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: attrition corrasion hydraulic action The processes of erosion highlighted by the activity: 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. Figure 12: traction saltation suspension The processes of transportation highlighted by the activity: 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 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.