International audienceLittle is known about the Himalayan glaciers, although they are of particular interest in terms of future water supply, regional climate change and sea-level rise. In 2002, a long-term monitoring programme was started on Chhota Shigri Glacier (32.2°N, 77.5°E; 15.7 km2, 6263-4050 m a.s.l., 9 km long) located in Lahaul and Spiti Valley, Himachal Pradesh, India. This glacier lies in the monsoon-arid transition zone (western Himalaya) which is alternately influenced by Asian monsoon in summer and the mid-latitude westerlies in winter. Here we present the results of a 4 year study of mass balance and surface velocity. Overall specific mass balances are mostly negative during the study period and vary from a minimum value of -1.4 m w.e. in 2002/03 and 2005/06 (equilibrium-line altitude (ELA) ∼5180 m a.s.l.) to a maximum value of +0.1 m w.e. in 2004/05 (ELA 4855 m a.s.l.). Chhota Shigri Glacier seems similar to mid-latitude glaciers, with an ablation season limited to the summer months and a mean vertical gradient of mass balance in the ablation zone (debris-free part) of 0.7 m w.e. (100 m)-1, similar to those reported in the Alps. Mass balance is strongly dependent on debris cover, exposure and the shading effect of surrounding steep slopes

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An analytical study of major cations and anions of the glacier fed Alaknanda River and its tributaries has been carried out to assess the weathering and geochemical processes in the high altitude river basin. The Alaknanda and its tributaries were sampled during pre-monsoon (June 1992) and post-monsoon (October 1992) season for the present hydrogeochemical study. Calcium and magnesium are the major cations, accounting for 85% of the total cations. Among the anions, bicarbonate is the most dominant (78%) with minor contribution from sulphate (19%) and chloride (3%). The average (Ca+Mg)/HC0 3 equivalent ratio of 1.2, a relatively high contribution of (Ca+Mg) to the total cations (TZ +) and high (Ca+Mg)/(Na+K) ratio indicate that the carbonate weathering could be the primary source of the major ions to these waters. Wide seasonal and spatial variations are observed in the total dissolved solids (TDS) and total suspended matter (TSM) concentration in the Alaknanda river basin. The chemical denudation rate of Alaknanda is 133 t km" 2 year 1 , nearly double that of the Ganga river basin and three times the world average. The sediment erosion rate of the basin is 356 t km 2 year 4 , double that of the world physical denudation rate. The high rates of denudation are attributed to high relief, intense monsoon rainfall, landslides and glacial erosion in the Himalaya. Chimie des éléments majeurs et érosion dans un bassin de haute montagne (Alaknanda, Garhwal Himalaya, Inde) Résumé Une étude des cations et anions majeurs de la rivière Alaknanda d'origine glaciaire et de ses affluents a été entreprise en vue d'évaluer les processus d'érosion physique et chimique dans un bassin de haute montagne. Pour cette étude hydrogéochimique, la rivière Alaknanda et ses affluents ont été échantillonnés avant (juin 1992) et après (octobre 1992) la mousson. Les principaux cations sont le calcium et le magnésium qui représentent 85% du total des cations. En ce qui concerne les anions, on note principalement la présence des bicarbonates (78%) et accessoirement des sulfates (19%) et des chlorures (3%). Un rapport moyen (Ca+Mg)/HC0 3 égal à 1.2, une contribution relativement élevée de la somme (Ca+Mg) au total des cations (TZ +) et un rapport (Ca+Mg)/(Na+K) important indiquent que l'érosion des carbonates pourrait être à l'origine des ions majeurs de ces eaux. On observe d'importantes variations saisonnières et spatiales des matières dissoutes totales et des matières en suspension dans le bassin de la rivière Alaknanda. L'érosion chimique de ce bassin est de l'ordre de 133 t km" 2 an" 1 , soit près du double de celle du bassin du Gange et du triple de la moyenne mondiale. L'érosion mécanique est de 3561 km 2 an" 1 , le double de la moyenne mondiale. Ces valeurs très élevées doivent être attribuées à l'altitude, à l'intensité de la mousson, aux glissements de terrains et à l'érosion glaciaire de l'Himalaya.

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