What happens hydrologically when we add a dam to a river profile at the point when the stream enters the reservoir?

Question

BlackTom AI · Accepted Answer

When a dam forms a reservoir the flow entering the impounded area slows dramatically, so the river’s ability to carry sediment (its transport capacity) decreases.

1) Relevant knowledge points and concepts
- Flow velocity, channel gradient and sediment transport: sediment transport capacity depends strongly on velocity and slope; higher velocity => higher capacity and competence (largest particle size transportable).
- Reservoir/backwater effect: upstream of a dam the water surface is raised and the local gradient becomes much gentler (backwater zone), producing lower flow velocities.
- Deposition vs. erosion: reduced velocity promotes deposition (sediment settling) rather than erosion.

2) Step‑by‑step reasoning
- Installing a dam raises the water level upstream, flattening the local channel slope (gentler gradient).
- Continuity (Q = A · v) means for the same discharge Q, an increase in cross‑sectional area A (flooded reservoir) leads to a lower mean velocity v.
- Sediment transport rates fall steeply as velocity falls, so the mass of sediment that can be carried downstream from that point declines.
- Result: sediment settles in the reservoir (forming a delta), reducing downstream sediment delivery and reducing the stream’s transport capacity at the reservoir entrance.

3) Calculations / quantitative relations (intermediate steps)
- Qualitative relation: sediment transport capacity ∝ v^n (n > 1, often large), so even modest reductions in v reduce capacity substantially.
- Using continuity: if A increases (flooded area) and Q constant, v = Q/A decreases → capacity decreases.

4) Why the correct answer is correct
Because the dam raises upstream water level and reduces local slope and velocity, the stream’s sediment‑transport capacity at the reservoir entrance drops; sediments deposit rather than being transported.

5) Why the other options are incorrect
- "erosion will occur": incorrect — reduced velocity promotes deposition upstream of the dam, not erosion (erosion may increase downstream of the dam due to sediment‑starved water, but not at the entrance to the reservoir).
- "velocity will increase": incorrect — velocity decreases in the reservoir/backwater zone because area increases and gradient is reduced.
- "competance will increase": incorrect — competence (max particle size transportable) decreases with lower velocity; coarser particles settle out first.

6) Conclusion
Therefore, the correct hydrological effect at the stream’s entry to the reservoir is that the sediment transport capacity will decrease, matching the provided answer: "capacity will decrease."

What happens hydrologically when we add a dam to a river profile at the point when the stream enters the reservoir? Single choice

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