Why is river water typically less clear at the mouth than at the source?

• A. Because evaporation at the mouth increases water clarity.
• B. Because the river accumulates more sediments and pollutants as it flows downstream.
• C. Because the source contains more dissolved minerals that cloud the water.
• D. Because the water at the mouth is colder, causing particles to settle.

Answer: (B)

Verified step by step guidance

1. Understand the question: The problem is asking why river water is typically less clear at the mouth compared to the source. This involves understanding the factors that affect water clarity as the river flows downstream.
2. Clarify the concept of sediment accumulation: As a river flows downstream, it picks up sediments, organic matter, and pollutants from the surrounding environment, including soil erosion, runoff, and human activities. These materials contribute to reduced water clarity at the mouth.
3. Evaluate the incorrect options: The statement about evaporation increasing water clarity is incorrect because evaporation primarily affects water volume, not clarity. Similarly, the idea that colder water at the mouth causes particles to settle is not accurate, as temperature changes do not significantly impact sediment suspension in this context.
4. Analyze the source of the river: At the source, the water is typically clearer because it has not yet traveled far enough to accumulate significant sediments or pollutants. Dissolved minerals at the source may exist, but they do not cloud the water as much as suspended particles do downstream.
5. Conclude with the correct reasoning: The correct answer is that river water at the mouth is less clear because the river accumulates more sediments and pollutants as it flows downstream, which reduces water clarity.