Isotope Investigations of Major Rivers of Indus Basin, Pakistan

Indus River is one of the longest rivers in the World. It has five major tributaries joining from eastern side, the Bias, Satlej, Ravi, Chenab and Jhelum, while a number of small rivers join the Indus on the west side. These perennial rivers originate from mountains covered with glaciers. Isotopic (18O, 2H, 3H) monitoring of these rivers was carried out between 2002 and 2005 to study temporal variations of these isotopes at different control points and to understand water cycles and hydrological processes in the catchments of these rivers. The headwaters of the main Indus River (the Hunza, Gilgit and Kachura tributaries) display the most depleted δ18O (-14.5 to -11.0 per mille ) and δ2H (-106.1 to -72.6 per mille ) values due to precipitation at very high altitudes and very low temperatures. Generally these waters have low d-excess, which indicates that the moisture source is the Indian Ocean. The high d-excess in some winter (November-February) samples from the Hunza and Gilgit indicate the dominance of Mediterranean moisture sources. Kachura station has a δ18O- δ2H line with a slope of 4 and low d-excess, indicating an evaporation effect in Kachura Lake. The Indus River become enriched in isotopes going downstream towards the Arabian Sea because of contributions from rains at low altitude plains and a baseflow mainly recharged by local rains. At tail stations, evaporation and contribution from baseflow are responsible for isotopic enrichment. Low tritium in some samples also indicates a baseflow contribution of (relatively older groundwater). The Chenab River has the widest variation in δ18O and δ2H, and the slope of the δ18O- δ2H line is 6.1, which is due to the variable contribution of snowmelt at high altitudes and rains at low altitudes. High d-excess in snowmelt and low d-excess in monsoons show that the moisture source in winter is generally western (Mediterranean) and that monsoon conditions predominantly originate from the Indian Ocean. The Kabul River has experiences a wide variation in isotopic values, probably due to variable contribution from the Swat River, which carries snowmelt. High d-excess associated with depleted isotopic values indicates a dominance of Mediterranean moisture sources in winter, while a low d-excess associated with enriched isotopic values, which typically occurs during summer monsoons points to the Bay of Bengal as the major source of moisture. The Jhelum River and its tributaries upstream of Mangla Lake experience enriched δ18O and δ2H due to the low altitude of its catchment. Precipitation in the catchment results from Indian Ocean moisture sources, and there is not any significant evaporation effect. Isotopic variations in the river Ravi appears to be mainly due to water diversions from the western rivers through link canals. (author)