Geography · Ch 3 — Drainage System
The Himalayan Drainage
The Himalayan Drainage
The photograph shows a mountain river in full spate, its water churning white as it surges over and around large boulders strewn across the channel. The scene is unmistakably a steep Himalayan reach — the kind of stretch where the river has not yet lost the energy it gained from the high gradient. The water is not smooth; it breaks into foam and spray, tumbling in short, violent steps as it forces its way past the obstructions. There are no banks visible in the frame, no flat ground, no vegetation — just rock, water, and turbulence. The image is a single, unlabelled black-and-white shot, with no arrows, panels, or annotations. Its caption, "Rapids," is the only text attached to it.
What the photograph teaches is the physical meaning of a rapid: a short, steep section of a river where the bed gradient increases sharply and the flow becomes turbulent, usually because the channel is littered with boulders or the bedrock is uneven. Unlike a waterfall, where water drops vertically, a rapid is a sloping, broken cascade — the water stays in contact with the bed, but it is violently agitated. In the Himalayan context, rapids form because the rivers are young, flowing through freshly uplifted, rugged terrain. The same erosional energy that carves deep gorges and V-shaped valleys also produces these rough, boulder-strewn stretches. The photograph therefore illustrates the very first stage of a river's journey — the turbulent mountain course — before it loses gradient, slows down, and begins to deposit the sediment …
The Himalayan drainage system is the product of a long geological history, shaped by the same forces that raised the mountains themselves. It is built around three great river basins — the Indus, the Ganga and the Brahmaputra — and every major feature of these rivers, from their gorges to their shifting plains courses, follows directly from how they were born and how they are fed.
Because these rivers draw water from two sources — the melting of snow in the high mountains and rainfall — they flow throughout the year. This makes them perennial rivers, unlike the rain-fed rivers of the peninsula that swell and shrink with the monsoon.
The most striking feature of these rivers in the mountains is the giant gorges they have carved. These gorges are not simply the result of erosion; they were cut while the Himalayas were still rising. The rivers eroded downward at the same time the land was being uplifted, so the channels stayed deep and narrow even as the mountains grew taller. Alongside these gorges, the rivers also form V-shaped valleys and rapids as they tumble down steep gradients.
Once these rivers leave the mountains and enter the plains, their behaviour changes completely. The same rivers that cut deep gorges in the hills now build depositional features — flat valleys, ox-bow lakes, flood plains, braided channels and, near the river mouth, deltas. In the Himalayan reaches their courses are highly tortuous (twisting and winding), but on the plains they show a strong meandering tendency and shift their courses frequently.
The classic example of this instability is the Kosi river, known as the 'sorrow of Bihar'. The Kosi has been notorious for frequently changing its course. The reason lies in the sediment it carries. The river brings a huge quantity of sediment from its upper reaches and deposits it on the plains. This deposited sediment blocks the river's course, and the river, finding its path obstructed, changes direction. The cycle then repeats — more sediment, another blockage, another shift. …