Erosion and Stream flow

The Play-Debref-Replay method of science is a good format for this activity (see the resource) when student groups each have a set of materials.
As a demonstration, group discussion is aligned with what the activity shows each moment. Students can direct the teacher where to place rocks, and try to model other ideas students mention e.g. tsunami, landslide.

To set up activity: Pile the sand up at one end of the tray, and clip the binder clip to the edge of the tray. Set up the siphon system by submerging the weighted end of the tubing in the large jug of water, then pushing the other end of the tubing through the binder clip at the end of the tray, so that the end hangs over the top of the pile of the sand. Use a squeeze bottle with the air expelled to suck on the tubing and remove the air, to get the water flowing. The water flow slowly runs down the slope of sand.
(Another way to set up this activity is to simply allow students to pour water over a pile of sand. As the flow rate is faster, the more subtle erosion patterns will not be seen, but the sand is still washed down the slope demonstrating erosion.)

Ask that the students simply watch the water flow for a while. This will be tricky if they have their own materials - when the water flow has been started, try requiring that the students watch the flow without touching until they receive their first rock to place in its path. Then pass out rocks one by one, for students to place in the path of the water to change its course, or split the river in two.

Actions for the students to try that focus on the water flow (rather than moving sand around):
Can you make the stream split into two?
Can you find the bank of a stream is washing away as the stream takes a new course?
Can you make a waterfall?
If the sand is made of different coloured particles, how do they separate out?
Where is the sand that is washed downstream being deposited?

After 5 or so mins of water flow, the teacher will need to raise the siphon system up, placing a book or block under the jug, to keep the flow rate up, then again after another little while. The activity ends when the water flow stops.

Group discussion of what students found, during the activity if it is a demonstration, or after students have completed the activity:

• The water flow makes channels in the sand. River valleys are formed the same way - the overlying soil, then the underlying rock are worn away by water (as well as ice and wind). Streams and rivers carve out our landscape to make valleys with mountains on either side, though over a much longer period of time than this activity. The process of sediment removal is called erosion.
• Sand is deposited at the edge of the pool of water at the bottom of the sand hill. Sediment is moved where water flow is faster, and deposited where the flow is slower, so wide shallow bays (deltas) are formed where rivers meet the ocean. Show this image of the Horton River delta in the Northwest Territories: https://www.google.ca/maps/@69.9505943,-126.8071953,12934m/data=!3m1!1e3 (showing a classic delta shape: a triangle, and named after the greek letter delta; the old path of the river can also be clearly seen). As sediment builds up on ocean beds it is compressed by further layering and eventually forms sedimentary rocks.
• The water flow can move the small sand particles but not the larger rocks. In the same way, small rock and soil particles are washed down rivers whereas large boulders remain, sometimes creating waterfalls. Students may notice sand particle colours separating as they are deposited - the differently-sized particles are carried and deposited at different rates.
• Changing the direction of the stream by placing rocks in its path models a rockslide or human structures e.g. dams or dykes, which change the path of rivers.
• Water will find a way down a mountainside, whether over, under or around rocks and other obstacles in its path. Flow of water underground forms cave systems. When we block the path of water with a dam, the stored water has energy, that can be converted to electrical energy as it falls over the turbines of a hydroelectric power station.

To include life sciences: streams and rivers bring life-essential water to animals and plants, bring food to animals that feed on aquatic life, move minerals around that are needed by living things, and provide habitats and homes for plants and animals.