Civilisation exists by geological consent, subject to change without notice. – Will Durant
Rivers are the most commonly accessible of natural water resources. But rivers do not flow everywhere, nor do all rivers carry equal amounts of water. Each river is an individual entity – not living, but certainly dynamic. A river, like any other natural feature, has shape, size, context, as well as a history and process. The combination of these elements is distinctive, making each river unique. It is also dynamic, continuously evolving, and no river's behaviour can be reflected in simple deterministic models. Likewise, any operation on the river – removing water, impeding the flow – will produce effects that do not follow simple arithmetical rules. These changes induce change in the river's process, and therefore have cascading consequences. In other words, humans need to take the river itself into consideration as a participant while intervening in its process.
Let us begin with some potentially oft-overlooked basics. A river begins when water drops on high ground, and the natural level of rest for all water on the earth's surface is the sea. Water arrives at high ground through precipitation in the form of rain, or as snow, which melts, and this transportation process is powered by solar energy. At the high point of its trajectory, this water has a large amount of potential energy, which is proportional to its height above sea level, and it is this energy that ultimately drives the water downhill until it reaches the sea. This entire flow mechanism is essentially a process in which the potential energy is expended.
Rivers typically have three stages. The first takes place in the higher reaches, where the slopes are steep and 'fresh' rocks are eroded by the river. Second, as the water flow through the plains, some silt is deposited and some is picked up, the net effect of which is sediment transport. Third, and finally, as a river approaches its lowest point, where energy drops to near zero, this silt is again deposited. In the case of Himalayan rivers, these transitions are particularly pronounced, due to the extremely high levels of relief that are part of this process. Most Himalayan streams begin from glaciers or glacial lakes at very high altitudes, up to 6000 metres above sea level. From there, they follow tortuous courses, along and across the long grain of structural alignments, until they emerge onto the plains.