[en] Pakistan lies between latitudes 24 deg. and 37 deg. North and longitudes 61 deg. to 76 deg. East. It possesses quite complicated and attractive physiographical features. There are very often a series of mountain ranges possessing deep broad valleys in-between. It includes the famous valley of the Indus that has been the cradle of ancient civilization like those of the delta area of Nile and the valley of the Tigris Euphrates. Indus River is one of the longest rivers in the World. It has five major tributaries viz. Bias, Satlej, Ravi, Chenab and Jhelum joining from eastern side, while a number of small rivers join the Indus on the right side. All these main rivers are perennial. They originate from the mountains. Physiography and climate of the catchments of these rivers vary widely. Going from the catchment of the River Satlej to the catchment of Indus River, altitude increases and temperature decreases. In Northern Areas, mountains are covered with glaciers and some of the peaks are higher than 8000m, which get snowfall even in summer season. The basic sources of these rivers are snowmelt, rainfall and under certain conditions seepage from the formations. For certain rivers the source of snow is seasonal which falls in winter and melts in summer. From the middle of March to the breaking of monsoon, in mid July, river water is drawn from the melting of snow. During monsoon, rainfall run-off is added to the rivers over and above that from melting of snow so their discharge increases manifold. During 1980-84, samples were collected on monthly basis from the river Satlej at Sulimanki, the river Ravi at Baloki (upstream including Qadirabad-Baloki Link Canal originating from the river Chenab) and Sidnai including two link canals originating from Trimu Headworks just after the confluence of the rivers Chenab and Jhelum, the rivers Chenab at Marala, mixed water of the rivers Chenab and Jhelum, the river Indus at Taunsa and Panjnad (after joining the other tributaries). The samples were analyzed for ¹⁸O, ²H and ³H isotopes. All the rivers have wide ranges of stable isotopes and tritium. The river Indus at Taunsa has relatively the most depleted values of δ¹⁸O and δ²H because of major contribution of snowmelt coming from glaciated peaks in Northern Areas. Tritium is also higher due to some contribution of snow fallen during high tritium period in 1960s. Isotopic data of pure snowmelt collected during 1992-94 show that δ¹⁸O (-15.9 to -12.2 per mille) and δ²H (-115 to -82 per mille) are even more depleted along with still high tritium ranging from 25 to 65TU, which supports the above finding. Isotopic signatures of the river Indus at Panjnad get enriched due to contribution of other tributaries, which have enriched isotopic values. The rivers Sutlej and Ravi have he most enriched values of δ¹⁸O and δ²H because their catchments have relatively low altitude and contribution of snowmelt is also less. River Chenab at Marala has the widest ranges of δ¹⁸O and δ²H because of mixing of snowmelt originating from higher altitudes and rainfall of piedmont areas. Data of Trimu, which show the combined effect of the rivers Chenab and Jhelum is almost similar to that of Marala. The δ¹⁸O and δ²H monitored at both of these stations i.e. Marala and Trimu during 1990-93 have average values of -10 per mille and -61 per mille and -9.4 and 59 per mille respectively, which are slightly different than the previous record. It also observed that temporal variations of both the δ¹⁸O and δ²H in rivers are cyclic especially in the rivers Indus, Jhelum and Chenab depending on the contributions of snowmelt and rains i.e. enriched during monsoon. The δ¹⁸O and δ²H data also give information about source of moisture. The winter runoff and snowmelt have relatively depleted isotopic signatures and higher d-excess indicating the source of moisture from the West (Mediterranean Sea) while the d-excess in monsoon is relatively less along with enriched isotopic values, which is also confirmed by the meteorological information. The correlations between δ¹⁸O and δ²H for all the give the slope lees than 8, which is mainly due to high d-excess in winter precipitation or snowmelt and low d-excess of monsoonal/summer rains. The lowest slopes for Taunsa at Indus and Marala at Chenab are attributed to high variations in discharge contributions of resulting from both the sources of moisture through precipitations