1. RIVERS
AN INTRODUCTION

2. DEFINITIONS
Drainage Basin – the area drained by a river and its tributaries. Water reaches rivers from rainfall and from water flowing over land (surface runoff) and through soil and rocks (throughflow and groundwater flow). All of the rainwater that falls on a drainage basin is carried to the sea by the tributaries and main channel of one river.
Tributary – a smaller river which joins a bigger river, usually at an acute angle. In the case of tributaries that are part of a trellised drainage pattern, the tributaries join at right angles.
Confluence – where two rivers meet.

3. Definitions Source – where the river starts.
Mouth – where the river meets the sea. Most rivers flow downhill to the mouth.
Discharge – the volume of water flowing within the river channel past a certain point and at a certain time. It is measured in cubic metres per second (m³/sec) or cumecs. The discharge of a river can vary considerably in areas with a wet and dry climate or in areas which receive snowmelt in spring. Tributaries add water to the main river so the discharge increases further downstream, nearer the mouth. (Remember the Bradshaw Model)

4. Drainage Basin
All rivers receive a water supply and the area of land that this comes from is known as a drainage basin.
The boundaries of the basin are known as the watershed and will usually be marked by areas of high land.

5. Water Divide

6. Water Divide
Two adjacent drainage basins are separated from each other by a water divide – usually an area of high land.

8. The Bradshaw Model
The Bradshaw Model is a geographical model that shows how a river's characteristics change and vary between the lower and upper courses.
The left side represents the source of the river in the upper course and the right side represents the mouth.
The model looks at discharge, occupied channel width, channel depth, mean velocity, volume of load, load particle size, channel bed roughness and gradient.

9. The Bradshaw Model

10. River Variables - Discharge
Discharge:  ​The volume of water that passes through a stream's cross section in a given time period. 
Discharge = cross-sectional area x velocity
The cross sectional area of the river is:
average river depth x river width.
This gets larger as the river progresses from the source to the mouth, therefore, as the cross sectional area increases so to does the discharge.
River discharge increases as water enters the river channel from tributaries, surface runoff, throughflow and baseflow across the drainage basin.

11. Variables in a River

15. River Variables - Velocity
Mean Velocity: This is the speed the river is travelling at and is measure in m/s.
Velocity increases as a river progresses towards the mouth (although commonly believed otherwise) as more water is added to rivers via tributaries. This means that less of the water is in contact with the bed of the river and the banks so there is less energy used to overcome friction. Hence rivers flow progressively faster on their journey downstream.
In the upper course of the river, the cross sectional area is smaller and therefore, velocity is lower with a lot of energy lost due to friction.

16. River Variables – Velocity (2)
Gradient can have an impact on velocity
Rivers tend to be very shallow and narrow in their upper courses, which increases the friction acting on the water and slows it down despite the steep gradient.  
Velocity is also highly variable from location to location on a river, and is highly influenced by channel shape or form.  
Wider shallow channels have larger wetted perimeters (so more friction) and hence flow slower than narrower deeper channels.  
Velocity profiles also differ between symmetric and asymmetric.
The white foam that appears as the river flows is due to friction and does not indicate the speed of the water. 

17. A cross-section of a river will show that the velocity varies from one side to the other.
The lowest velocities are found where the river is shallow and so friction is greatest.
The maximum velocity is found near the river’s surface where the channel is deep. At this point, friction with both the air and the channel is minimal.
This point is often directly above the river’s thalweg.
Thalweg: A line running along a river’s profile linking its deepest points. A river’s fastest flow normally runs along it.

18. River Variables – Velocity (3)
Uniform Channel Velocity
Asymmetric Channel Velocity

20. River Variables – Roughness (1)
Channel Bed Roughness: Decreases as the river flows from the upper to lower course of the river.
Increased roughness causes the wetted perimeter to be higher in proportion to the area of the river. This increases friction and reduces the velocity of the river.
Pebbles, stones and boulders on the beds and banks increase the roughness of the channel.

21. River Variables – Roughness (2)

24. River Variables - Gradient
Gradient: The gradient of the river decreases from source to mouth.
The sources of rivers are often found at watersheds (high ridges of land separating adjacent drainage basins from each other) and the water starts to flow downhill.
This changes because the river changes from eroding vertically at the source, to laterally further downstream (together with the effects of deposition further downstream too).
The changes in gradient are related to discharge which increases as the gradient decreases.

