Hydrodynamics of a Supraglacial Lake and its Effect on the Basin Expansion: Tsho Rolpa, Rolwaling Valley, Nepal Himalaya

Abstract

The thermal structure and hydrodynamics of supraglacial Tsho Rolpa Lake (27°51′N, 86°29′E) in the Nepal Himalaya were examined in the premonsoon of 1996. We continuously measured flow velocity, water temperature, and turbidity with moored self-recording current meters, temperature data loggers, and turbidimeters. Vertical measurements (every 0.2 m in depth) of water temperature and turbidity were also made by lowering a self-recording sonde. Tsho Rolpa Lake (surface area, 1.39 km² at present) has increased in size since late 1950s (surface area, 0.23 km² in 1958) by both glacial ice melt below the lake bottom and the retreat of the glacier terminus. Lake stratification is defined by suspended sediment concentration (SSC) rather than by temperature. The suspended sediment is mostly silt and clay (d < 15.6μm), which is supplied by the meltwater discharge from a subaqueous tunnel portal into the lake at the glacier terminus. The sediment is transported to the deepest zone of the lake by underflows and diffused by advection by the other currents into the upper zone. The observations revealed that a diurnal valley wind produces a vertical water circulation in the quasi-isopycnal surface layer, which is about 27 m deep. This circulation transports the surface water, heated mostly by solar radiation, toward the glacier terminus and consequently forces relatively warm water (>~5°C) into continual contact with the glacier terminus. This warm water contact could induce calving of the upper glacier-ice by increasingly melting the subaqueous lower part.