Rivers: Key Terms, Types and the Work of a River – Global Drainage Systems (Senior 6 Geography) | Ganzaa

Global Drainage Systems

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Senior 6 · Geography

Global Drainage Systems

5 lessons

Lesson 1 of 5

Rivers: Key Terms, Types and the Work of a River

25 min

Two colours of water

Below Manaus in Brazil, two great rivers meet and refuse to mix for several kilometres. The Solimões arrives pale and muddy, loaded with silt eroded from the Andes; the Rio Negro arrives dark and clear, stained by dissolved plant matter from the forest but carrying little sediment. The difference in colour is a difference in load, and load is the record of a river's work: what it has eroded upstream, what it is carrying now and what it will deposit downstream. This lesson sets out the terms used to describe rivers and the three processes that make up their work.

Definition

Drainage basin (catchment): the area of land drained by a river and its tributaries. Watershed (divide): the high ground that separates one drainage basin from the next; Rwanda's Congo–Nile Divide is an example.

Source: where a river begins. Mouth: where it ends, in a sea, lake or larger river. Tributary: a stream that joins a larger river; the junction is a confluence. Distributary: a channel that branches off a river and does not rejoin it, as on a delta.

Discharge ( $Q$ ): the volume of water passing a point each second: $Q = A \times v$ , where $A$ is the cross-sectional area of the channel (m²) and $v$ the mean velocity (m s⁻¹). Unit: cumecs (m³ s⁻¹).

Load: the material a river carries. Base level: the lowest level to which a river can erode, normally sea level; a lake gives a temporary (local) base level.

River regime: the pattern of a river's discharge through the year.

Types of rivers

Basis	Type	Meaning and example
Flow through the year	Perennial	Flows all year, fed by rain in all seasons or by groundwater, snow or a lake: the Nyabarongo, the Congo
	Intermittent (seasonal)	Flows only in the wet season
	Ephemeral	Flows only for hours or days after a storm, typical of deserts (a wadi)
Climate crossed	Exotic	Rises in a wet region and flows through a desert without tributaries, losing water to evaporation: the Nile in Sudan and Egypt
Relation to the slope and rock structure	Consequent	Follows the original slope of the land
	Subsequent	Develops later along a band of weak rock, often at right angles to the consequent
	Obsequent	Flows in the opposite direction to the consequent
	Resequent	Flows in the same direction as the consequent but develops later, at a lower level

Most of Rwanda drains east to the Nile through the Nyabarongo and the Akagera, while a narrower western strip drains to Lake Kivu and the Rusizi, and so to the Congo. The line between them is the Congo–Nile Divide, the high ridge that runs north to south through the west of the country.

Worked example 1: measuring discharge

Problem. A river channel is 24 m wide with a mean depth of 3.5 m, and floats show a mean velocity of 1.2 m s⁻¹. Find the discharge. (Illustrative measurements.)

Solution.

A = 24 \times 3.5 = 84\,\text{m}^2, \qquad Q = A \times v = 84 \times 1.2 = 100.8 \approx 101\,\text{m}^3\,\text{s}^{-1}

Floats travel at the surface, where water is fastest, so field workers often multiply the surface velocity by about 0.8 to estimate the mean. Ignoring this step would overestimate the discharge.

Worked example 2: which channel is more efficient?

Problem. Channel X is 20 m wide and 2 m deep; channel Y is 40 m wide and 1 m deep. Both are rectangular. Compare their hydraulic radius $R = \frac{A}{P}$ , where $P$ , the wetted perimeter, is the length of bed and banks in contact with water.

Solution.

\text{X}: A = 40\,\text{m}^2,\ P = 20 + 2 \times 2 = 24\,\text{m},\ R = \frac{40}{24} \approx 1.67\,\text{m}

\text{Y}: A = 40\,\text{m}^2,\ P = 40 + 2 \times 1 = 42\,\text{m},\ R = \frac{40}{42} \approx 0.95\,\text{m}

Both carry the same area of water, but X loses less energy to friction with its bed and banks. With the same slope, the deeper, narrower channel flows faster and has more energy for erosion and transport.

The work of a river 1: erosion

Process	What happens
Hydraulic action	The force of moving water pushes into cracks in the bed and banks; air trapped in cracks is compressed and released, weakening the rock
Abrasion (corrasion)	Load carried by the river is hurled against the bed and banks, scraping them like sandpaper; the main process deepening most channels
Attrition	Particles in the load knock against each other and break into smaller, rounder pieces; it wears the load, not the channel
Solution (corrosion)	Slightly acidic water dissolves soluble rock such as limestone

Erosion acts in three directions: vertical erosion deepens the channel (strongest near the source, far above base level), lateral erosion widens it (strongest in the lower course), and headward erosion cuts back the source and waterfalls upstream.

