Secrets of Past Monsoons from sediments in Gujarat

All are ears when grandpa tells that his grandpa used to say, 'in our times it used to rain so heavily for days that one felt as if the deluge is here'. The curiosity becomes deeper as one tries to go deeper in to the past. Often one ponders about the intensity of the monsoons during the history or even the pre-history! Reason is simple for this curiosity. The monsoons do affect our lives. Despite all the technological innovations if the monsoon is intense floods take their toll, if there is no monsoon then the drought makes the farmer commit suicide and many remain hungry.

N. Juyal, R. Bhushan and A.K. Singhvi of Physical Research Laboratory (PRL), Ahmedabad and Prof L.S. Chamyal and S. Bhandari of Department of Geology, M.S. University of Baroda, Vadodara, India carried out intense research in parts of Gujarat and published their results in Quaternary Science Reviews in 2006. 

Gujarat is a state bounded by the Arabian Sea to the west and southwest and Pakistan to the north. The states of Rajasthan, Madhya Pradesh and Union Territories of Dadra and Nagar Haveli have borders with Gujarat on the northeast, east and south and southeast respectively. Being a coastal state it gets good share of southwest monsoon.

The study of Juyal and his co-workers was aimed at collecting the evidences from the land about the variability of the southwest monsoon over Gujarat in the past 130 to 120 thousand years (Ka). For their study they chose an area between Sabarmati River basin in the north and Orsang River basin in the south. The former has around 750mm mean annual precipitation and falls under the sub-humid zone in the present day. The latter has a mean annual precipitation of more than 1000 mm and falls in the humid climate zone. Around 72% of days between May and September each year are windy days with an average wind speed of 38km/hour. These winds are the carriers of the southwest monsoon system, which starts from mid June and lasts till mid-September. These winds have been blowing regularly since the days of the yore is also confirmed by the direction of the sand dunes.

Gujarat is one province which is always geologically and politically hyperactive. Thus while the politicians have been tinkering with the development of the state, tectonic forces have been active in deciding the river courses. The river courses anywhere have lots of tale to share with the students of geology and geomorphology. In the area studied by Juyal and his co-workers the river courses are governed by the tectonic forces considerably. While the earlier workers suggested that the present course of the Sabarmati River was set by tectonic activities around 300Ka ago, quoting recent (1997) work of S. K. Tandon an eminent Professor of Geology from Delhi University Juyal et al say that it was set some 39Ka ago. The work of Tandon since it is based on the modern technique of Thermo-luminescence has found more favor with the later workers.

Gujarat has faced some of the worst earthquakes from historical times till recently (26th January, 2001). But the work of Chamyal et al published in 2003 says that tectonic movements during the past 1.75 million years are responsible for the present configuration of the Orsang River.

These rivers originate in the Aravali mountains and drain in to Gulf of Cambay. During their not very long journey they have been since long past brining sediments and depositing them in layers. More rains mean more and coarser sediments and in case the stream flow is slow due to any reason there is a tendency of fine clay to be deposited. In Mahi and Orsang River basins the sediments include from bottom to top a succession of clay, coarse gravel, silt and sand, sand, coarse gravel, clay with soil, red soil, coarse gravel, silt and sand with soil, wind borne sand transported by water and wind borne sand. 

Studies by Juyal in 2003 indicated that the wind borne sediments were deposited some 26Ka ago and their deposition continued intermittently till 6Ka. 

The rivers have been migrating, changing their courses in the alluvial plains of Gujarat since times immemorial. Various causes have been attributed to this character of the rivers. However, Juyal and his co-workers in their present study have taken in to account the tectonic forces that have been active in Gujarat, by virtue of its situation and climate. 

Rivers are extremely sensitive to climatic changes. For example the sediment load carried by a river during an inter-glacial phase is quite different from that during the glacial phase. Obviously in the interglacial phase the warming induces melting of glaciers and the volume of water in the river increases many folds. More volume means more sediment load. The situation is compounded if there is a slope disturbance due to sudden ground movement. During the inter-glacial periods the monsoon pattern also changes. In other words more water falls from the heavens thus changing the river geometry and pattern.

Quoting S.Clemens et al Juyal and his colleagues state in their paper 'the southwest monsoon is caused by differential heating of the oceans and the land mass. This leads to seasonal formation of low atmospheric pressure over the Asian plateau and high atmospheric pressure over the relatively subtropical Indian ocean' The southwesterly monsoon is induced over the Arabian Sea because of this pressure difference. The pressure difference also creates upwelling currents in the ocean which brings nutrient rich waters from the depths to the shallower zones. Based on the premise scientists attempting to dig the past climates in other countries have used the data from the oceans as proxies. They found an enriched concentration of foraminifers and cocolithophores brought to the shallower zones due to upwelling currents. Drawback from such inferences about the conditions in the sea is that it does not reflect about how much the precipitation on the land was. On the other hand the information obtained from the lake level changes, dune construction activity, changes in the sediment load in the rivers etc provide a direct record of changes in the rainfall amount and its distribution in space and time.

From their study Juyal and his coworkers conclude that between 130 to 120 Ka there were strong monsoon conditions as seen by the evidences of rivers meandering through the region and their wide flood plains.

With the passage of time between 120 to 100 Ka the meandering streams became like the braids of a maid. Such features develop when the volume of water is reduced. Obviously the monsoon had weakened. The climatic conditions never remain constant. Once again between 100 to 70Ka flood plains became wider and lime rich clays were deposited, indicating enhancement of monsoon. There was a deterioration of monsoon and between 70 to 60 Ka the rivers once again developed braided channels. During the period less than 60 Ka and till 30 Ka again extensive flooding took place as confirmed by development of wide flood plains, indicating once again enhanced monsoon. 

It is amazing that no wind borne sediments were found older than 30 Ka. But between the periods slightly less than 30 Ka to 11 Ka wind borne sediments started to get deposited on the plains indicating a decline in the monsoonal condition.
Near coasts eustatic changes took place due to the influence of Last Glacial Maxima (LGM). In simpler terms it means that around 18 Ka while the glacial activity was at its peak the discharge of the rivers was influenced accordingly. But when the ice began to melt the rivers began dumping more and more load into the oceans. Imagine a wooden block floating in a tub of water. Press the block from one side, the other side would rise. Likewise when the load of sediments on the ocean floor suddenly increases there are eustatic changes on the land surface and it rises. The process of course is not as simple as described here, but just in order to make the point reach home a simpler explanation has been cited. These changes did cause changes in the stream flow near shore, but in distant locations the streams were more affected by the rainfall and also neo-tectonic activity.

The river systems do respond to climatic changes but the response is not overnight. It takes couple of thousand years for a river to show a lasting change.

Such studies are of great importance in today's context when more and more habitations are coming up on the river banks or along the flood pathways of the rivers. There are many rivers at present which have a limited discharge due to several localized factors. Most of the states and developers presume that the river is now doomed and the land on the banks can be developed in to a dividend paying habitat. Well yes they might earn some money out of such 'development', but the future generations of those living there will be imperilled.
Never take a river for granted.  


More by :  V. K. Joshi (Bijji)

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