‘Investigating Riverbeds’
Another version of this article is published: Pratchett (1999) ‘Investigating
Riverbeds’ in Teaching Geography Vol. 24, No. 2 pp.82-84. Sheffield:
Geographical Association.
The Geography National Curriculum Context
The Key Stage 2 National Curriculum programme of study states:
Theme: Rivers
7b "In studying rivers and their effects on the landscape pupils should be
taught: how rivers erode, transport and deposit materials."
Skills
3b "Pupils should be taught to:
undertake fieldwork, including the use of instruments to make
measurements."
3a “use appropriate geographical vocabulary”.
In the following activities the children:
• used a stone board to measure and categorise pebbles on the riverbed;
• used vocabulary to describe and explain the shape and appearance of these
pebbles and to explain variations across the riverbed;
• discussed how these pebbles were transported, deposited and eroded each
other and the river banks.
Key questions:
•
•
•
•
•
What material is to be found on a river bed?
What is this material like and why?
Does this material vary in different places across and along the river bed
and if so why?
Where does this material come from?
Is there any evidence of erosion, transportation and deposition on the
river bed and where does it occur?
Key concepts:
erosion, transportation, deposition, attrition, traction, saltation, abrasion,
suspension, corrasion, hydraulic action, river-bed, flow, channel, bend,
bank, radius, diameter, circumference, length, width, depth, shape.
Figure 11:
The processes of erosion highlighted by the activity:
attrition
the process by which material moves along the river bed colliding
with other material and breaking up into smaller pieces.
the process by which the material carried by the water abrades the
river banks undercutting them and causing collapse.
the driving force of the water hitting a bank and causing erosion.
corrasion
hydraulic
action
Figure 12:
The processes of transportation highlighted by the activity:
traction
the process by which large boulders and rocks are rolled over and
over along a river-bed.
the way in which the river makes smaller stones bounce and skip
along the river-bed
saltation
suspension
the carrying of particles of material which are light enough to "hang"
in the water, giving a river its colour and muddy appearance.
A walk along the river bank
"Observation and recording will lead pupils to recognise the processes that
are taking place around them, thereby developing their understanding of
patterns and processes through first hand experiences of similarity and
difference."
Foley & Janikoun 1996 p. 108.
Prior to this activity, the children were taken on a walk along the river bank. The class
teacher gave emphasis to observation, questioning and discussion. The children's
attention was drawn to chunks of bank which had collapsed into the river and become
little islands. When the children were asked what these were and how they might have
got there, one child noticed what looked like a bite which had been taken out of the
bank. "It's come from here", she said, "the river's made it fall in". The children
were asked to find evidence of any other bits of bank which had fallen in or were
about to do so. At various places, deep cracks like crevasses were found in the turf on
the bank. It was concluded that these "bites" would soon suffer the same fate. In many
cases the turf roots were left exposed and dangling in mid air where the river had
washed away the surrounding earth. This highlighted the dangers of walking too close
to a river bank.
The children were asked to compare the two banks on a bend in the river and noticed
that it was the outside of the bend that was undercut and collapsing and that the
inside was a gently sloping bank of fine sand particles. A teacher-led discussion took
place about what the river was doing on each side and the concept of erosion ("eating
or wearing away") and deposition ("dropping and leaving behind") were introduced.
The reason for these processes was later explored using floating oranges to measure
surface speed in different parts of the river. Also embedded in the undercut bank were
small, medium and large lumps of granite, some of which were literally falling out of
their earth surround. These were compared and related to the bedload of stones, rocks
and boulders in the river and the hypothesis formulated that, as one child said, the
material on the river bed had “fallen out of the banks” because it had been “washed
out by the water” and “bashed by the pebbles”, (concept of hydraulic action and
corrasion).
Using a stone-board to measure and analyse material on the riverbed
The children were split into groups and given a stone board to measure, analyse and
categorise stones on the river-bed, (see photos ). I had made the boards with a semicircle chart for measuring the stones' radius and curvature, a ruler for measuring their
long and short axis and a shape chart for assessing roundness or angularity, (see figure
13). This board was covered with clear fablon for waterproofing and had a wooden
ledge at the bottom to prevent a stone slipping off while it was being measured. Some
children used the stone board to measure the radius of the stones but a group of more
able mathematicians converted these measurements into circumferences. They also
made fieldwork notes about the colour of each stone and used these later to enhance
their line drawings, (see figure 14 & 15).
"Teach geographical skills through fieldwork on physical geography. This
gives purpose to learning to observe, measure and record. At the same time
it encourages accuracy and the collection of detailed evidence."
(NCC. 1993 p.17)
To ensure that the children took a random sample of stones across the river-bed, they
took two paces, closed their eyes and picked up a stone in front of their foot. They
recorded their observations on a recording grid, (see figure 14 & 15).
Making deductions: what caused the stones to be round and different sizes in
different parts of the river?
"Emphasise the processes which produce features as well as descriptions of
features."
(NCC. 1993 p.17)
One group added an interesting comment to their grid:
"We noticed that the fast current carries away the smaller stones, wears the
larger stones down and leaves the large stones behind."
(Bradley, Lee, & Aaron 11)
This reflects some of their oral comments made while in the river; that the stones
were "bigger" in parts of the river "where the water goes faster" because as they
said, "little ones get taken away", (concept of transportation).The children
categorised most of the stones as "rounded" or "sub-rounded”on their charts. Their
reference to the "wearing down" of stones, (concept of erosion), arises from a
discussion of why they were rounded rather than angular shapes. We collected some
pebbles and demonstrated how they roll or skip along the river-bed, bumping into
each other and rubbing together. (Ideally, the opportunity to place angular stones in a
stone polishing drum back at school and to observe the way they become rounded
would help to reinforce this concept of abrasion).
One group of children sampled stones across a bend in the river. They noticed that on
the inside of the bend the stones were very tiny but increased in size as they
crossed towards the undercut bank on the outside of the bend. Through
discussion, the children began to grasp the idea that where the water slows down it
drops small stones and where it flows faster the small ones get washed away, leaving
only larger ones behind. This hypothesis was tested by floating oranges to see if speed
of flow was slower on the inside of the bend than the outside.
At another location where the river was straight and shallow, a group hypothesised
that,
"The stones were mostly the same all the way across the river because the
river was mostly shallow."
(Jennie, Kyleigh, Kelly, Roxanne 11)
Simulating a bend in the river
To reinforce the idea further, children can be lined up along a rope in the playground
and asked to move in an arc to give them a kinaesthetic experience of how much
faster they have to run if they are on one end (the outside of the bend) to keep up with
those on the other end (the inside of the bend).
Respect and concern for the environment:
For environmental reasons, it was felt that the stones should be returned after
inspection to the river-bed to minimise disturbance to fauna such as caddis fly larvae.
This was considered part of developing the children's concern and care for their
environment.
Photographs and examples of children’s work related to the investigation of the
Cherry Brook river bed on Dartmoor.
Note that the children are not wearing gloves as protection against waterborne
diseases. This is not good practice. The school involved left the gloves behind but
decided to proceed rather than abandon the fieldtrip. Any children with cuts on their
hands were not allowed to put them in the water.