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It is believed by some of the geologists that there was a mighty river named Siwalik or the Indo-Brahma. It flowed north-west from Assam to Punjab in the longitudinal valley along the foot of the Himalayas. From there, it took a southerly turn to join a gulf which occupied parts of the Sindh and the lower Punjab during the Miocene period.

It is also believed that this mighty river carried the combined flow of the Himalaya rivers and the main streams of the Brahmaputra, Ganga and the Indus.

The sediments brought by this mighty river formed the Siwalik range. Due to the uplift of the western Himalayas, the mighty stream dismembered into the following river systems and sub-systems :

(i) The Indus system.

(ii) The five tributaries of the Indus in Punjab.

(iii) The Ganga and its Himalayan tributaries.

This dismemberence changed direction of present Ganga and Yamuna. According to this theory, the Yamuna was the tributary of Indus before this event took place and had southwesterly course. This interchange of tributaries between the Indus and the Ganga systems in the region lying between the headstreams of the Sutlej and Yamuna has been a common phenomenon in the geologic recent past.

Arguments Against the Theory : The theory of Indo-Brahma has not been accepted universally. It has been challenged on the following grounds :

Firstly, the concept of such a mighty river Indo-Brahma flowing all along the longitudinal

extent of the Himalayas is not possible. The occurrence of Siwalik alluvial deposits might have been formed by successive deposition of alluvial fans by the rivers flowing down the Himalayan slopes.

Secondly, the evidence furnished by the depositional history in the Ganga delta and in the Assam does not prove this concept as the deposits in the gap lying between the two projections of the peninsular block-Rajmahal hills and Shillong plateau, have been composed of thick alluvium older than that suggested by the Indo-Brahma theory

Thirdly, the evidences of Tipan sandstorm of Assam has not been accepted by this theory.

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Radial drainage : When the rivers radiats from a hill, the pattern is called radial.

Centripetal drainage : The rivers flowing into a depression or a lake, is called a centripetal drainage. The desert is characterised by the centripetal drainage pattern.

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The distinction:

(ii)Cumecs : When the water flowing in the river is measured in cubic metre per second, it is called cumecs.

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Gorge : The rivers in the mountainous region cut deep valleys and flow through these valleys. These are called gorges. The gorges are steep-sloped and deep.

Rift valley : Rift valleys are formed due to faulting. Due to the internal disturbances of the earth, some regions sink and form a valley. This valley is called a rift valley.

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The flow pattern of Himalayan rivers and peninsular rivers is controlled by the climatic conditions.

The Ganga : As its hydrograph shows that the flow pattern of the Ganga attains the sizeable flow in the early summer because of snow-melting. Its maximum flow is either in August or September and minimum in the period of December and January.

The Godavari : As its hydrograph shows that the flow pattern of Godavari river has also seasonal effects. Its flow is low until May. It has two inaximas-one in May-June and other in July-August. After August, its flow pattern falls sharply but it has higher flow in October and November also.

The Narmada : The hydrograph shows that the flow pattern of Narmada is also seasonal. It is high in the. months of June to September and October during the rainy season. During winter it is low.

The Jhelum : The flow pattern of Jhelum is high throughout the year, because it is due to snow melting and rainfall in the region.