The Play-Debref-Replay method of science is a good format for this activity (see the resource).
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 suck on the other end of the tubing to get the water flowing. Push the tubing through the binder clip at the end of the tray so that the water slowly runs down the slope of sand.
(Not as ideal: students simply pour water over a pile of sand, scooping more water from the end of the tray.)
Allow time for free experimentation, so that students can see how how the water flows and moves the sand.
They can place the rocks as they wish to affect the water flow.
Draw what happens, to report to the group.
After about 5 mins, the teacher will need to raise the siphon system up, on a book or something, to keep the flow rate up, then again after another little while.
If students need ideas of actions to focus on the water flow:
If the sand is made of different coloured particles, how do they separate out?
Where is the sand being deposited in the pool of water?
Can you make a waterfall?
Can you made the stream split into two?
Can you find a place where the bank of a stream is washed away?
Group discussion of what students found. Write up summaries of their findings, using terminology relevant to stream flow and erosion:
Students see channels forming in the sand, through which the water flows. River valleys are formed the same way - the overlying soil, then the underlying rock are worn away by water. Streams and rivers carve out our landscape to make valleys with mountains on either side.
The rate of flow of the stream depends on how wide it is.
Students change the direction of the stream by placing rocks in its path. Similarly, rockslides or human structures e.g. dams change the path and flow rate of rivers.
Students may notice that water 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. The process of sediment removal is called erosion. Erosion of river banks makes curves in the path of the river.
Students may notice sand being deposited into the shallow pool at the bottom of the tray. Sediment is moved where water flow is faster, and deposited where the flow is slower, so wide shallow bays are formed at where rivers meet the ocean. Students may notice sand particle colours separating as they are deposited. In the same way, differently-sized sediments are separated out as a river slows down.
To include life sciences: streams and rivers bring life-essential water to animals and plants, bring food to animals that feed on aquatic life, and provide habitats and homes for plants and animals.