SEQUENCE OF VALLEY DEVELOPMENT OF ALAKNANDA AND ITS TRIBUTATIES & QUATERNARY SEDIMENTATION, GARHWAL HIMALAYA, PARTS OF CHAMOLI TEHRI UTTAKASHI & PAURI UTTAR PRADESH (UTTRARKHAND STATE ) INDIA
Authors:   Ex. Director Geological Survey of India, Ex. Director Rajeev Gandhi Prodyogiki Mahavidhyalaya Bhoplal M.P. India., Khan A. A. , Chaterjee A.C ,   Ex Head of Department School of Studies in Geology Vikram University Ujjain M.P. India.
Published: July 2023

The sedimentological study in Alaknanda and its tributaries in upper Ganga basin has been attempted in parts of de Uttarkashi,Chamoli, Pauri and Tehri districts in parts of QA sheet 53Jand 53 N on 1:50000 scale of Garhwal Himalaya U.P presently known as Uttrakhand State of Union of India. The area of Upper Ganga basin consisting of Alaknanda,Bhagirathi,Bhilangna, Nandakini, Mandakini, Pindar, Dhauli- Ganga Bal- Ganga , Madhmeshwar Ganga and Berhi Ganga The Alaknanda is characterized by six terraces AT1_AT6 followed by Bhagirathi with five terraces BT1_BT5, Bhilangna BHT1_BHT4 Nandakini four terraces NT1_NT5, Mandakini MT1_MT6 /Pindar PT1_PT3 /Dhauli-Ganga DGT1_ DGT3 /Balganga three terraces BGT1, to BGT3, Madhmshwar Ganga MDT1, to MDT2Ganga two terraces and Berhi Ganga one terrace BRT1, amidst these Alaknanda is trunk stream and others are tributaries. The sequence of valley development and Quaternary sedimentation in Alalknanda and its tributaries is established in stratigraphic sequence has been attempted for the first time. The area genetically comprised of terraces of three domains, viz. Glacial, Fluvio-glacia and Fluvial which represent distinct environment of sedimentation of Pleistocene, late Pleistocene and Holocene time during Quaternary period. The Glacial terraces are identified at an average elevation of 1150 m above MSL, the fluvio-glacial terraces at an average elevation of 975m above MSL and fluvial terraces at an average elevation between 650 to 900 m above m.s.l, type area is attempted in to understand the nature of Quaternary sedimentation in these valleys in increasing antiquity. The statistical parameters of sediment of fluvial terraces of Alaknanda revealed tha the average mean size for AT1 is 1.550 Ã˜ (medium sand) and it varies from -0.3240 Ã˜ to 2.250Ã˜ (coarse sand to fine sand), for AT2 average mean size is -0.5350Ã˜ (coarse sand) and it ranges from -1.742Ã˜ to 1.565Ã˜ (very coarse to medium sand),AT3 the average value is 1.760Ã˜ whereas it ranges between -0248 to 2,650 Ã˜ The average and range values of means size of Alaknanda terraces (ATo to AT3) predominantly consist of both coarser and finer sediments, whereas younger terraces (ATo to AT1) primarily consist of very fine sand. The mean size of sediment in sequence of terraces except in AT2 has strong tendency to decrease in decreasing antiquity i.e. from AT3 to AT1 towards the younger terraces. The progressive increase of finer sediments towards younger terrace revealed the constant and steady decrease in load carrying capacity of Alaknanda from early to late history of river sedimentation. The anomalous values of mean size in the terrace AT3 indicate sudden reactivation in energy condition of the channel system during sedimentation perhaps due to tectonic dislocation in watershed region of Alaknanda which is also manifested in disposition of sediment in vertical column of AT3 terrace in the valley. The average value of standard deviation for terrace AT0 is 0.3740 Ã˜ (well sorted), whereas it varies from 0.2530 to 0.4665Ã˜ (well sorted). The average value for AT1 is. 0.3.320Ã˜ it ranges from 0.255 Ã˜ to 0.440 Ã˜, average value for AT2 â€“ AT6 is 0.3.35 Ã˜ and it varies from 0.348 to 0.555 Ã˜ . The average value for terrace AT3 is 0.6120Ã˜ (moderately sorted) and it varies from 0.540 Ã˜ to 0.710Ã˜ (moderately sorted). The average and range value of (S) indicate that sediments are poorly sorted to very well sorted. The sorting index of sediments appears to be related with size of sediments. The correlation between these two parameters revealed that the higher terraces AT2, AT-3 contrastingly consist of coarser clastic and are mostly moderately sorted, as compared to AT1 to AT2, AT3, AT3 which comprised of medium to fine, sediments and show significant improvement in sorting. The size distribution curve indicates variable unimodel to bimodal nature of sediments. The probability plots indicate that the sediments load of AT2 to AT3 consist of traction (average 82%) and little of suspension whereas AT1 to A2 both of traction and suspension (average 55% and 45% respectively). This variation from AT1 to AT3 revealed strong fluctuation in the current velocity of the channel and steady decrease in the kinetics of river from early to late history of sedimentation (Khan1975). The average value of standard deviation (S) from AT3 to AT1 shows sharp progressive improvement in sorting of sediments which may be due to (i) due to repeated reworking of sediments (ii) due to increase load in mean size of sediments, (iii) progressive static environments of sedimentation towards later phases, perhaps due to low energy condition of channel during sedimentation. The average value of skewness is + 0.123Ã˜ positive skewed), for skewness AT1 skewness is (positive skewed), where it varies from -0.626 to 0.225 Ã˜ (negative skewed to very negative skewed), for AT2 AT3, AT3, AT4) the average is +0.385 (positive skewed) and varies from 0.440 to -0.975 Ã˜ (very negative skewed) and varies from 0.145 to -0.328 (negative skewed to positive skewed). The average value of skewness revealed that the sediments of older terraces are strongly negative skewed, whereas the sediments of younger terraces are positive skewed. The negative skewness is considered to be the resultant of high energy environment and the winnowing action of the sand, whereas the positive is the reversal of former. The relative skewness range of the sediments from negative skewed to positive skewed from older to younger terrace revealed the steady decrease in the load carrying capacity of channel and current velocity towards the late history of river sedimentation in the Narmada valley ( Khan 1987) The average value of Kurtosis for terrace AT1 is 0.385 Ã˜ (very platykurtic) it varies from 0.285 Ã˜ to 0.370 Ã˜ i.e. (very platykurtic), for terrace AT2 the average value is 0.385 Ã˜ (very platykurtic) and it varies from 0.3600 Ã˜ to 0.4560 Ã˜ (very platykurtic), for terrace AT2 , AT3, AT4) the average value is 0.5240 Ã˜ (very platykurtic) and varies from 0.385 Ã˜ to 0.681 Ã˜ (platykurtic to very platykurtic) for terrace AT3, the average value is 0.639 Ã˜ (very platykurtic), and varies from 0.6325 Ã˜ to 0.755 Ã˜ (platykurtic to very platykurtic). The younger terraces ATo to AT1 show the lower values of Kurtosis whereas the older AT2 to AT1 the higher values. The values decrease from older to younger terraces which suggest that the sediments from older to younger terraces become mesokurtic to very platykurtic. The relatively lower values of Kurtosis towards younger terraces indicate that the sediments were derived from more than one source.