Lacustrine sedimentation in a monsoon environment: the record from Phewa Tal, middle mountain region of Nepal

A watershed-scale approach to the sedimentary environment of the Phewa Tal drainage basin is used to assess rates and patterns of sediment accumulation within Phewa Tal reservoir. The susceptibility of surficial materials to erosion and transportation is highest in the early part of the monsoon season but major erosional events also occur during the monsoon proper. Thus, sediment delivery to the reservoir is at least two orders of magnitude higher during the monsoon than the dry season. In the reservoir, suspended sediment is distributed mainly by interflows at 4 to 6 m and 8 to 12 m depth. Phewa Tal has four distinct sedimentary environments as interpreted from the sedimentary record in cores. (1) The deltaic environment, which receives sediment directly from the main inflowing stream, the Harpan Khola, and from up-valley wetlands and agricultural fields, experiences rates of sedimentation of up to 1 m a−1. (2) The delta-proximal region where clearly varved sediments are deposited in response to the annual monsoon, has a mean rate of sedimentation is 23.5 mm a−1. (3) The distal environment is beyond the influence of sediment-charged interflows, and is characterised by accumulation from suspension to form largely massive deposits, so that annual rates of sedimentation cannot be determined stratigraphically. (4) The region near the Pardi Dam is dominated by seasonal flushing and erosion of fine sediments during the monsoon. The sedimentary record documents the geomorphic processes of the drainage basin, especially as controlled by the monsoon, and indicates the impact of human occupance in the drainage basin in the twentieth century. The useable life of the reservoir is estimated as 360 years.