The diversion of the part of the course of a river by another river is called stream diversion stream/river capture or stream piracy. River capture is a natural process, which is more active in the youthful stage of the valley development because the streams are actively engaged in headward erosion and valley lengthening but it can also occur during mature and senile stages of the valley development through the process of lateral erosion and meander intersection. The drainage system refers to the origin and development of streams and their channels through time. It is believed that the present drainage system of the Himalayas is the outcome of progressive river piracy during various stages of drainage development. Headward erosion has been the most active process of river capture in the Himalayas. River capturing has been a frequent phenomenon in the evolution of the present drainage system of the Himalayas which can be understood with the help of the following examples:

• Yarlung Tsangpo was sequentially captured by the paleo-Red, Irrawaddy and Lohit Rivers, before its final capture by the Brahmaputra.
• The Arun Kosi, a head tributary of the Kosi River, has captured the Phung Cho, a southern tributary of the Tsangpo (the upper part of Brahmaputra) river.
• Two head tributaries of the Ganga e.g. the Bhagirathi and the Vishnuganga have captured the source tributaries of the Sutlej river.
• The deviation in the course of the Saraswati River is believed to be caused by river piracy. The headward erosion by the Yamuna cut the channel deeper and deeper eventually leading to the capture of the flow of the Saraswati.
• The water divide between the tributaries of the Song River (a tributary of the Ganga River) and the Asan River (a tributary of the Yamuna River) is only a few meters wide near Dehradun. It is expected that the Song river may capture the Asan river and thus the upper course of the Yamuna may be diverted to the Ganga via the Asan and the Song rivers.

In the initial stage of their development, most of the streams and their tributaries are engaged in active headward erosion resulting in continuous creeping or shifting of water divides and lengthening of their valley thalwegs. The stronger and more powerful streams (in terms of channel gradient, stream velocity discharge, and kinetic energy) capture the upper courses of weak and sluggish streams.