Correct Option: B. Fall in gradient at the base of the hill Explanation of the Correct Answer: When we talk about deposition in a river, we are referring to the process where sediments, soil, and other materials carried by the river are dropped or settled in a new location. This process is heavily influenced by the gradient or slope of the riverbed.

1. Understanding Gradient:
2. The gradient of a river is the steepness or incline of its channel. A steep gradient means the river flows quickly, which allows it to carry more sediment. Conversely, a gentle gradient means the river flows more slowly, which reduces its ability to carry sediment.
3. Fall in Gradient:
4. In the case of River Amy, if there is a fall in gradient at the base of the hill, the river will slow down significantly. This reduction in speed means that the river can no longer carry all the sediment it has picked up upstream. As a result, the excess sediment is deposited along the riverbed and banks, leading to the formation of features such as bars, islands, or deltas.
5. Geological Context:
6. When a river exits a steep area (like a hill) and enters a flatter area, the sudden decrease in energy causes sediment to settle. This is a common occurrence in river systems and is a fundamental concept in fluvial geomorphology.

Why the Other Options are Incorrect: A. A construction of a dam across the river: - While a dam can cause deposition upstream due to the creation of a reservoir, it does not directly explain deposition along the channel of the river downstream. In fact, dams often lead to sediment being trapped, which can reduce deposition downstream. Therefore, this option does not apply to the scenario described. C. Marked increase in the river's erosive power: - An increase in erosive power would typically lead to more sediment being carried away rather than deposited. If the river is eroding more, it is likely to be transporting sediment downstream rather than allowing it to settle. Thus, this option contradicts the concept of deposition. D. Channelization of the river course: - Channelization refers to the alteration of a river's natural course, often to control flooding or improve navigation. While channelization can affect sediment transport, it does not inherently lead to deposition unless it results in a significant change in gradient or flow velocity. In this case, it does not directly explain the deposition observed along River Amy. Summary of Key Points:

• Deposition occurs when a river slows down and loses energy, often due to a decrease in gradient.
• A fall in gradient at the base of a hill leads to reduced flow velocity, causing sediment to settle.
• Other options (dam construction, increased erosive power, channelization) do not adequately explain the deposition process in this context.
• Understanding the relationship between gradient and sediment transport is crucial in fluvial geomorphology.

This thorough understanding of how gradient affects river behavior will help you in both theoretical and practical aspects of geography.