The Ganga Geology - The Ganga: Water Use in the Indian Subcontinent - page 18

power of the rivers, leading to a faster formation of delta about 15,000 years ago.
Both the Pleistocene and later sediments were deposited by the GBM river sys-
tem and formed a complex alluvial mass. Continuous deposition took place in the
form of back swamp, meander-belt and channel-fill deposits. Lateral planations and
shift of Ganga-Brahmaputra as well as their tributaries and distributaries led to their
overlapping and burial by later deposits.
The Ganga discharges into the Basin from north-west after draining the foothills
of the Himalayan range across more than 2,400 km. The Brahmaputra enters the
Basin from north-east and drains the northern slope of the Himalayas, taking the
name of its main Tibetan tributary, Tsang Po; their combined length is about
2,900 km. The Meghna drains the Sylhet Basin and parts of the adjacent plateau
and Tripura hills, together draining about 0.15 million (150,000) cusecs. This huge
discharge brings enormous sediments and helps form recent deposits. The bulk of
the deltaic southern Bengal Basin is probably not more than 10,000 years old.
The Himalayan rivers originated from the still young and friable range.
Frequency of earthquakes owing to the geological state of the region and heavy
rains in the catchments because of elevation and general direction of the monsoon
wind, cause frequent landslides and soil-erosion. Variation of extreme temperature
and the friable nature of the Himalayan rocks enhance silt movement from the catch-
ment. All these result in high silt charge in the mountain rivers. The non-Himalayan
rivers in the Ganga sub-basin, on the contrary, originate at much lower altitudes and
in less rainfall zones; they drain geologically more stable regions. Some of these
rivers, particularly the major Vindhyan rivers like the Sone and the Tons, cascade
where they cross sandstones, thereby reducing their effective gradient. As a result,
these rivers carry less silt than the Himalayan rivers and therefore, run more stable
courses.
The drainage pattern of the Himalayas reflects their geological past too. Instead
of originating from the mighty Himalayas, the main drainages originate either in the
northern plateau of the Tibetan Himalayas, or from the southern fans of the lesser
of the Indian Himalayas. According to the theory of 'antecedent drainage', rivers
like the Arun Kosi of the Ganga were flowing before the Himalayas took the present
shape. Simultaneously with the mountain-building process in the Himalayas, these
river-valleys are said to have upheaved and rejuvenated again and again. The theory
explains, how some of the great rivers of the region drain not only the southern
slopes of the mountains but do it on the northern slopes too, the limit of the valley
being much further north than the main axis of the great range. It also explains
the configuration and the enormous depth of the gorges, as the uplift of the ranges
and the erosion of the upheaved valleys occurred at the same time. The Himalayan
rivers often change their courses and alter bed-gradients, caused not only by the
differential erosion, capture of one river by another, land-slips, glaciations etc. but
by quakes too along the faulting belt, which characterises the southern flank of the
Himalayas.
The Pleistocene alluvial terraces formed in four major regions and several small
outliners that topographically stood above active flood plains. Of these four areas,
two flank the basin on the east and the west; the other two lie within it. The latter two
areas are known as the Barind and the Madhupur jingle. The Pleistocene sediments