Drainage Systems: Factors Shaping Water Movement in Geographical Contexts

The term “drainage” refers to the movement of water through well-defined channels, and the collective system of these channels is referred to as a “drainage system.” 

The specific pattern of drainage in a given area is a result of various factors, including the geological history of the region, the type and composition of rocks, the topographical features, the slope of the land, the volume of water flowing through, and the regularity of the flow over time.

How does a River Basin differ from a Watershed in the context of a Drainage System?

• Catchment Area definition: A river serves as the drainage outlet for water collected from a specific region known as its “catchment area.”
• Understanding Drainage Basin: An area encompassing a river and its tributaries, which collectively contribute to the river’s water flow, is termed a “drainage basin.”
• Significance of Watershed: The demarcation line that separates one drainage basin from another is referred to as the “watershed.”
  • In practice, catchments of larger rivers are commonly referred to as “river basins,” whereas those of smaller rivulets and rills are often termed “watersheds.”
• The role of Water Divides: Any elevated area, such as a mountain or an upland, separates two drainage basins. Such an upland is known as a water divide (Refer Figure).
• River Basins: Large Geographic Regions: It generally covers large geographic areas compared to watersheds, which are relatively smaller.
  • Both river basins and watersheds share a fundamental characteristic:
  • They exhibit unity within themselves, what occurs in one part of a river basin or watershed has a direct impact on other parts and on the entire unit.

• This interconnectedness makes them ideal units for micro, meso, or macro-level planning and management considerations in Drainage Systems. As actions in one area can have far-reaching consequences throughout the entire basin or watershed.

The Indian Drainage System: Size, Origin and Discharge

The Indian drainage system can be distinguished on the basis of discharge of water, size of watershed and on the basis of origin i.e. Himalayan or Peninsular (Refer Figure).

India’s Water Divide: Arabian Sea vs. Bay of Bengal Drainage System

• The Indian drainage system can be grouped into two, the Arabian Sea drainage and the Bay of Bengal drainage. 
  • Geographical Dividers : Both the drainage systems are separated through the Delhi ridge, the Aravallis and the Sahyadris.
• Flowing Westward: The Arabian Sea Drainage system: Nearly 23 percent, comprising the Indus, the Narmada, the Tapi, the Mahi and the Periyar systems, and discharging their waters in the Arabian Sea.
• Eastward Streams: The Bay of Bengal Drainage system: 77 percent of the drainage area, consisting of the Ganga, the Brahmaputra, the Mahanadi, the Krishna, etc. and is oriented towards the Bay of Bengal.

Organizing Indian Drainage system: Major, Medium, and Minor River basins by size

On the basis of the size of the watershed, the drainage basins of India are grouped into three categories:

• Major river basins with Extensive Catchment Areas: These are river basins with more than 20,000 sq. km of catchment area.
  • It includes 14 drainage basins such as the Ganga, the Brahmaputra, the Krishna, the Tapi, the Narmada, the Mahi, the Pennar, the Sabarmati, the Barak, etc
• Medium river basins with Balanced Catchment Areas: These are river basins with catchment areas between 2,000-20,000 sq km.
  • It  incorporates 44 river basins such as the Kalindi, the Periyar, the Meghna, etc.
• Minor river basins: Small but Significant: These are river basins with catchment areas of less than 2,000 sq. km.
  • A fairly good number of rivers flowing in the area of low rainfall.

Himalayan and Peninsular Drainage systems: Understanding the Origin and Nature

On the basis of the mode of origin, nature and characteristics, the Indian drainage system may also be classified into the Himalayan drainage and the Peninsular drainage.