The quality of Nahr Beirut's water, supplying water to Beirut and the suburbs, has been studied. A complete algology and chemical study has been conducted. The diatoms are considered one of the best bioindicators of the water's surface because they react directly with organic pollutants and they need a very ecological medium. The use of some diatomic indicators and the polluo-sensible ones (IPS) is widespread in the French hydrographic network. The methodology applied in our biotope showed good results around the head-water and dramatic deterioration around Beirut city. Water samples were collected periodically and analyzed for HCO, Cl, and HNO, and for the major ions/Ca, Mg, Na and K, and for some trace elements. Atomic absorption spectrometry using flame acetylene was used to measure the alkaline metals, whereas graphite furnace was used for the determination of trace elements. It has been found that the major composition in rivers is predominantly influenced by the composition of parent rocks in Lebanon. The slightly elevated values of PH (7.7) in the rivers and the large amount of carbonate HCO, dolomite CaMg (CO) and limestone. These compounds lead to elevated concentration of Ca and Mg in the studied stations. The concentration of the major ions varied with the cationic dominance order of Ca > Mg > Na > K which is compatible with the common natural major ion assemblages established for world rivers. The concentration of the major ions are also predominantly influenced by the Chemical weathering of rocks and minerals. The mean concentration of nitrates increased in urban and agriculture areas. The mean concentration of trace elements in the freshwater increased with increasing distance from the headwater, particularly at Ain el Delbe and the end station of Jisr Bacha, a consequence of anthropogenic inputs. In fact the above mentioned stations are located in industrial and popular areas. (author)