Question

How did the lab activities help you answer the lesson question: What factors affect the power of a river to cause erosion? What conclusions can you make from your observations?

Answer 1

The lab activities (stream table / flow-rate trials where we changed slope, flow rate, sediment size and channel obstacles) let us directly see how different variables change erosion and deposition. By changing one factor at a time and watching where sediment moved or accumulated, we could link cause and effect instead of just theorizing. Key observations and conclusions:

What we did and observed
- Slope (gradient): when we tilted the table steeper, the water ran faster and cut deeper channels. More sediment was carried downstream and the bed was scoured more rapidly.
Conclusion: steeper slope increases the river’s erosive power.

- Flow rate / discharge: increasing the pump flow made the stream wider, faster, and able to pick up and carry larger particles. Lower flow deposited sediment and built bars.
Conclusion: higher discharge increases erosive capacity and transport; low discharge favors deposition.

- Velocity and channel shape: faster flow concentrated in the thalweg (deepest part) and caused erosion at outer bends; slower flow on inner bends deposited point bars.
Conclusion: velocity distribution controls where erosion and deposition occur (outer bank erosion, inner-bank deposition forming meanders).

- Sediment size and amount (load): fine sand and silt moved easily and were carried far; gravel and coarse pebbles moved only under high flow and tended to armor the bed when abundant. When we added lots of loose sediment, downstream deposition increased.
Conclusion: particle size and availability strongly affect transport and where sediment is deposited; more available sediment can enhance downstream deposition.

- Bed/bank cohesion and vegetation (simulated roots/cover): areas with “vegetation” or cohesive material resisted erosion; bare, loose banks eroded quickly.
Conclusion: bank strength and surface cover reduce erosion; vegetation stabilizes banks.

- Obstacles and roughness: added rocks or roughness elements created turbulence, local scour downstream of obstacles, and changed flow paths.
Conclusion: channel roughness and obstacles alter local velocity patterns and can locally increase erosion.

General conclusion
- The power of a river to cause erosion depends mainly on discharge and slope (which together control velocity), modified by sediment size/amount, channel shape, bed/bank cohesion and roughness. In short: higher slope and higher discharge → higher velocity → greater erosive power and transport capacity, but the actual erosion pattern also depends on sediment characteristics and bank strength. The lab reproduced classic features (cut banks, point bars, sorting of sediments), supporting these relationships.