Suspended sediment from the Gangotri Glacier: Quantification, variability and associations with discharge and air temperature

Abstract

To understand the sediment delivery variation for a Himalayan Glacier (Gangotri Glacier, Garhwal Himalayas) and to determine its relationship with discharge and air temperature, suspended sediment samples and discharge data were collected near the glacier snout (4000 m) for four melt seasons during the period 2000–2003. These data were used to estimate suspended sediment concentration (SSC), suspended sediment load (SSL), sediment yield and erosion rate in the glacier melt stream (Bhagirathi). The monthly distribution of suspended sediment and its variability from year to year have been examined. Mean monthly SSC for May, June, July, August, September and October were found to be 1942, 2063, 3658, 2551, 734 and 136 mg l −1, respectively. Maximum SSC in meltwater was observed in July followed by August. It was found that the cumulative percentage delivery of SSC precedes discharge throughout the melt season. Mean monthly total SSL for May, June, July, August, September and October during the study period was found to be 149, 423, 1220, 746, 143 and 5×10 3 ton, respectively. The strong variability is found in SSL ( C v=1.1) than SSC ( C v=0.8) because computation of SSL includes both discharge ( C v=0.6) and SSC. Delivery response of SSL in terms of percentage of total load is less in the early part of the melt season than in the later stage in comparison to that of discharge. This may be due to the fact that in the beginning of the melt season low melt rate conditions prevails and thus, the low discharge velocity could not flush out stored glacial sediment. It has been observed that 59–64% of the sediment passed through the channel by the time 50% of the total discharge passed. The average suspended sediment yield for the whole melt season from the study area was estimated to be about 4834 ton km −2 and corresponding erosion rate was 1.8 mm. The relationship between mean monthly SSC and discharge ( R 2=0.99) is much better than the daily SSC and discharge ( R 2=0.40) because variability of both parameters is averaged-out on monthly scale. Mean monthly SSC and mean monthly SSL provide a good exponential relationship with mean monthly air temperature. These results are relevant for planning and management of water resources in the high altitude areas and for designing hydropower projects.