In physical geography, an estuary is a place where which of the following occurs near a river mouth?

Introduction / Context:
Rivers can form different types of landforms at their mouths, such as deltas and estuaries. These features are important in physical geography because they affect navigation, ecosystems, and human settlement. This question asks for the correct definition of an estuary, focusing on what happens near the mouth of a river when it meets the sea. Understanding the difference between a delta and an estuary is essential for exam questions on river landforms.

Given Data / Assumptions:

• The landform in question is an estuary.
• All options describe conditions near the mouth of a river.
• We assume standard geography textbook definitions of delta and estuary.
• The key idea is the interaction between river water and seawater.

Concept / Approach:
An estuary is a partially enclosed coastal body of water where fresh water from rivers mixes with salt water from the sea. It is usually deep and funnel shaped, allowing tides to influence the river mouth. In contrast, a delta is formed where a river slows down and deposits large amounts of sediment, creating a triangular or fan shaped area of land. Therefore, the correct approach is to identify the option that refers to mixing of fresh and salt water rather than just silt deposition or triangular land formation.

Step-by-Step Solution:
Step 1: Recall the textbook definition of an estuary as a place where river fresh water meets and mixes with sea salt water. Step 2: Examine option a, which refers to a triangular piece of land made by mud deposits; this describes a delta, not an estuary. Step 3: Examine option b, which mentions silt deposition near the river mouth; this is also more characteristic of delta formation. Step 4: Examine option c, which says fresh water collects at the mouth; while rivers do bring fresh water, this does not capture the key idea of mixing with sea water. Step 5: Examine option d, which mentions fresh and salt water mixing near the river mouth; this is exactly what defines an estuary. Step 6: Select option d as the correct definition of an estuary.

Verification / Alternative check:
Verification can be done by consulting any school geography text or reference book. Estuaries are often described as drowned river valleys or funnel shaped inlets where tidal action is strong and where brackish water is found. Examples include the Thames Estuary in England and the Hudson Estuary in the United States. These descriptions always emphasise the mixing of river and sea water. Diagrams of deltas, on the other hand, show sediment deposition and land building, confirming the distinction and supporting option d as the correct one for estuaries.

Why Other Options Are Wrong:
Deposits of mud forming a triangular piece of land at a river mouth describe a delta, not an estuary, so option a is incorrect. Silt deposition near the mouth of a river is again typical of delta formation and does not capture the main feature of an estuary, so option b is incorrect. Fresh water collecting at the mouth, as in option c, misses the essential mixing process with sea water and therefore does not define an estuary.

Common Pitfalls:
The main pitfall is confusing deltas with estuaries. Because both features occur at river mouths, many students treat them as similar or interchangeable. However, the physical processes are different. Deltas build land by deposition where the sea is relatively calm, while estuaries occur where tidal forces dominate and sediment is not deposited in the same way. Focusing on the phrase fresh and salt water mix will help you quickly identify an estuary in multiple choice questions.

Final Answer:
An estuary is a place where fresh and salt water mix near the mouth of a river.