Drainage System

Drainage System

Drainage refers to the flow of water through well-defined natural channels such as rivers, nalas, and smaller streams that drain excess water during the rainy season. These channels play a crucial role in preventing large-scale flooding by efficiently carrying away surplus water. When these channels are ill-defined or blocked, flooding becomes a common and destructive phenomenon. The flow of water through these channels is called 'drainage', and the entire network of such channels in a region is known as a 'drainage system'. The drainage pattern of an area is influenced by several factors including the geological time period, the nature and structure of rocks, topography, slope, the amount of water flowing, and the periodicity of the flow. A river drains water collected from a specific area called its 'catchment area'. The area drained by a river and its tributaries is known as a 'drainage basin'. The boundary line separating one drainage basin from another is called a 'watershed'. While river basins cover large areas, watersheds are relatively smaller catchment areas. Both river basins and watersheds are units of hydrological unity, meaning that what happens in one part of the basin affects the entire basin. This makes them important units for micro, meso, or macro-level planning. Drainage patterns describe the arrangement of rivers and streams in a particular area and are influenced by the underlying geological structure and topography. The main drainage patterns include: (i) Dendritic Pattern: This resembles the branches of a tree and is the most common drainage pattern. Rivers of the northern plains are examples. (ii) Radial Pattern: Rivers originate from a central hill and flow in all directions. The rivers originating from the Amarkantak range are examples. (iii) Trellis Pattern: Primary tributaries flow parallel to each other and secondary tributaries join them at right angles. This pattern is common in folded mountain regions. (iv) Centripetal Pattern: Rivers discharge their waters from all directions into a central lake or depression. Understanding drainage patterns helps in studying river behavior, flood management, and planning water resources.

• Drainage is the flow of water through natural channels like rivers and nalas.
• A drainage system is a network of such channels in a region.
• Catchment area is the area drained by a river and its tributaries.
• Drainage basin is the larger area drained by a major river system; watershed is a smaller catchment area.
• Drainage patterns depend on geology, topography, slope, and water flow.
• Main drainage patterns: dendritic, radial, trellis, and centripetal.
• 📌 Drainage: Flow of water through well-defined natural channels.
• 📌 Drainage system: Network of rivers and streams in an area.
• 📌 Catchment area: Area drained by a river and its tributaries.

Indian Drainage System: Classification and Overview

The Indian drainage system is vast and complex, shaped by the country's diverse physiographic units and precipitation patterns. It can be classified on various bases. One important classification is based on the direction of water discharge into the sea: (i) Arabian Sea Drainage: Rivers that discharge their waters into the Arabian Sea. These include the Indus, Narmada, Tapi, Mahi, and Periyar among others. (ii) Bay of Bengal Drainage: Rivers that discharge into the Bay of Bengal. This group includes the Ganga, Brahmaputra, Mahanadi, Krishna, Godavari, and many others. The dividing line between these two drainage systems is formed by the Delhi Ridge, the Aravalis, and the Sahyadris, which act as a water divide. In terms of area, about 77% of the drainage area of India, including the Ganga, Brahmaputra, Mahanadi, and Krishna, drains into the Bay of Bengal, while the remaining 23% drains into the Arabian Sea. Another classification is based on the size of the watershed: (i) Major River Basins: Catchment area more than 20,000 sq. km. There are 14 such basins including the Ganga, Brahmaputra, Krishna, Tapi, Narmada, and others. (ii) Medium River Basins: Catchment area between 2,000 and 20,000 sq. km. There are 44 such basins. (iii) Minor River Basins: Catchment area less than 2,000 sq. km. These are numerous and mostly found in low rainfall areas. Geographically, many rivers originate in the Himalayas and flow either eastwards into the Bay of Bengal or westwards into the Arabian Sea. Large rivers on the Peninsular plateau mostly originate from the Western Ghats and flow eastwards into the Bay of Bengal. The Narmada and Tapi are exceptions, flowing westwards into the Arabian Sea. Based on origin, nature, and characteristics, the Indian drainage system is broadly classified into two major systems: - Himalayan Drainage System - Peninsular Drainage System This classification, though with some exceptions, is widely accepted and forms the basis of further study.

• Indian rivers discharge into two main seas: Arabian Sea and Bay of Bengal.
• 77% of drainage area drains into Bay of Bengal; 23% into Arabian Sea.
• Major river basins have catchment areas > 20,000 sq. km; medium and minor basins are smaller.
• Himalayan rivers mostly perennial; Peninsular rivers mostly non-perennial.
• Narmada and Tapi flow westwards, unlike most Peninsular rivers.
• Classification based on origin: Himalayan and Peninsular drainage systems.
• 📌 Water divide: A ridge or elevated area separating drainage basins.
• 📌 Major river basin: Large catchment area > 20,000 sq. km.
• 📌 Himalayan drainage: Rivers originating in the Himalayas, perennial.