In the context of Indian rivers, which of the following rivers does not form an estuary at its mouth but is better known for forming a delta as it enters the sea?

Introduction / Context:
India has both west flowing and east flowing rivers, and they show different behaviour at their mouths. Some form estuaries where the river mouth is drowned by the sea, while others build deltas by depositing large quantities of silt. Understanding which rivers form estuaries and which form deltas is a standard topic in Indian physical geography.

Given Data / Assumptions:

• The question asks which river does not make an estuary.
• Options are Narmada, Tapi, Mandovi, and Mahanadi.
• We assume basic knowledge that many west coast rivers, especially those flowing into the Arabian Sea, tend to form estuaries.

Concept / Approach:
The Narmada and Tapi are major west flowing rivers that empty into the Arabian Sea. Due to strong tidal action and deeper coastal waters in the Gulf of Khambhat region, they form estuaries rather than extensive deltas. Mandovi is a smaller west flowing river in Goa that has an estuarine mouth. In contrast, the Mahanadi is an east flowing river that enters the Bay of Bengal in Odisha and is known for forming a broad delta. Therefore, Mahanadi does not create an estuary; it creates a deltaic coastline instead.

Step-by-Step Solution:
Step 1: Classify the rivers by direction of flow and coast of entry.Step 2: Recall that Narmada and Tapi both flow westwards and empty into the Gulf of Khambhat, forming estuaries.Step 3: Remember that Mandovi flows into the Arabian Sea near Goa and is described as an estuarine river.Step 4: Identify that Mahanadi flows east and enters the Bay of Bengal, forming the Mahanadi delta in Odisha.Step 5: Conclude that Mahanadi is the only river among the options that does not make an estuary.

Verification / Alternative check:
A physical map of India showing deltas and estuaries highlights the deltas of the east coast rivers such as the Ganga, Brahmaputra, Godavari, Krishna, Kaveri, and Mahanadi. The west coast is marked more by estuaries and rias rather than large deltas. Seeing the Mahanadi delta on such a map confirms that the river forms a delta and not an estuary at its mouth.

Why Other Options Are Wrong:
The Narmada forms a wide estuary in the Gulf of Khambhat and has no extensive delta. The Tapi similarly ends in an estuarine mouth. Mandovi is well known as an estuarine river flowing through Goa and supports riverine harbours. None of these are primarily associated with delta formation, so they do not satisfy the condition stated in the question.

Common Pitfalls:
Students sometimes assume that all major Indian rivers form deltas, forgetting that western coastal conditions favour estuaries. Another pitfall is confusing Mandovi with some east flowing rivers because of less familiarity. Keeping a simple rule in mind that most large east coast rivers form deltas and many west coast rivers form estuaries helps answer such questions quickly.

Final Answer:
The river that does not make an estuary but is known for forming a delta is the Mahanadi.