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 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 to get the water flowing, so that the water slowly runs down the slope of sand.
(Another way to set up this activity is to simply allow students to pout 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.)
Ask that the students simply watch the water flow for a while. This will be tricky - when the water flow has been started, require that the students watch the flow without touching until they receive their first rock to place in its path.
Pass out rocks. Ask students to place them in the path of the water flow 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 made the stream split into two?
Can you find a place where the bank of a stream is washed away?
Can you make a waterfall?
Can you make a lake on the hill?
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.
Group discussion of what students found, 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 (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.
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. When sediment collects in oceans it is compressed by further layering and forms sedimentary rocks.
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. Students may notice sand particle colours separating as they are deposited - the differently-sized particles are carried and deposited at different rates.
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.
Water will find a way down a mountainside, whether over, under or around rocks and other obstacles in its path. 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.