HPAS 2024 GS1 Question 5

HPAS 2024 Mains GS-1 Question 5

Why is the western coastal plain devoid of any delta? Explain.

Solution:

The western coastal plain is devoid of deltas primarily because the west-flowing rivers form estuaries, not deltas. This is a direct consequence of the region’s geography, geology, and hydrology, which stand in sharp contrast to the conditions on the eastern coast.

A delta is a landform created by the deposition of sediment carried by a river as the flow leaves its mouth and enters slower-moving or standing water. This requires a large sediment load and a shallow sea. An estuary is a partially enclosed coastal body of water where fresh water from rivers mixes with salt water from the sea, typically characterized by a deep, funnel-shaped mouth.

Key Reasons for the Absence of Deltas

The conditions on the western coast are unfavorable for delta formation for the following reasons:

• Steep Gradient and Short Course: The Western Ghats, which are the source of most west-flowing rivers, are very high (a fault scarp) and run close and parallel to the coast. This results in:
  • The rivers having very short courses.
  • A very steep gradient, leading to high water velocity.
  This high velocity gives the rivers immense erosive power but no time or suitable terrain to deposit silt. The sediment is carried swiftly out into the sea.
• Hard Rock Terrain: The west-flowing rivers primarily flow over the hard, crystalline rocks of the Deccan Plateau (Deccan Traps). This hard rock is not easily eroded. As a result, the rivers carry a very low sediment load. Deltas are built from sediment, and these rivers simply lack the raw material.
• Narrow Coastal Plain and Continental Shelf: The western coastal plain is a narrow strip of land (a submerged coastline). More importantly, the continental shelf off the western coast is narrow and drops off steeply. Any sediment that does reach the coast is quickly washed into deep water by waves and currents, preventing the formation of a depositional landform like a delta.

The Special Case: Narmada and Tapti

One might ask why the Narmada and Tapti, which are much longer rivers, also do not form deltas. This is because:

They flow through rift valleys (grabens). These valleys are formed by faulting and are composed of hard rock.

Flowing through these hard-rock troughs means that even these long rivers do not pick up significant amounts of silt. They also maintain a strong current, ultimately emptying into the wide estuaries of the Gulf of Khambhat rather than depositing sediment to form a delta.

Contrast with the Eastern Coast

In contrast, east-flowing rivers like the Godavari, Krishna, Mahanadi, and Kaveri all form massive deltas (e.g., the Sundarbans, a joint delta of the Ganga and Brahmaputra, is the world’s largest).

This is because they:

• Have very long courses, flowing gently across wide plains.
• Flow over softer, alluvial terrain, picking up a huge sediment load.
• Empty onto a wide, emergent coastal plain and a broad, shallow continental shelf, which is ideal for sediment deposition.

Conclusion

The absence of deltas on the western coast is a classic example of geological and geographical determinism. The combination of a steep gradient, hard rock, and a narrow, deep continental shelf makes delta formation impossible and favors the creation of deep estuaries.

Concise Model Answer (150-Word Limit)

The western coastal plain is devoid of deltas because its rivers form estuaries. This is due to three main factors:

1. Steep Gradient & Short Course: The Western Ghats are high and very close to the coast. This makes the rivers short, swift, and straight, giving them high velocity that prevents sediment deposition.
2. Hard Rock Terrain: Rivers flow over the hard crystalline rocks of the Deccan Plateau, which are not easily eroded. This results in a very low sediment load—the primary building material for a delta.
3. Narrow Continental Shelf: The western plain is narrow and the shelf is steep. Any sediment carried by the rivers is quickly dispersed into the deep sea, not deposited at the river’s mouth.

Even the longer rivers, like the Narmada and Tapti, flow through hard-rock rift valleys, limiting their silt load and causing them to form estuaries in the Gulf of Khambhat.