Characterization of subglacial pathways draining two tributary meltwater streams through the lower ablation zone of Gangotri glacier system, Garhwal Himalaya, India

An attempt has been made to characterize the subglacial pathways that transport the meltwaters from Chaturangi and Raktavarn glaciers through the lower ablation zone of the Gangotri glacier, Indian Himalaya, by means of dye tracer experiments. These inactive tributaries of the Gangotri glacier contribute significantly to the proglacial discharge, which is dominantly controlled by air temperature (0–20°C at the measurement site) rather than rainfall (117 mm from June to September 2008). The breakthrough flow velocities, ranging from 0.6 to 1.7 ms−1 and dispersivity values varying between 2.1 and 17.5 m, decreased with bulk discharges in the proglacial stream (R2 = 0.66), indicating flow through highly efficient subglacial channels that develop during the melt season with increased meltwater influx. However, the dye return curves giving high velocities in a narrow range from 1.3 to 1.7 ms−1 as seasonal discharges were peaking, are interpreted as indicative of flow through pressurized channel/s. In general, the subglacial channels routing meltwater from these input points are arguably controlled by recharge hydrographs rather than channel geometry during the ablation season. Closely spaced dye tests need to be carried out to better understand the factors that control the diurnal and seasonal development of these hydrological pathways, which are important for downstream water management.