Abstract The authors translate and comment a digression from the Kit?b al-???r al-b?qiya on several hydraulic hydrological subjects. passage reveals al-B?r?n?’s understanding of fluvial regimes, water physical behaviour, handful peculiar natural phenomena. Al-B?r?n? departs discussion weather forecast seasonal regimes Tigris, Euphrates, Oxus, Nile. main concern al-B?r?n? is to defend principle ...