The work of a river 2: transport

Large boulders are rolled along the bed (traction); pebbles and sand bounce along it (saltation); fine silt and clay are held up in the water (suspension, which gives a river its colour); and dissolved minerals are carried invisibly (solution). A river's competence is the largest particle it can carry; its capacity is the total load it can carry. Both rise steeply with velocity and discharge, so most transport happens during floods.

The work of a river 3: deposition

A river deposits when it loses energy: where it enters a lake or the sea, on the inside of a bend, where its gradient falls, when floodwater spreads over the valley floor, or as discharge falls after a flood. The heaviest particles are dropped first, so deposits are sorted by size.

Figure 1: the Hjulström curve, a graph of velocity against particle size. It is drawn here as a sketch of the original 1935 diagram, so read it for its shape rather than exact values.

Two features of the graph need explaining. First, clay and silt need higher velocities to be eroded than sand, because their tiny particles stick together (cohesion) and lie in a smooth bed. Second, once lifted, clay settles only in almost still water, which is why fine sediment travels all the way to lakes and deltas.

Warning

Common mistakes

Confusing abrasion and attrition. Abrasion wears the channel; attrition wears the load.
Confusing load and discharge. Discharge is water (m³ s⁻¹); load is sediment carried by that water.
"Small particles are always the easiest to erode." The Hjulström curve shows that fine clay needs a higher velocity than sand, because of cohesion.

Tip

Memory hook. Erosion: "HAAS" (hydraulic action, abrasion, attrition, solution). Transport, from biggest to smallest: "TSSS" (traction, saltation, suspension, solution). In an explanation, link each process to energy: more velocity, more erosion and transport; less velocity, deposition.

Geography Hawks: "River erosion, transportation & deposition". Pause at each process and match it with the tables above; note which processes act on the channel and which on the load.

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Practice

Define drainage basin, watershed, confluence and base level.
Distinguish between a perennial, an intermittent and an ephemeral river.
A stream 6 m wide and 0.5 m deep flows at 0.8 m s⁻¹. Calculate its discharge.
Explain why the Nile is described as an exotic river in Egypt.
Describe the four processes of river erosion.
Using the Hjulström curve, explain why a river at 20 cm s⁻¹ may erode sand but not clay.
A river's discharge doubles in a flood. Explain what happens to its competence, its capacity and its erosion, and why.

Show answersHide answers

Drainage basin: the area drained by a river and its tributaries. Watershed: the high ground separating basins. Confluence: where two rivers join. Base level: the lowest level to which a river can erode, usually sea level.
Perennial: flows all year; intermittent: flows only in the wet season; ephemeral: flows only briefly after storms.
$Q = 6 \times 0.5 \times 0.8 = 2.4\,\text{m}^3\,\text{s}^{-1}$ .
It rises in the wet East African and Ethiopian highlands and crosses the Sahara without gaining tributaries, losing water to evaporation along the way.
Hydraulic action (force of water and compressed air in cracks), abrasion (load scraping the bed and banks), attrition (load particles colliding and breaking), solution (dissolving soluble rock).
Sand of about 0.1–1 mm needs only about 20 cm s⁻¹ to be eroded, the lowest velocity on the curve; clay particles are cohesive and lie in a smooth bed, so they need a much higher velocity.
Both rise: with more water the river flows faster and deeper, so it can move larger particles (competence) and more of them (capacity). More load and more energy mean more abrasion and hydraulic action, so most erosion occurs in floods.

Summary

A river drains a basin separated from others by a watershed; discharge $Q = A \times v$ .
Rivers are classified by flow (perennial, intermittent, ephemeral, exotic) and by relation to slope and structure (consequent, subsequent, obsequent, resequent).
Erosion: hydraulic action, abrasion, attrition, solution. Transport: traction, saltation, suspension, solution. Deposition follows any loss of energy.
The Hjulström curve links velocity, particle size and the three processes.

Sources and credits

River, Drainage basin, Hjulström curve and Meeting of Waters: Wikipedia contributors, CC BY-SA 4.0, adapted by Ganzaa.

Photo: Meeting of waters from the air manaus brazil (Immelman284, CC BY-SA 3.0), via Wikimedia Commons.

Video: "3) River erosion, transportation & deposition", Geography Hawks, embedded from YouTube.

Figure 1, the worked examples and the exercises were drawn and written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Two colours of water

2Types of rivers

3The work of a river 1: erosion

4The work of a river 2: transport

5The work of a river 3: deposition

Two colours of water

Types of rivers

The work of a river 1: erosion

The work of a river 2: transport

The work of a river 3: deposition