25. How they are transported
River Variables - Load
Load: this is the total mass of material transported by a river.
The way in which material is moved depends on its size. There are downstream changes in the amount and the mean particle size of load.
Type of load
Type of particles
Diameter of particles
How they are transported
Bed load
Sand, pebbles
Over 0.1mm
Saltation and traction
Suspended load
Clay and silt mm
Suspension
Dissolved load
Soluble material -
Solution

26. River Variables – Load (2)
The competence of flowing water (or the river) is the maximum size of particle that the river can transport.
The capacity of a river is not the amount of water it contains (this is the discharge) but the maximum amount of load that the river can transport.
Therefore, a river's competence increases as the water velocity increases. At low velocities only clay and silt can be transported. As the velocity increases, larger particles such as sand and pebbles, can be transported. However, the relationship between velocity and the size of particles transported is not a simple positive correlation.
This relationship is seen in a Hjulström Curve

27. River Variables – Load (3)
The mean particle size decreases with distance downstream. This is not because the competence of the river has decreased. Instead smaller particles have become a proportionately higher component of the load. Why?
More time for erosion - the major source of pebbles and stones in the river is from the river's upper course. The further these rocks are carried downstream, the more time there will have been for them to have been eroded by attrition and abrasion. Abrasion and attrition makes rocks both smaller and rounder.
More time for weathering - much of the river's erosion occurs at times of high discharge. During times of low flow, stones are stored on the beds or banks. The longer the stones spend in storage, the longer they will be affected by weathering processes (such as frost shattering).
Sorting - the river sorts particles of different sizes. Smaller particles are carried at lower velocities. These particles remain in the water flow during periods of low flow when larger particles are deposited.

29. Hjulström Curve
The relationship between erosion, transport and deposition of sediment is complex and can be shown by the Hjulstrom diagram.

30. River Variables – Cross-Sectional Area
Cross sectional area is another useful measure, and it shows in m² the surface area of a river from bank to bank.  
This can be calculated in 2 ways:
the average depth can be multiplied by the width to provide an approximate but not perfect measurement.    
If you have enough depth measurements, you could plot an accurate to scale cross section on graph paper and from that you will be able to calculate a more accurate cross sectional area.  
The CSA should increase downstream as water feeds into main streams from tributary rivers.

31. River Variables – Wetted Perimeter
Wetted Perimeter: is the linear measure of how much water contact there is with the bed and the banks of a river.  
This should increase downstream as water feeds into main streams from tributary rivers.

32. River Variables – Hydraulic Radius
Hydraulic radius is a measure of how efficient the channel is at transporting water and sediment. 
According to the Bradshaw model, this should increase as the stream increases in size and thus power, and the channel bed should get less rough/turbulent due to the effect of erosion. 
Hydraulic Radius is calculated by dividing the cross sectional area by the wetted perimeter.

35. The Schumm model

37. Long Profile Gradient refers to how steep an area of land is.
The long profile of a river shows the change in gradient of the land through which a river flows. Rivers usually start on high land and flow downhill to the sea.
In the upper course, rivers flow down steep gradients to reach lower land.
The lower course is more gentle as the river is closer to sea level. When the river reaches sea level, the channel flows over flat land.

39. Cross-Sections or Cross-Profiles
A river cross-section or cross-profile shows the shape of a river channel and valley at certain points in the river’s course.
The cross profile of a river changes as it moves from the upper to lower course as a result of changes in the river’s energy and the processes that the river carries out.
The river bed is the bottom of the channel.
The river banks are the sides of the channel.
When the water reaches the top of the banks the river is at bankfull and the volume of water is bankfull discharge.
If bankfull discharge is exceeded, the water flows over the top of the banks and onto the surrounding channel. This is a flood.

41. Cross-Sections or Cross-Profiles
In the upper course, the valley and channel are narrow and deep as a result of the large amount of vertical erosion and little lateral erosion.
The sides of a river’s valley in the upper course are very steep earning these valleys the nickname “V-Shaped Valley” since they look like a letter V.
The river’s valley can be anything from a few meters to a few hundred metres in width depending on the geology and structure but the channel rarely more than 5m or 6m wide.

42. Upper Course Valleys

43. Cross-Sections or Cross-Profiles
In the middle course, the valley has increased in width due to an increase in lateral erosion but its depth hasn’t changed significantly because vertical erosion has slowed down.
Similarly, the channel’s width has increased but it’s still roughly the same depth.
The land to either side of the channel in the valley is now the river’s floodplain and the valley’s sides are much more gentle.

44. Middle Course Valleys

45. Cross-Sections or Cross-Profiles
In the lower course the valley is now very wide (often several kilometres) and the floodplain has increased greatly in size.
The channel is a little wider but not much deeper.

46. Lower Course Valleys

47. Lower Course Valleys