Vol. 1, Issue 10, Part K (2015)

Abstract
River Siang is the one of the major rivers of Arunachal Pradesh and also an important influent tributary leading to the mighty River Brahmaputra. River Siang is Hill-Stream 1st order river; has colluvial valley segment and pool-riffle type of reach. This reach type is most commonly associated with small to mid-size streams and is a quite prevalent type of reach in the rheophilic zones. Pools and riffles alternating with cascades are generally found to dominate the micro-habitat type with frequent occurrence of trench pools. The substrate type has been found to be dominated by bedrocks, boulders, gravels and cobbles. River with its tributaries is a unique type of ecosystem which generally covers different types of climatic zones, landscapes and bio geographical regions. River is the natural drainage system of the land mass of the earth which move continuously. Near the source, the river is small, straight and swift while in downstream the velocity of water decreases and meandering of river begins in the plains.