Drainage
Senior 1 · Geography
Drainage
Lesson 1 of 4
25 min
Study Figure 1 before you read on. Find the orange dashed line. Rain falling to the west of it runs into Lake Kivu; rain falling to the east of it runs into the Nyabarongo. Use the arrows at the edges of the map to answer: which sea does each raindrop finally reach?
Figure 1. Rwanda's rivers, lakes and the Congo–Nile divide, drawn from Natural Earth and OpenStreetMap data. The divide is drawn approximately.
Rain that reaches the ground soaks in, evaporates or flows away over the surface. The water that flows away gathers into streams, the streams join into rivers, and the rivers carry it into lakes, swamps and finally the sea. Geographers call this whole system of moving and stored surface water the drainage of an area. The rain itself, and how it is measured, was studied in Unit 8; this unit follows the water once it is on the ground.
Definition
Drainage: the way water flows off the land and is stored on it, through a system of streams, rivers, lakes and swamps. We also speak of the drainage of an area: all the rivers and water bodies in it. Example: the drainage of eastern Rwanda includes the Akagera River, Lake Ihema and the swamps around it.
Definition
Drainage basin (catchment): the whole area of land drained by one river and all its tributaries. Every raindrop that falls inside it ends up in that river. Example: the Nile basin covers about four fifths of Rwanda.
Definition
Watershed (divide): the line of high ground that separates two neighbouring drainage basins. Example: the Congo–Nile divide, which runs along the ridge of the western highlands (Figure 1).
| Term | Meaning | Rwandan example |
|---|---|---|
| Source | Where a river begins, often a spring or a marsh in high ground | The Rukarara, a headwater of the Nyabarongo, rises in Nyungwe forest |
| Tributary | A smaller river that flows into a larger one | The Akanyaru and the Nyabugogo are tributaries of the Nyabarongo |
| Confluence | The place where two rivers meet | The Mbirurume and Mwogo meet to form the Nyabarongo |
| Mouth | Where a river ends, in a lake, a sea or another river | The Nyabarongo ends in Lake Rweru; the Rusizi ends in Lake Tanganyika |
| Distributary | A channel that splits off from a river and does not rejoin it, as in a delta | The branches of the small Nyabarongo delta at Lake Rweru |
A water body is any large, lasting mass of water on the Earth's surface. They differ in size, in whether the water is salty or fresh, and in whether it moves or lies still.
| Water body | What it is | Example |
|---|---|---|
| Ocean | One of the five huge bodies of salt water that cover about 71% of the Earth: Pacific, Atlantic, Indian, Southern and Arctic | The Indian Ocean, about 1,000 km from Rwanda at its nearest point |
| Sea | A smaller area of salt water, partly enclosed by land, often at the edge of an ocean | The Mediterranean Sea, where the Nile ends; the Red Sea |
| Lake | A large body of still water in a hollow on land, usually fresh | Lake Kivu (about 2,700 km², shared with the DR Congo); Lake Muhazi |
| Swamp (marsh, wetland) | Low land that is waterlogged all or most of the year, covered with water-loving plants such as papyrus | The Rugezi marsh; the Akanyaru and Nyabarongo wetlands |
| River | A large natural stream of fresh water flowing in a channel towards a lake, a sea or another river | The Nyabarongo, 351 km long, the longest river entirely inside Rwanda |
Definition
Swamp (wetland): an area of low, flat land where the soil is soaked with water for all or most of the year and water-loving plants grow. A wetland stores flood water and releases it slowly, and filters out mud. Example: the Rugezi marsh, a high-altitude peat bog at about 2,100 m in Burera District.
Worked example 1: how much of Rwanda drains to the Nile?
Rwanda's area is 26,338 km². About 80% of it drains to the Nile and about 20% to the Congo. Find each area.
Step 1 Change the percentage to a decimal: 80% = 0.80.
Step 2 The rest drains to the Congo:
Answer. About 21,000 km² drains to the Nile and about 5,300 km² to the Congo. Because the divide lies far to the west, the Nile side is four times as big.
Worked example 2: trace a raindrop on Figure 1
A raindrop falls near Musanze, east of the divide. Name, in order, the water bodies it passes through to reach the sea.
Step 1 Musanze lies east of the orange line, so the drop is in the Nile basin.
Step 2 Follow the blue lines downhill: a small stream → the Mukungwa → the Nyabarongo, which flows east past Kigali and is joined by the Akanyaru → Lake Rweru.
Step 3 Leave Figure 1 by the arrow: the Akagera, north along the Tanzanian border, over Rusumo Falls → Lake Victoria → the Nile → the Mediterranean Sea.
Answer. Mukungwa, Nyabarongo, Lake Rweru, Akagera, Lake Victoria, Nile, Mediterranean Sea: a journey of more than 6,000 km.
Important
Case study: Rwanda's two great drainage systems
Warning
A watershed is a line, not a basin. In British English "watershed" means the high ground between basins (the divide); in American English it is often used for the basin itself. In your exams, a watershed is the divide, such as the Congo–Nile ridge. The area it encloses is the drainage basin (catchment).
The (river) drainage basin, by Mrs Geography (5 min). As you watch, find each labelled part (watershed, source, tributary, confluence, mouth) on Figure 1.
Rwanda's biggest river flows away to Lake Victoria and Egypt, and Lake Kivu's water leaves for the Congo. Why might it matter to Uganda, Tanzania, Egypt and Burundi what Rwandan farmers and towns do to their rivers and swamps? Give one example for each basin.
Tip
Source to mouth: "S-T-C-M". Source, Tributary, Confluence, Mouth, in the order a river meets them. And Rwanda's rain has two doors: "East door, Nile, 80; West door, Congo, 20."
Practice
Remember
Sources and credits
The lesson text, map, tables, worked examples and practice are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
