Which of the following groups of rivers in India is likely to show the least erosional activity in its courses?

Introduction / Context:
The erosional activity of a river depends on factors such as gradient, volume of water, sediment load, and the nature of rocks or sediments it flows over. Rivers in different physiographic regions of India therefore show different intensities of erosion, transportation and deposition. This question asks which group of rivers is likely to show the least erosional activity, that is, they erode their beds and banks the least.

Given Data / Assumptions:
• Four groups of rivers are mentioned: Himalayan rivers, rivers rising in the Great Plain, thin streams flowing into the Arabian Sea, and east flowing peninsular rivers. • Erosional activity is linked to steepness of slope and river energy. • We assume generalised behaviour of each group across its course. • Least erosional activity implies more deposition and gentler gradients.

Concept / Approach:
Himalayan rivers originate in high mountains with steep gradients and large discharges, so they are powerful agents of erosion, especially in their upper and middle courses. Short streams flowing into the Arabian Sea from the Western Ghats usually have steep slopes and cut deep valleys, again showing strong erosion. East flowing peninsular rivers like Godavari and Krishna flow over old hard rocks and can also cut valleys in their upper reaches. In contrast, rivers that originate entirely within the Great Plain of India start on an already low relief alluvial surface, have very gentle slopes, and are dominated more by lateral shifting and deposition than by vigorous vertical erosion. Therefore, they show the least erosional activity overall.

Step-by-Step Solution:
Step 1: Consider Himalayan rivers such as Ganga, Indus tributaries and Brahmaputra. In their upper courses they are youthful, steep and highly erosive. Step 2: Think about small west flowing coastal streams that rise in the Western Ghats and flow quickly to the Arabian Sea. Their short length and steep gradient produce strong local erosion. Step 3: Note that east flowing peninsular rivers rise on the plateau, where slopes are moderate, and many of them have incised valleys, indicating significant erosion, at least in upper courses. Step 4: Rivers that originate within the Great Plain, however, are born on alluvial terrain with slight slope and mainly adjust their channels by deposition and lateral migration rather than aggressive down cutting. Step 5: Hence, they can be described as showing the least erosional activity of the options given, so this group is the correct answer.

Verification / Alternative check:
Geomorphology texts classify rivers on plains as typically mature to old stage rivers with lower energy available for vertical erosion. They mostly rework and deposit sediments rather than carve out deep valleys. In contrast, mountain rivers and ghats streams are used as classical examples of vigorous erosion. This supports the choice of rivers rising in the Great Plain as the group with the least erosional activity.

Why Other Options Are Wrong:
The Himalayan rivers are youthful, high energy rivers in their upper reaches and are heavily involved in vertical and lateral erosion, so they cannot be described as least erosive. Thin short streams flowing into the Arabian Sea from the Western Ghats have steep gradients and tend to be torrential during the monsoon, which increases their erosive power. East flowing peninsular rivers flow over hard rock plateaus and show significant erosion, especially where they cut through hill ranges before entering the coastal plains.

Common Pitfalls:
A common mistake is to assume that short streams have less erosional activity simply because they are small in size. In reality, steep gradients and concentrated discharge can make such streams very effective agents of erosion. Another error is to think that older rivers on plateaus erode less, but many peninsular rivers still cut valleys in their upper reaches. Remember that rivers starting on flat alluvial plains have the gentlest slopes and therefore the least erosive power overall.

Final Answer:
The group of rivers that is likely to show the least erosional activity is rivers that rise within the Great Plain of India.