforms: Waterfalls, Meanders, Oxbow Lakes, Levees and More, by Keducate (3 min). Pause at each landform and say which stage of Figure 1 it belongs to.
Families farm the rich silt of the Nyabarongo flood plain, but the same land floods in the rainy seasons. Using Figure 1 and the landforms in this lesson, explain why the flood plain is both the most fertile and the most flood-prone land along the river. Should houses be built there?
Tip
Down, sideways, drop. Youthful rivers cut down (V-valleys, waterfalls); mature rivers cut sideways (meanders); old rivers drop their load (flood plains, levees, ox-bows, deltas).
Practice
Remember
Sources and credits
The lesson text, long profile, cross-sections, ox-bow sequence, worked examples and practice are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
