Fluvio sedimentary environment of the Pagladiya and Puthimari rivers an investigation of their lower catchments in Assam

Abstract

The study was carried out in Pagladiya and Puthimari rivers, two major north bank tributaries of the Brahmaputra river, to examine the various aspects of the fluvio-sedimentary environment of the two river systems in their Assam section. Sediment analysis and lithofacies studies have been carried out on the point bars of both the rivers. Geological, geomorphological, soil, land use/land cover, channel migration study have also been done in the field as well as in the laboratory. From the study of aerial photographs and satellite images along with other ancillary data it has been found that the Bhutan Hills in the upper catchments of the rivers are composed of loose and weathered rocks with poor vegetation cover making the area highly susceptible to erosion. Rainfall increases from the Brahmaputra valley towards the Himalayas, which helps in carrying large amount of sediments to the plains. Most of the soils in the catchments in Assam are of transported type. The rivers have changed their courses during recorded history since 1893. Great changes occurred due to the 1897 Assam Earthquake. Since the earthquake had occurred, the Puthimari river is more or less stable though changes occurred in the Pagladiya. From the study of grain-size analysis of the Pagladiya and the Puthimari river sediments, it has been observed that overall grain-size decreases in the downcurrent direction. In bars of both the rivers it is observed that the surface sediments (lateral distribution) are coarser towards the rivers and finer away from the river courses towards the river banks. The mean size of sediments varies from 1.28 to 4.83 (average 2.92) for the Pagladiya river sediments; and 2.05 to 4.41 (average 3.31) for the Puthimari river sediments, i.e. in general, the sediments of the Puthimari river are finer than that of the pagladiya river. From the mean size variation it may be concluded that average grain size of the batrs falls in the fine sand category. The Pagladiya sediments are fine sand, moderately sorted, very positive...