A dam is built on a river that has a riverbed that is primarily sand. You have a house just downstream of the dam, and you like to go trout fishing in the river in front of your house. A few years after the dam is built, it is likely that:

Question

BlackTom AI · Accepted Answer

Because dams trap sediment, water released downstream becomes “sediment-starved” and thus more erosive. That erosive, clear water will pick up and carry away the sand from a sandy riverbed, lowering the channel elevation.

1. Relevant knowledge points and concepts
- Reservoirs behind dams trap incoming sediment (sand, silt, gravel), so little of that sediment passes downstream.  
- The downstream flow, now carrying less sediment than its transport capacity, is called “hungry” or sediment-starved water and will erode bed and banks to pick up sediment.  
- Sandy beds are especially susceptible to erosion (sand grains are easily entrained), so channel incision (downcutting) is common downstream of dams.  
- Channel lowering can undercut banks and reduce river elevation relative to adjacent land, making former access paths steeper.

2. Step-by-step reasoning
- Before dam: river carries its natural sediment load; bed elevation in equilibrium.  
- After dam construction: reservoir traps most upstream sand and silt.  
- Water released downstream is relatively clear and has energy exceeding the sediment it carries.  
- This water erodes the sandy riverbed, transporting sand downstream and lowering the bed (incision).  
- Over a few years the channel in front of the house becomes deeper and banks steeper; the previous gentle path to the water is now a drop-off.

3. Intermediate steps (process detail)
- Sediment supply upstream decreases → sediment transport capacity > sediment supply → net erosion of bed.  
- Sandy bed grains are mobilized easily → measurable lowering of bed elevation over years.  
- Result: need for a ladder or steep path to reach lower water level.

4. Why the correct answer is correct
The first answer describes exactly the hungry-water erosion effect: clean (sediment-poor) releases from the dam wash sand away and lower the river elevation, forcing homeowners to build ladders or steep access. This matches well-established geomorphic responses downstream of dams.

5. Why other options are incorrect
- Option 2 (sand deposits bury house): opposite of expected effect—downstream of a dam typically has sediment deficit, not deposition that buries banks.  
- Option 3 (huge rocks released build up bed): dams trap coarse material too; floods do not suddenly release huge rocks that build the bed—net effect is erosion, not rock deposition.  
- Options 4 and 5 (Diet Pepsi dissolves house/sand): nonscientific and irrelevant.

6. Conclusion
Therefore the correct choice is the first option: you will likely have built a ladder or steep path because the clean water released by the dam will have washed sand away and lowered the river elevation. This matches the provided answer.

A dam is built on a river that has a riverbed that is primarily sand. You have a house just downstream of the dam, and you like to go trout fishing in the river in front of your house. A few years after the dam is built, it is likely that: 单项选择题

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