IBGeographyDPSLRevision NotesFreshwater: Drainage BasinsDrainage Basin Hydrology & GeomorphologyRiver Discharge & Channel Characteristics

River Discharge & Channel Characteristics (DP IB Geography): Revision Note

Briley Habib

Written by: Briley Habib

Reviewed by: Bridgette Barrett

Updated on

DP Geography: SLDP Geography: HL

River Discharge & Stream Flow

  • River discharge is the volume of water passing a given point over a set time

  • As rivers move downstream the characteristic features change

Bradshaw model

Diagram with blue gradient arrows showing changes in river characteristics from upstream to downstream, including discharge, channel width, depth, velocity, load quantity, particle size, bed roughness, and slope angle.
Bradshaw model

River Channel Characteristics

Component

Definition

Discharge

The volume of water passing a specific point in the river per unit of time increases downstream due to tributary contributions

Occupied channel width

The width of the river channel typically increases downstream as more water from tributaries is added

Channel depth

The depth of the river channel increases downstream as more water accumulates

Average velocity

The speed at which water flows within the river generally increases downstream with a greater volume of water and steeper gradients

Load quantity

Load quantity increases as the material is made smaller through erosion

Load particle size

Load particle size becomes smaller as the material is made smaller through erosion

Channel bed roughness

Channel bed roughness decreases as the river’s energy decreases, allowing for accumulation of finer sediments, leading to a smoother channel downstream

Slope angle 

The slope angle decreases as a river moves downstream

Hydraulic radius

A cross-sectional area of the flow divided by the wetted perimeter

How to measure discharge in a river

Diagram showing stream cross-section and labeled elements: measured stream length, velocity, cross-sectional area, and water depth. Formula: discharge = area_cs × velocity.
Measuring river discharge

Worked Example

A student wanted to calculate the discharge of a river. The measured stream length was 10 metres. The width of the channel was 4 metres.

To calculate discharge

c r o s s short dash s e c t i o n a l space a r e a space x space v e l o c i t y space equals space m ³ divided by s space left parenthesis c u m e c s right parenthesis

  • NB: all units of measurement should be the same in metres

Step one: find the mean depth

  • Calculate the mean depth by adding the measurements together and divide by the number of sites

Depth measurements for Site 1

 

1

2

3

4

5

6

7

8

Mean

Depth in mm

0.05

0.12

0.17

0.23

0.30

0.35

0.28

0.18

0.21

0.05 space plus space 0.12 space plus space 0.17 space plus space 0.23 space plus space 0.30 space plus space 0.35 space plus space 0.28 space plus space 0.18 space equals space 1.68 space

M e a n space d e p t h space equals space 1.68 space divided by space 8 space equals space 0.21 space m

Step two: calculate the cross-sectional area

  • Cross section (m²) = mean depth x width

  • The width is 4 m

C r o s s space s e c t i o n space equals space 0.21 space m space x space 4 space m space equals space 0.84 space m squared

Step three: find the velocity

  • Velocity = distance/time (V = d/t)

  • Distance is 10 m

Time measurements for Site 1

Time Measurement

Left

Centre

Right

1st

35

28

37

2nd

42

30

39

3rd

36

27

45

Mean

37.7

28.3

40.3

  • First find the mean time it takes for the float to travel 10 metres at site 1

  1. Add the total mean times together

  2. Divide the total by the number of positions

37.7 plus 28.3 plus 40.3 equals 106.3

M e a n space t i m e space equals space 106.3 space divided by space 3 space equals space 35.43 space s e c o n d s

  • Secondly, to find the velocity in m/s at site 1, divide the distance by the mean time (v = d/t)

space v e l o c i t y equals space fraction numerator d i s tan c e over denominator m e a n space t i m e end fraction space fraction numerator 10 space m over denominator 35.43 space s end fraction space equals 0.282 space m divided by s

  • The surface velocity for Site 1 is 0.28 m/s

  • Do not divide time by distance, as this will give you an incorrect answer.

Step four: discharge

  • Discharge = cross-sectional area x velocity in m³/s (cumecs)

D i s c h a r g e space equals space 0.84 space m ² space x space 0.28 space m divided by s space equals space 0.2352

  • Discharge at Site 1 = 0.24 m3/s (cumecs)

Factors affecting stream flow

  • Hydraulics is the study of water flow in channels

  • Water flow is determined by gravity and frictional resistance with the channel bed and banks

  • Channel volume and shape affect the stream's energy

  • When water flow is turbulent there will be eddying patterns

  • Turbulence supports the lifting and suspension of fine particles

  • Turbulent flow conditions include complex channel shapes, high velocities and cavitation

  • Laminar flow is characterized by smooth and layered movements and is common in groundwater and glaciers but not in rivers

  • Laminar flow occurs in shallow, smooth, straight channels with low velocities

  • Rivers' sediments remain undisturbed on the bed under laminar flow conditions

  • When water velocity is low turbulence is reduced

  • When water levels rise the mean velocity and the hydraulic radius enable the stream to appear to be more turbulent

Velocity

  • Friction causes uneven velocity distribution in a stream

  • The water closest to the bed and banks moves slowly

  • Water in the centre of the channel travels the fastest

  • Maximum velocity occurs mid-stream, about one-third down

  • Channel shape influences the velocity

Channel shape

  • Stream efficiency is measured using hydraulic radius (cross-sectional area divided by wetted perimeter)

  • Higher ratios indicate greater efficiency and less frictional loss

  • Channel shape is influenced by both channel material and river forces

  • Solid rock leads to slow changes and alluvium allows rapid changes

  • Silt and clay create steep, deep, narrow valleys, while sand and gravel promote wide, shallow channels

Channel roughness

  • Channel roughness introduces friction, reducing water velocity

  • Friction arises from bed irregularities, boulders, trees, vegetation and water-bed and bank contact

  • Manning's n is a formula describing the relationship between channel roughness and velocity

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Author
Reviewer
Briley Habib

Author: Briley Habib

Expertise: Geography Content Creator

Briley is an experienced Head of Geography. With more than 16 years of teaching experience, Briley was awarded a PGCE from the University of Lancaster and has a degree in European Studies and Human Geography. Briley has worked in a range of schools around the world and has experience of teaching at all levels. Briley is a member of the Geographical Association’s special interest group on diversity and inclusion. She has also written articles for the Teaching Geography Journal, a book chapter on Place-Based Education and a report on Decolonising IB Geography.

Bridgette Barrett

Reviewer: Bridgette Barrett

Expertise: Geography, History, Religious Studies & Environmental Studies Subject Lead

After graduating with a degree in Geography, Bridgette completed a PGCE over 30 years ago. She later gained an MA Learning, Technology and Education from the University of Nottingham focussing on online learning. At a time when the study of geography has never been more important, Bridgette is passionate about creating content which supports students in achieving their potential in geography and builds their confidence.