The Hindu Kush-Himalayan region - River hydrology (2/3)
With respect to the following rough descriptions we have to bear in mind that diverse local environmental processes provide several morphological modifications.
Mountain rivers and streams
Mountain streams of the mid-altitude and high mountains comprise many types of rivers and streams. Small and mid-sized streams flow through canyons or trough valley and constrained channel courses due to the steep gradients. Floodplains, if occurring at all, are usually small, often less than 10 m. The stream bottom is characterized by boulders, bedrock, and cobbles. The streams show a high variation of discharge during the wet and dry season of the monsoon. Anabranching channels and braided stretches are common, occasionally meandering sections occur. During the rainy season and at the end of the dry season they carry large volumes of sediment and snowmelt. Hence, the rivers and streams of the mid-altitude and high mountains have a permanent but highly dynamic hydrological regime controlled by the monsoon climate and the origin of their water, either snow-fed or spring-fed.
Rivers in the Siwalik range
Running waters of the low mountain range predominately flow in a constrained or weakly sinuate channel, depending on the shape of the valley. Steep valleys lead to a more constrained course, whereas U-shaped or meander valleys bear sinuate or meandering rivers. In general the riverbed comprises coarse mineral material (pebbles and cobbles). Fine sediments (sand and clay) appear in areas with low current velocities. Annual discharge alters noticeable with occurrence of extreme discharges (flash floods) during the rainy season. Except of the rainy season many rivers in the Siwalik ranges are dry (ephemeral).
Rivers in the lowland
In general lowland rivers flow in a sinuous or meandering course in a meander valley or a plain floodplain. Riverbed substrata predominately consist of sand, clay, or other fine material. Organic substrates cover between 10 and 50 % of the bottom and comprise considerable amounts of large wood (logs, debris dams). Large wood causes high substrate and current diversity, allowing both erosive and depositional zones. In the dry season the flow picture is predominately calm with significant alterations of discharge during the rainy season.