Watersheds located in semi-arid areas such as the eastern Mediterranean are particularly sensitive to the impact of climate change. To gain knowledge on the hydrogeochemical processes occurring in the Nahr Ibrahim watershed, a Critical Zone Observatory in Lebanon, we analyze the isotopic composition of the river water as well as the concentrations of the major ions exported (Ca 2+ , Mg 2+ , HCO 3-, Na + , Cl-, K + , SO 4 2-). Sampling campaigns were conducted from March 2014 to August 2016 to capture contrasting hydrological conditions. The results indicate that the carbonate lithology of the watershed is the predominant source of Ca 2+ , Mg 2+ and HCO 3-, whereas the low contents of Na + , Cl-, K + , SO 4 2mainly originate from sea spray. Except in the headwaters, the Nahr Ibrahim River is oversaturated with respect to calcite and dolomite. During wet seasons, calcite and dolomite weathering contribute in an equivalent manner to the solute budget, whereas during dry seasons, calcite precipitates in the river. The isotopic composition of the river water reveals little seasonal dependency, the groundwater recharge by snowmelt infiltration leading to spring waters depleted in heavier isotopes during the dry seasons. A Carbonate Weathering rate of about 176 t/km 2 /yr was determined at the outlet of the Nahr Ibrahim watershed. The calculated values of CO 2 partial pressure, on average twice the atmospheric pressure, suggest that the river is a significant source of CO 2 to the atmosphere (111 t/yr).

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The population of Ephemeroptera was studied in three selected stations of Rawanduz River (Gali Ali Beg water fall, Rawanduz River and after the junction of these two waters) during the three seasons of spring, summer and autumn in 2016. In addition,sixteen physicochemical parameters (pH, EC, turbidity, DO, BOD5, NO3, TDN, TDP, HCO3-, Hardness, Ca2+, Mg2+, Na+, K+, Cl-, SO42-, Na% and SAR) of water in these stations were estimated and used to calculate the Canadian Council of Ministers of the Environment Water Quality Index (CCME WQI). Eleven species of aquatic insects were identified,which belong to four families of the order Ephemeroptera. Three of the studied species are described for the first time. According to water quality index, the water was ranked as poor for drinking and marginal for irrigation purposes.