The Volume of water in the rivers is subject to vast seasonal and monthly fluctuations. It is small in winter, and increases gradually with the approach of summer, as the snows in the mountainous catchment areas begin to melt. The volume of water in the rivers in the early summer months varies with their size, altitude, situation with respect to the monsoons, the height of the snow line, and heritage of glaciers from past eras in the respective catchment areas.

In the Indus, Jhelum and Chenab, the volume of water increases appreciably after March, but this increase come later in the eastern rivers. The Indus, in which an early rise is mostly marked, draws its supply from two large groups of glaciers, the Hindu Kush and the Karakoram, being larger than those of any other mountains outside the Polar region.

The approach of the rainy season at the end of June or early July is marked by a great increase in flow. The period of high flow terminates in the Indus and Jhelum in September, but continues for another month in the eastern rivers. The decrease in flow after the peak month is as sharp, or even sharper, than the rise before it. Even months with high mean discharges are characterized by wide fluctuations in the daily discharge. Floods generally occur in the early part of the rainy season in the western rivers, and later in the eastern.

Because about 60 percent of the flow in the Indus system is concentrated in the three rainy months, there is a great need for reservoirs and dams to regulate the flow, reduce floods and loss to the sea, and provide more water for irrigation. Further, the flow of the Ravi, Beas, and Sutlej has been lost to India, and must be replaced from other sources. Storage dams and replacement works are discussed later, in the

Drainage Pattern of Baluchistan

In Baluchistan, the main rivers sprawl out in all directions from the axis of high land formed by the Quetta node, the Central Brahui Mountains, and the Central Makran Range. Rivers draining to the north-east and east of the main divide generally join the Indus system. These include the Zhob, with its main tributary the Kundar, the Loralai, and the Kulachi. The Bolan and Mula Rivers flowing south or south-east from the main divide, dissipate themselves in the Karach-Sibi Plain. Southward flowing rivers drain to the Arabian Sea. The Hab, Porali, Hingol, with its main tributary the Mashkai, are the chief of these. Rivers flowing west or south-west generally dissipate their water in shallow depressions of varying size called hamuns. The more important of the rivers draining into the inland basins are the Pishin Lora, the Baddo, and the Rakhshan.

The rivers of Baluchistan generally flow only during the rainy season, and some small rivers are dry not only for the great part of the year, but for many consecutive years. Some of the larger rivers, such as the Zhob, Loralai, Pishin Lora, Hingol, Porali, and Hab, are perennial only in their lower reaches, and the volume of water is small except in the rainy season. Some rivers, for example, the Indgol and the Bolan, flow intermittently above and below ground, and disappear underground in limestone regions.