Studies on the sediments of vembanad lake, Kerala state: Part III - distribution amd interpretation of bottom sediments

16-27Percentage distributions of different size fractions and the grain size parameters of the sediments have been studied with a view to understanding the depositional processes operating in different parts of the lake. These studies have revealed that (1) in the southern half of the lake, coarser fractions are confined to the western margin of the lake and the finer fractions occupy the rest of the area while in the northern half, finer fractions are restricted to the estuarine region and the coarser fractions occupy the rest including the rivers; (2) well sorted and negatively skewed sediments are present in the Muvattipuzha and Ittupuzha rivers and in the neighbourhood of these river mouths and some of the subsidiary channels while poorly sorted and positively skewed sediments are present in the rest of the lake; and (3) sediments deposited as bed load are confined to the Muvattipuzha and Ittupuzha rivers and to the subsidiary channel between Arakuttiand Shertallai, those deposited as rolling and suspension population(with higher percentage of rolling population towards the western margin in the southern half) occupy the area between the southern end of Kumbalam Island and the southern end of the lake and those deposited as suspended load are restricted to the estuarine region. The presence of finer sediments in the southern half of the lake has been attributed to (1) the shallowness of the area, (2) the presence of a number of big and small rivers that supply the finer material to the lake during the monsoon months, and (3) the presence of fresh water conditions for the major part of the year, while the finer sediments in the estuarine region are due to the deposition of suspended load by flocculation during the post and premonsoon months and the supply of fine material from the sea through tidal currents during these months