Physiographic Feature Parallel to Shiwaliks Explained

The question asks to identify a specific physiographic feature that runs parallel to the Shiwaliks and measures approximately $8$ to $16$ km in width. Let's analyze the geography involved.

Understanding the Shiwalik Context

The Shiwaliks, also known as the outer Himalayas, form the southernmost or lower Himalayan range. They are characterized by young, folded mountains. Physiographic features found near them often result from the processes occurring at the transition zone between the mountains and the plains.

Defining the Bhabar Region

The Bhabar is a region characterized by a series of gravelly fans and porous beds formed by the deposition of boulders, pebbles, and coarse sand by the streams descending from the Himalayas. Key features include:

• Location: It lies parallel to the Shiwalik range, marking the northernmost part of the Indo-Gangetic plain.
• Width: The Bhabar tract is typically narrow, generally ranging from $8$ to $16$ km in width.
• Characteristics: Due to the coarse porous nature of the deposits, most streams flowing into the Bhabar disappear underground, reappearing further south in the Terai region.

This description perfectly matches the conditions mentioned in the question.

Evaluating Other Options

• Terai: The Terai region lies south of the Bhabar. It is characterized by marshy land, dense forests, and a higher water table, as the underground streams from the Bhabar re-emerge here. It is typically wider and wetter than the Bhabar.
• Delta: Deltas are landforms created at the mouth of a river where it enters a larger body of water (like the sea or a lake) due to sediment deposition. This is geographically distinct from the area described relative to the Shiwaliks.
• Doab: A Doab is the region of land situated between two confluent rivers. While significant in the Indo-Gangetic plain, it doesn't specifically describe a feature running parallel to the Shiwaliks with the given width characteristics.

Therefore, the physiographic feature that is parallel to the Shiwaliks and about $8$-$16$ km wide is the Bhabar.