Science 7–10 · Year 8
River in a gutter: erosion, transport and deposition
Science understanding: Earth and space sciences (NSW Stage 4 focus area: Change)
The idea
Flowing water erodes, carries and drops sediment according to its speed, so a steeper or fuller channel moves larger grains and a slowing flow builds fans and deltas.
What you need
- 1 length of square-section gutter (1 to 2 m) or a shallow plastic box lined with a plastic sheet, half filled with washed sand and some small pebbles
- a water supply: a 2 L jug or a hose with a slow tap; a bucket to catch the outflow
- 1 protractor or ruler and wooden spacers to set the slope; 1 stopwatch; 1 measuring jug for flow rate
- a pinch of iron filings or crushed dark heavy mineral for the placer deposit extension
How to do it
- Set the gutter on a low slope, make a straight finger channel and pour 1 L of water slowly at the top over 60 seconds; watch and record which moves first, sand or pebbles, and where the sand is dropped.
- Repeat pouring 1 L in 20 seconds (three times the flow) and record the changes: channel deepening, braiding, pebbles rolling along the bed, the fan or delta at the outlet.
- Raise the slope (double the height) and repeat the slow pour; record the differences.
- Add iron filings to the top sand and note where they collect after a pour.
- Sketch the final channel and label erosion, transport and deposition zones; measure the distance the largest moved pebble travelled in each run.
What you should see
Sand moves first and pebbles only when the flow is fast or the slope steep (the Earthlearningidea notes that a 10 percent rise in velocity raises the flow's energy by 21 percent); the slow pour on the low slope moves sand by sliding and rolling and drops it a short way down, the fast pour deepens the channel, cuts side channels into a braided pattern and pushes pebbles, and the outflow builds a fan or micro-delta where the water slows. Dense filings collect where the flow slows, modelling a placer deposit. The learner knows it worked when the largest pebble moves further in the fast run and on the steep slope than in the slow, low run.
What changes
- What you change
- flow rate and slope
- What you measure
- grain size moved and distance moved, channel shape
- What you keep the same
- sand bed depth
- water volume
- starting channel shape
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Rivers only carry mud; they move gravel and boulders in flood.
- Sediment is dropped where the river stops; it is dropped wherever the water slows, including inside the channel.
Safety card
Hazards
- water on the floor
- sand in drains
Controls
- run outdoors or over a tray with a catch bucket
- sand never poured down a sink
Note
No hazardous chemical and no flame or heating apparatus: a generic classroom risk assessment (CSIS 1.7 or RiskAssess) covers trips, spills, warm lamps and sharp edges.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7-10 Syllabus (2023), NSW Education Standards Authority (implemented from 2026; codes read at curriculum.nsw.edu.au on 22 and 23 September 2026)SC4-CHG-01SC4-WS-03SC4-WS-04
- Australian Curriculum v9AC9S8U04AC9S8I02AC9S8I04
Sources
The pages the author read to write this activity.