The irrigation of the Sawad lands which depended on the Tigris River are presented as the subject of this paper. In this paper one of the largest irrigation systems, not only of the old world but even today is described; this is the Nahrawn Canal System which irrigated the whole alluvial lands east of the Tigris extending as far east as the foothills of Persia and from Al- Dur above Samarra towards the south for almost 300 kilometers. From the archeological findings and historical writings, we know that this system had existed for thousands of years. The construction of the old Nimrud dam on the River Tigris was associated with the excavation and construction of this canal, and they were both attributed to the days of Hammurabi the famous king of the Chaldeans which had been mentioned previously although the canal may have been excavated in many stages and by successive kings. The canal and its whole systems were kept in good operational conditions by the successive empires who ruled the country as agriculture in this vast area of land had depended entirely on its water supply. The revenue drawn from this agriculture was a major part of the income of these empires which had fueled their progress, prosperity and their wars. Full description is given of the canal itself and the two feeder canals which had supplied it with water from the Tigris south of Sammara. The third feeder excavated by Khusraw Anushirwan which took off from al- Dur north of Tikrit which was called Katul Kisrawi is also described. More canals which were associated with the Nahrawn Canal such as al- Qawrach and Nahr abu el- Jund are detailed with the circumstances that led to their construction. The trunk canal course and the two massive gravity dams, which served as diversion structures on the Adhaim and Diyala Rivers are described, which were obstructing the passage of the course of the canal. Maps and sketches also supported these descriptions. The towns and settlements along the course of al-Nahrawn Canal are detailed with descriptions based on the writings of the Muslim geographers of the Abbasid period who had reported in length about the canal and the state of the country it had served. The canal was also the main source of water supply to the eastern quarters of Baghdad, which had stretched over considerable area during the days of Khalifah al- Mansour and the following Khalifahs. The distributaries of the Nahrawn, which served the eastern quarters of city on the right side of the Tigris, are, therefore, described in details and the dense network of the secondary canals and watercourses branching from them to serve the various quarters of the city are given full attention. In addition to this, the notes and sketches that were made by two British travelers who travelled along the whole course of the canal in three separate occasions at the end of the nineteenth century are documented with their remarks and notes. In addition, the comments of Sir William Willcocks, the famous irrigation British engineer of the same period, are also added. In summary, the story of one of the greatest engineering achievements of the old people of Iraq in addition to narrating the events that led to its decline and final collapse during the last period of the Abbasid Khalifah.
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