Which feature is commonly formed in the lower course of a river?

In the lower course of a river, a delta is a commonly formed feature due to the deposition of sediment as the river reaches its mouth and slows down. As the river approaches larger bodies of water, such as seas or oceans, its velocity decreases significantly, leading to the sediment it carries being deposited. Over time, this accumulation of sediment builds up to create a landform known as a delta, characterized by its triangular shape. Deltas are typically rich in nutrients and are often associated with fertile agricultural areas, making them important for both ecosystems and human activities.

Other features, such as waterfalls and gorges, are more commonly found in the upper course of a river where there is greater gradient and energy. An estuary, while also a feature associated with river mouths, represents a transitional zone where freshwater from rivers meets saltwater from the sea, but it does not involve the formation of land as delta does. Thus, the unique depositing process in the lower course unmistakably leads to the formation of a delta, underscoring why this is the correct answer.