Which region acts as the water divide between the Ganga and Indus river systems?

Introduction / Context:
A water divide or watershed is a geographical highland that separates the drainage basins of different river systems. In northern India, it is important to know the region that separates the catchment areas of the Ganga and Indus systems, since this influences river flow directions, irrigation patterns, and flood behaviour in the plains. This question asks which region serves as the water divide between these two major river systems.

Given Data / Assumptions:

• The question concerns the water divide between the Ganga and Indus river systems.
• The options include Haridwar, Namcha Barwa, Alaknanda valley, Ambala region, and Patna region.
• We seek the region where rivers on one side flow towards the Ganga system and on the other side towards the Indus system.

Concept / Approach:
On the Indo Gangetic plain, the Ambala region in Haryana is widely known as a subtle but important water divide. From this area, some rivers and canals flow westwards towards the Indus basin, while others flow eastwards towards the Ganga basin. It is not a tall mountain ridge but a slight elevation and slope difference on the plain. Other locations like Haridwar, Namcha Barwa, Alaknanda valley, and Patna lie clearly within one river system and do not function as the main divide.

Step-by-Step Solution:
Step 1: Recall that on the plains, water divides can be gentle and are sometimes referred to in exam books, especially Ambala as a classic example. Step 2: Ambala in Haryana is described in geography texts as a region from which the Yamuna and Ghaggar systems separate, linking to Ganga and Indus basins respectively. Step 3: Haridwar is located on the Ganga itself and does not separate Ganga and Indus systems. Step 4: Namcha Barwa is a mountain peak near the Great Bend of the Brahmaputra and unrelated to the Ganga Indus divide. Step 5: Alaknanda valley and Patna region lie firmly within the Ganga basin and do not act as divides between basins.

Verification / Alternative check:
Maps of river basins in northern India often mark Ambala as the drainage divide between the Ganga and Indus systems. Water and land management studies in Haryana also describe how streams and canals from this region are directed into different basins. This repeated acknowledgement in multiple sources confirms that Ambala region in Haryana is the correct answer.

Why Other Options Are Wrong:

• Haridwar region in Uttarakhand: Located on the Ganga River and not acting as a divide with the Indus basin.
• Namcha Barwa region in Tibet: Associated with the Brahmaputra Great Bend, not with Ganga Indus drainage separation.
• Alaknanda valley in Uttarakhand: Entirely within the Ganga system through the Alaknanda and Bhagirathi tributaries.
• Patna region in Bihar: Located deep in the Ganga plains with no role in dividing the Indus basin.

Common Pitfalls:
Because the term water divide is often associated with high mountains, some students mistakenly look for Himalayan locations such as Namcha Barwa or Haridwar. They overlook that subtle elevation differences on plains can also act as divides. Another mistake is not revising the specific example of Ambala that is often highlighted in textbooks. Close reading of the drainage chapter usually prevents this error.

Final Answer:
The region that serves as the water divide between the Ganga and Indus river systems is the Ambala region in Haryana.