The study determines rates of sedimentation and estimates the useful life of Dal-Nagin lake situated in Srinagar, Jammu and Kashmir using radiometric techniques. In spite of various ecological problems the lake is the largest freshwater lake in India. It has four major sub-basins viz., Hazratbal, Bod Dal, Garibal and Nagin. The lake is fraught with the problem of growth of underwater weeds and increasing rate of sedimentation.
The sediment cores collected from various locations in all the four sub-basins comprise of considerable organic matter. It is found that the organic matter contributes to the extent of 25-30 per cent in the lake sedimentation process for all the sub-basins. The incoming water to the lake contributes less than 10 per cent of organic matter, the rest being contributed by underwater weeds.
The rates of sedimentation are higher in the Hazratbal sub-basin as a result of the silt load entering into the lake through Telbal drain. The pattern of sedimentation in Dal lake depends more on the distance of sampling points from the Telbal drain vis a vis the depth of the lake at the sampling locations. It is to be noted that the average rate of sedimentation has reduced considerably (0.58 ± 0.10 cm/yr) in Hazratbal sub-basin after 1986-87 in comparison to the rate of sedimentation since 1963-64 (1.60 ± 0.13 cm/yr) and 1978-79 (1.0 ± 0.18 cm/yr) as evident clearly from the case of core D-2.
This may be due to the effect of settling basin, which has come into existence from the year 1989. On comparison of 137Cs and 210Pb patterns in different cores, it is found that the core D-1 does not represent the true sedimentary environment. It appears that due to the proximity of the sampling location to Telbal drain, the top portion of the sediment deposit has been eroded as a consequence of flow from the Telbal drain with higher velocity.
The effect of settling basin is also reflected in case of useful life of Dal lake estimated by using average rates of sedimentation. The estimated useful life based on post-1987 (0.22±0.03 cm/yr) rate of sedimentation is about 364±50 yr and that based on post-1964 (0.52±0.04 cm/yr) rate of sedimentation is about 154±12 yr.
However, the useful lives of Hazratbal and Nagin sub-basins, estimated using the bathymetric maps, are about 786±86 years and 521±48 years respectively based on the rate of sedimentation prevailing after 1987. The variation in the estimated life is due to the variation in measured volume or mean depth of the lake in both cases.
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