The correct option is (b).
Explanation
Soil conservation involves various methods to prevent soil erosion and degradation. Rock dams are a mechanical measure specifically designed to manage water runoff and stabilize soil in areas prone to water erosion.
Detailed Analysis
Rock dams consist of rocks piled across a channel or drainage line. Their primary function is to obstruct the flow of water, thereby reducing its velocity. By slowing down the runoff, the erosive capacity of the water is significantly diminished. This process is crucial for:
- Preventing Gullies: High-velocity water cuts deep channels into the soil, forming gullies. Rock dams check this flow, preventing the formation of new gullies and stabilizing existing ones.
- Reducing Soil Loss: By mitigating the force of flowing water, the displacement of topsoil is minimized.
Comparison with other options:
- Option (a): Retaining soil moisture is primarily achieved through methods like mulching (covering bare ground with organic matter).
- Option (c): Protecting soil from rain wash by growing alternate crops refers to intercropping.
- Option (d): Checking wind movement is the purpose of shelterbelts (rows of trees planted to break wind speed).
Key Takeaway: Rock dams are barriers constructed to slow down water flow, specifically to prevent the formation and expansion of gullies and subsequent soil loss.