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Climate History Archived from River Sediments!
|by V. K. Joshi (Bijji)|
Archives of history in river sediments! Sounds strange! Yes it does sound weird but it is true. In school level geography we are taught that rivers carry rocks dislodged from mountain sides. During transportation in the river these rock boulders are tossed and turned and in the process they acquire sub-rounded to rounded shapes. The sizes of the boulders, their rock types and their shapes have stories to tell about their source, their mineral constitution and the velocity of water which carried them. These boulders get worn down to pebble cobble, sand, silt and clay sizes and get deposited on river banks in layers. A geologist just studies these layers and is able to unlock the archives of the rivers’ past condition. A river exists only because it gets sufficient water. If there are fewer rains there is less water in the river and naturally it can not carry large boulders and things like that. Such records are better preserved in the alluvial river basins. That is low land areas where the river bed and banks are made up of lose sediments and soil. The layers of varieties of sediments on these river banks present a better picture of the past climates.
To facilitate the researchers modern tools like dating equipment have evolved so much that it is possible to find out the when the river borne sediments were deposited. Thus making the history of the river more interesting as quite a bit of it is related to human history as well. One can through the dates deciphered find out during the reign of so and so in the past there was a drought or there were devastating floods.
Alpa Sridhar and L.S. Chamyal, researchers from Maharaja Sayajirao University of Baroda, Vadodara, Gujarat chanced upon one such archive of sediments of Narmada River which has interesting tale about the climates of the past.
Rivers like Narmada have been largely affected by the climate throughout their history. Most part of the river flows through a sub-humid climate zone which is marked by variable rainfall and warm wet summers and cool dry winters. All these factors contribute towards sediment carrying capacity of the river. Narmada is not a small stream; it is a major River- 867 km long and drains 94,020 square kilometer area. Narmada rises in the Amarkantak plateau in Madhya Pradesh and flows through a rocky terrain for a considerable distance. The upper part of the river flows through constricted channel with water falls (Dhuandhar waterfall on Narmada River at Jabalpur is famous), rapids and scablands. Scablands are typical of Narmada River in the plateau terrain. While flowing through a plateau country it seems that the gush of water has removed chunks of rocks and transported them. The patches of land left denuded by this process are the scablands.
By the time the River reaches Gujarat, the scenario changes completely. The valley is broad there and valley sides are made of alluvium. The Narmada plains in Gujarat act as a sink for the material it carried from M.P. As the river enters plains at Garudeshwar it changes its character from a buoyant teenage girl full of energy to a mature woman, swaying gently through the plains. In other words the energy conditions of the river completely change due to change of gradient.
The fluvial deposits (sediments of the river) carry subtle impression of hydrological regime that Narmada River was influenced with in the past. The variety and pattern of river deposits indicate existence of high-magnitude floods in the past. Gujarat plains were formed during the beginning of the latest chapter of the Earth’s history, i.e. the Holocene (10000 years ago). This was the period when Gujarat area was under going a high tectonic stress. Due to a good monsoon, rainfall was high. Thus quantity of water available was high. The tectonic events were leading to sudden changes in the slopes and gradients. Flooded rivers in their fury cut through underlying 125 ka (125000) year old sediments of Late Pleistocene and also 11 ka wind born sediments.
The sizes of sediments carried by the Narmada River are indicative of two factors-either the rainfall was high and rivers were able to scour more from the source and carry the material with gusto to the places they were dumped. Or the ultimate destination of sediments was sinking or subsiding due to tectonic events and thus creating space for a higher volume of sediments. Naturally this forced the river to erode more at the source.
Sediments representing an archive of hydrological regime and climate during the last phase of Quaternary have been deposited as channel fill deposits, channel accretion deposits, floodplain and slack water deposits. Due to changes in hydrological parameters or due to tectonic stresses the river at places has not only cut through its own deposits but also through basement rocks 125000 years old.
As the river hurtles down towards plains it carries material of the size of gravel. The moment it enters the plains there is a sudden change in the velocity of the stream. In other words the energy conditions of the river change and the gravel carried by the stream get deposited in a fan shape. These are known as the fan gravels. Similarly in the plains whenever the energy conditions change the same gravel is deposited along longitudinal strips, giving a braided look to the river and hence called as braided gravels. For all such deposits a particular velocity and volume of water are required. In the Late Pleistocene times gravel fans and braids were deposited due tectonic conditions.
Once the Narmada began to flow through the plains the sediment archives indicate a higher discharge of the river. In this region the river was predominantly meandering, with a persistent flow and reduced depth-width ratio. In the beginning of the Holocene Period (Holocene is from present to 10000 years ago), the tectonic instability of the region gave a higher discharge to the river which enhanced the capacity of the river to erode more, leading to incision of older sediments. Narmada basin is known to be unstable therefore Alpa and Chamyal have all the reason to believe that the fluvial processes of deposition and neotectonic activity (tectonic activity in the last 10000 years) went hand in hand. During the past 10000 years Narmada has aggraded its channel twice. The first one was between 6.6 to 4 ka, when there was generally a drier phase marked with short-lived high precipitation events. The second one took place post 2 ka and may have continued until the Medieval Warm Period (AD 950-1250), after which the River incised older sediments at many places due to a comparatively humid climate.
It is interesting to know that at levels much higher than the present day river slack water deposits of the river dated to be 5-1.5 ka indicate high magnitude floods during that period. During the little Ice Age (AD 1400-1800) discharge of the river was considerably reduced. This caused large volume of sediment to accumulate in the river valley. These are the sediments with which the River plays with even now, carves them in to different forms, cuts through them and flows through them.
The sediment archives of Narmada strongly point towards an unpredictable nature of Narmada. This is because the basin through which it flows is shaky (Read Narmada: A River on an Unstable Basin). The history of the River reconstructed with the help of sediments further confirms the nature of the River. It is for the planners and developers of states along the Narmada to keep in mind the perky nature of the River before pushing huge amounts of tax payer’s money to harness the River.
|More by : V. K. Joshi (Bijji)|
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